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Old March 30th, 2012, 03:47 PM   #1
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How to Choose the Best Carburetor ?

I've been searching around for a cure to my carburetor problems. I've got a 72 Cutlass S with a 350, a th350 trans, 3.33's in the rear, an edelbrock 1406 Carburetor matched up with an Edelbrock Performer RPM intake.

Edelbrock specifically states matching up a 1406 carb and the performer RPM intake isn't recommended(apparently the last owner didn't get that memo).

Now the car just doesn't run right. If a floor the accelerator from a dead stop, the car hesitates for a second before it "comes to life". Roughly midway through first gear, I can feel a loss of power. It's not significant, but it's there. Almost like a small stumble. After I pass this rpm range, the car will shift and usually chirp second gear. The hesitation has been there since I bought the car, so it's not related to the dying accelerator pump(i don't think). If I'm driving at like 30mph and I stand on it, sometimes it'll get right up and go, sometimes it won't. Also, if I'm on the highway cruising, and I let off, I'll get a couple of "pops" out of the tailpipes. This is caused by a lean condition, right?

I haven't taken the 1406 apart yet to investigate. All I've done is sprayed the crap out of it with carb cleaner, gotten any gunk out that I could find.

The Carb Tech line at Edelbrock recommends I buy a calibration kit for my 1406. Part number 1487. He says it'll eliminate the hesitation mid way through. This, combined with a new accelerator pump, and he thinks I'll be good to go.

What do you Oldsmobile Experts think? Would you get a 1406 and re-calibrate it, or buy a 1405 and run it a little lean?

Anyone with a similar drive line want to share what they've done and how well it's worked?

Last edited by jpc647; March 30th, 2012 at 04:15 PM.
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Old March 30th, 2012, 04:51 PM   #2
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I don't understand why you can't use that carb on a performer manifold. I looked at both the carb and the manifold instructions and did not see anything saying you could not.

Is it the choke setup?

Have you tried moving your accelerator pump linkage up a notch? Are your primaries and secondaries opening all the way.

Also, I'm a firm believer that a lot of carb problems arise from ignition issues. What distributor are you running, is your dwell (if applicable)@ 30, timing and curve set properly?
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Old March 30th, 2012, 05:15 PM   #3
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If you are running a stock 72 350 (8 to 1 Cr and .400 cam) the RPM is not the correct intake. The Performer is a better choice, IMO. Sell the RPM, buy a Performer, keep your carb.
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Old March 30th, 2012, 05:16 PM   #4
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I don't understand why you can't use that carb on a performer manifold. I looked at both the carb and the manifold instructions and did not see anything saying you could not.

Is it the choke setup?
They don't recommend using a 1406 Carb with the RPM intake because the 1406 Carb is their "economy carb". Out of the box it is set up for fuel economy, not for performance, where as the RPM intake is a performance oriented intake.

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Have you tried moving your accelerator pump linkage up a notch? Are your primaries and secondaries opening all the way.

Also, I'm a firm believer that a lot of carb problems arise from ignition issues. What distributor are you running, is your dwell (if applicable)@ 30, timing and curve set properly?
Yup, the accelerator pump is on the top notch. I haven't taken it out to see if it's torn or anything, and I don't have the gasket kit yet, to be able to seal the top plate back on(pretty sure there is a gasket there). I started having problems last year, after the car sat for quite a while. When testing in the yard, the both the primaries did seem to be opening all the way. I played with the secondaries a bit, sprayed them good and proper with carb cleaner, and they seem to be opening right.

I have a GM HEI distributor. I don't know anything about curving a distributor. I did check the timing at the end of last season, I want to say it was 12* BTDC, but I could be mis-remembering. I thought I posted, and members said it was fine.

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If you are running a stock 72 350 (8 to 1 Cr and .400 cam) the RPM is not the correct intake. The Performer is a better choice, IMO. Sell the RPM, buy a Performer, keep your carb.
As far as I know the motor is stock. Any simple way to check? I certainly don't have a high-lift cam with a lumpy idle. You may be right. If that's the case, I'm probably better off having the old Q-Jet professionally done, acid dipped and all, and throwing the stock intake on. How much of a performance loss would I be taking at this point?

Side note, It'd have to find an OEM air cleaner, and notch out the back of it to fit around the HEI distributor.

Last edited by jpc647; March 30th, 2012 at 05:23 PM.
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Old March 30th, 2012, 07:11 PM   #5
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Just find an air cleaner from an HEI equipped V8, dime a dozen.

"As far as I know the motor is stock. Any simple way to check? I certainly don't have a high-lift cam with a lumpy idle. You may be right. If that's the case, I'm probably better off having the old Q-Jet professionally done, acid dipped and all, and throwing the stock intake on. How much of a performance loss would I be taking at this point?"

None. With a stock intake and rebuilt Q-jet, the car should run much better.
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Old March 30th, 2012, 07:59 PM   #6
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Since your car sat awhile did you remove the shooter and clean under it? There is a check valve there that holds the fuel up top ready to go out the shooters. If it's all varnished up it may leak allowing fuel to drain back and give you a flat spot initially. It should look like a square pointy rod with the point down. Edelbrock may have changed this to a check ball, I'm not sure.
If you're getting a pop when letting off it probably is too lean. Get the kit and richen the primary side a little. Unfortunately with that carb you have to change both rods and jets at the same time as doing only one will richen it too much (or lean it). GL
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Old March 31st, 2012, 06:02 AM   #7
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yeah at the very least i'd put a std. performer on it... or bump up the compression, cam it up a bit and put headers and dual exhaust on it and recalibrate the carb.

i guess what i'm trying to say is that intake is a bad choice for the way your engine is configured...
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Old March 31st, 2012, 06:18 AM   #8
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Does your mechanical timing advance stop at 32-36 deg BTDC @ 3000 - 3500 rpm with vac advance disconnected?
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Old March 31st, 2012, 01:01 PM   #9
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I'd be checking everything else before just laying blame on the carb. Make certain the distributor isn't messing up, check the advance Mech and vacuum, cap ,rotor, coil button and pickup and make certain the advance is hooked up right, ported vacuum pulls the advance on when floored, good, if it's hooked to manifold vacuum the timing falls off when floored, bad. If dist is OK then move onto the carb which probably just needs a minor jet/rod change. I'd also check the timing chain to make certain the cam isn't running 12 degrees behind the rest of the engine.
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Old March 31st, 2012, 05:01 PM   #10
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No matter where your vacuum advance is hooked up, when floored it's supposed to drop to 0. It's under lite throttle you get your vacuum advance. If you have it on ported (the passenger side port on the carb), you should have 0 vacuum at idle. With manifold (the drivers side port on the carb), you should have a high reading at idle.
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Old March 31st, 2012, 07:28 PM   #11
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fwiw i found ot that in some application running the unported vac. source for the dist. adv. will cure a flat spot from an idle. When i ran the 1405 on a very mild 350 i had . it would run bad on the ported side after some playing around i found out that the unported side so its pulling vac at idle helped it . I may have that ported unported vac. bacwards but the point being play with where you have the vac. line hooked up on the carb it may aid that initial flat spot., As for that lack of power while cruising that could be a lean surge due to not enough fuel. The rpm has more intake volume so it will require more fuel and air the 1406 is leaner than the 1405.
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Old March 31st, 2012, 09:36 PM   #12
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Here's a video of my car from launch http://www.youtube.com/watch?v=jeZAoQg035w
There is not much difference in the 1406 and the 1405. A simple jet change and presto, the carbs are virtually the same. In reference to the lean spot...you can use a stiffer step up spring (pink or silver) to help with the lean spot. Also, if you get the recalibration kit you can try a thinner metering rod. Doing both of these actions will give you more gas to eliminate a lean spot, if indeed that is your problem. I'm running the Edelbrock RPM manifold and a 1406 just like you are. You can see from this video, that my car works just fine with that set up. http://www.youtube.com/watch?v=jeZAoQg035w
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Old March 31st, 2012, 10:01 PM   #13
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i ran a borrowed 1406 for a while and it ran fine for me too on the rpm intake. The 1405 was a big difference and now the 650 holley turned it into a beast.
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Old March 31st, 2012, 11:22 PM   #14
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i ran a borrowed 1406 for a while and it ran fine for me too on the rpm intake. The 1405 was a big difference and now the 650 holley turned it into a beast.
I currently have a 1406. Thinking of buying a holley 650, which one do you recommend ? Double pumper?
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Old April 1st, 2012, 05:05 AM   #15
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The 650 will need a good stall speed i tried running a 650 before with a 2200 stall and it didnt like all that fuel that soon. It really somesdon to your combo and what it requires. I ran a 1405 on my current combo and it went 13.3 at 103 in the quarter mile . It also depends what you wanna do the 650 dp is a gas pig.
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Old April 1st, 2012, 09:49 AM   #16
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There is not much difference in the 1406 and the 1405. A simple jet change and presto, the carbs are virtually the same. In reference to the lean spot...you can use a stiffer step up spring (pink or silver) to help with the lean spot. Also, if you get the recalibration kit you can try a thinner metering rod. Doing both of these actions will give you more gas to eliminate a lean spot, if indeed that is your problem. I'm running the Edelbrock RPM manifold and a 1406 just like you are. You can see from this video, that my car works just fine with that set up. http://www.youtube.com/watch?v=jeZAoQg035w

If I understand this thread,
http://classicoldsmobile.com/forums/...tml#post380092
you are running a rebuilt engine with 10.5 to 1 CR, correct? How does that apply to a worn 8 to 1 engine? I will state again, the RPM is not the best intake for this application.
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Old April 1st, 2012, 05:39 PM   #17
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[QUOTE=oldcutlass;388880]No matter where your vacuum advance is hooked up, when floored it's supposed to drop to 0.

OOP's what am I thinking?? You are right. I researched this last fall and even change my car over for ported to manifold vacuum and it ran better.
Maybe my wife is right, it's time to drop me off at the park and see if I can find my way home.
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Old April 1st, 2012, 06:13 PM   #18
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So THAT's why I couldn't find my wife when we went to the park last week!

I guess it's okay - I met this other lady who seems quite nice... I just wonder where my wife and my house went...?
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Old April 2nd, 2012, 09:15 AM   #19
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Here's a video of my car from launch http://www.youtube.com/watch?v=jeZAoQg035w
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If I understand this thread,
http://classicoldsmobile.com/forums/...tml#post380092
you are running a rebuilt engine with 10.5 to 1 CR, correct? How does that apply to a worn 8 to 1 engine? I will state again, the RPM is not the best intake for this application.

The purpose of my post was to point out that the 1406 does work on an Edelbrock RPM intake. This was a concern to the OP, so I answered it with a fact and a video(after I posted it, someone else confirmed they too used the 1406 on an RPM manifold with no problem). I then gave the OP factual advice on what to do with an Edelbrock 1406 carb to help with a lean problem, if indeed he has one. The 1406 can be used with his 8:1 motor (just like my 10.5:1 motor) and if he so chooses, he can try out the RPM manifold as well. It's up to him. Pretty straightforward and simple to follow.
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Old April 2nd, 2012, 02:08 PM   #20
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The purpose of my post was to point out that the 1406 does work on an Edelbrock RPM intake. This was a concern to the OP, so I answered it with a fact and a video(after I posted it, someone else confirmed they too used the 1406 on an RPM manifold with no problem). I then gave the OP factual advice on what to do with an Edelbrock 1406 carb to help with a lean problem, if indeed he has one. The 1406 can be used with his 8:1 motor (just like my 10.5:1 motor) and if he so chooses, he can try out the RPM manifold as well. It's up to him. Pretty straightforward and simple to follow.
I think you are overlooking the fact that the engine with the higher compression and more cam makes the extra volume of the RPM a better choice. Showing a video of an engine with 2.5 to 1 higher CR and saying "Hey, works fine for me" really does this guy no good at all. The carb debate can continue, but what is the point if he has the incorrect intake on it? My 9 to 1 355 with a 210/216 cam ran noticably better with the performer,
http://www.realoldspower.com/phpBB2/...performer++rpm
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Old April 2nd, 2012, 07:25 PM   #21
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Here's a video of my car from launch http://www.youtube.com/watch?v=jeZAoQg035w
I'm not overlooking a thing, I simply said if he wants to use that manifold "It's up to him." It is his car.

You're obsessing over what intake this guy chooses. You've already told him in this thread (now 3 times) you think the regular Performer would be better. No one is disputing that, so you can relax. For the second, and hopefully last time: the point of my post was to correct the erroneous info that a 1406 does not work well with a RPM manifold. This correction helps future readers of this thread. Also, if the OP cannot afford another manifold, or doesn't want to change the RPM out, he can at least know there is no issue with using that combo (ie. linkage binding or whatever). Now let the guy choose what intake he wants. He will either take your advice or he won't.
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Old April 3rd, 2012, 10:02 AM   #22
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Since your car sat awhile did you remove the shooter and clean under it? There is a check valve there that holds the fuel up top ready to go out the shooters. If it's all varnished up it may leak allowing fuel to drain back and give you a flat spot initially. It should look like a square pointy rod with the point down. Edelbrock may have changed this to a check ball, I'm not sure.
If you're getting a pop when letting off it probably is too lean. Get the kit and richen the primary side a little. Unfortunately with that carb you have to change both rods and jets at the same time as doing only one will richen it too much (or lean it). GL
I didn't remove the accelerator pump. Not yet anyway. I know I need to buy a gasket/rebuild kit in order to take the top of the carb off. Looks like that's my best starting point. I'll order the kit from summit or jegs tonight.


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Does your mechanical timing advance stop at 32-36 deg BTDC @ 3000 - 3500 rpm with vac advance disconnected?
I honestly don't remember. I know I checked it last summer. I bought a timing light the other day, so I can check it again this weekend.


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I'd be checking everything else before just laying blame on the carb. Make certain the distributor isn't messing up, check the advance Mech and vacuum, cap ,rotor, coil button and pickup and make certain the advance is hooked up right, ported vacuum pulls the advance on when floored, good, if it's hooked to manifold vacuum the timing falls off when floored, bad. If dist is OK then move onto the cah probably just needs a minor jet/rod change. I'd also check the timing crb whichain to make certain the cam isn't running 12 degrees behind the rest of the engine.
The plugs, wires, cap, and rotor are all new end of last season. Maybe 100 miles on the car since. I think the carb is hooked up right, it hasn't been changed since I bought the car, and it used to run good. I will check timing this weekend.

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I'm not overlooking a thing, I simply said if he wants to use that manifold "It's up to him." It is his car.

You're obsessing over what intake this guy chooses. You've already told him in this thread (now 3 times) you think the regular Performer would be better. No one is disputing that, so you can relax. For the second, and hopefully last time: the point of my post was to correct the erroneous info that a 1406 does not work well with a RPM manifold. This correction helps future readers of this thread. Also, if the OP cannot afford another manifold, or doesn't want to change the RPM out, he can at least know there is no issue with using that combo (ie. linkage binding or whatever). Now let the guy choose what intake he wants. He will either take your advice or he won't.
In regards to the RPM or not, will a standard performer be better for my application after tuning my carb? Do I really need the extra volume the performer offers with a stock motor. If it works well with a 10.5:1 compression motor, and mine stock is 8:1, couldn't this intake be sucking too much air, and then I'm trying to richen up the car, when I don't really need to? Obviously the RPM intake can be used on a stock motor, but from a performance standpoint, would the performer intake be a better choice?

Or should I just go back to the OEM intake and Q-jet carb. I would think having an aftermarket intake and carb would gain some sort of a performance increase over the OEM equipment(when properly set up).
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Old April 3rd, 2012, 03:15 PM   #23
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Here's a video of my car from launch http://www.youtube.com/watch?v=jeZAoQg035w
Word for years from the personal experience of Olds owners, including myself, has been that the Performer is not worth the money, based on the virtually unnoticeable HP gains it produces over the stock manifold (though it does weigh less). In other words, I would not spend the money on a Performer if you don't have one b/c you would just be tossing the $$ away. Just use the stock manifold.
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Old April 3rd, 2012, 04:35 PM   #24
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I have used a Performer RPM on a stock 330 with a Holley and it worked great. I would say if you are going to continue doing upgrades to your car get the manifold that will be the one you need in the end, a Performer RPM is really necked down at the intake port out of the box (like 3/4" smaller) I think that is why they work fine in the stock engines, then if you do a lot of work down the road have the manifold gasket matched to the heads which is what I did with my rebuild. I don't believe a standard Performer is worth the money over a stock manifold, but an RPM is because you can tailor it to your needs as they increase. Just my opinion but it worked for me.
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Old April 3rd, 2012, 08:07 PM   #25
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Word for years from the personal experience of Olds owners, including myself, has been that the Performer is not worth the money, based on the virtually unnoticeable HP gains it produces over the stock manifold (though it does weigh less). In other words, I would not spend the money on a Performer if you don't have one b/c you would just be tossing the $$ away. Just use the stock manifold.
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I have used a Performer RPM on a stock 330 with a Holley and it worked great. I would say if you are going to continue doing upgrades to your car get the manifold that will be the one you need in the end, a Performer RPM is really necked down at the intake port out of the box (like 3/4" smaller) I think that is why they work fine in the stock engines, then if you do a lot of work down the road have the manifold gasket matched to the heads which is what I did with my rebuild. I don't believe a standard Performer is worth the money over a stock manifold, but an RPM is because you can tailor it to your needs as they increase. Just my opinion but it worked for me.

Thanks you both for the comments. It seems my best bet is to check my timing, and if that checks out, to take my carb apart, look at the shooter and recalibrate it with #1487 kit, to give it a better step up spring and metering rods. Probably should get the insulator gasket, while I'm at it.

I mean I have to buy the gasket/rebuild kit so I can look at/replace the accelerator pump, so I might as well order the accelerator pump at the same time. If nothing else, I have an extra one. It'd like to keep the RPM intake over the OEM one if possible, and if I'm not going to lose performance in any way.

Still wondering, with a stock 72 350, is the RPM overkill? Am I wastefully adding more fuel potential to my carb to compensate for the excessive intake? Or with it actually be beneficial for power delivery?

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Old April 4th, 2012, 06:53 AM   #26
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Still wondering, with a stock 72 350, is the RPM overkill? Am I wastefully adding more fuel potential to my carb to compensate for the excessive intake? Or with it actually be beneficial for power delivery?[/QUOTE]

I don't believe you have excessive intake you just have the wrong combo of intake and carb. Just rejet the carb to the performance specs of the non smog carb and start from there. I used that same intake on a stock 330 and it worked better than the stock manifold and carb.
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Old April 4th, 2012, 11:04 AM   #27
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I don't believe you have excessive intake you just have the wrong combo of intake and carb. Just rejet the carb to the performance specs of the non smog carb and start from there. I used that same intake on a stock 330 and it worked better than the stock manifold and carb.
Thank You. I had a jegs gift card, I bought a new accelerator pump and the thermal gasket(fuel is probably boiling).

I'll order the calibration kit and the gasket kit. I hate I need a whole gasket kit, just to get the gasket that mates the top of the carb to the body, but I'm cheap, and I digress.

Just wanted to be sure that I'm not wasting time and money trying to re-calibrate the 1405 to use on the RPM. I'll play with it this weekend and report back.
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Old April 4th, 2012, 02:28 PM   #28
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Still wondering, with a stock 72 350, is the RPM overkill? Am I wastefully adding more fuel potential to my carb to compensate for the excessive intake? Or with it actually be beneficial for power delivery?
Did you read the link I posted? My 355 was fresh, minor port work, headers, a full point more compression, a LOT more cam, more gear, lighter, better converter, and the performance increase with the Performer over the RPM was noticeable. A stock 72 350 has around 8 to 1 Cr and a .400/194 cam. FWIW, a 330 has over 10 to 1 compression. On a high compression engine with a little head work the RPM is a better choice. Not on a stock 350. IMHO.
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Old April 4th, 2012, 02:30 PM   #29
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Originally Posted by jag1886 View Post
Still wondering, with a stock 72 350, is the RPM overkill? Am I wastefully adding more fuel potential to my carb to compensate for the excessive intake? Or with it actually be beneficial for power delivery?
I don't believe you have excessive intake you just have the wrong combo of intake and carb. Just rejet the carb to the performance specs of the non smog carb and start from there. I used that same intake on a stock 330 and it worked better than the stock manifold and carb.[/QUOTE]

MUCH more compression than a 72 350. Again, you guys are comparing apples to oranges. Compression makes a HUGE difference in port velocity, IMHO.
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Old April 4th, 2012, 08:24 PM   #30
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. On a high compression engine with a little head work the RPM is a better choice. Not on a stock 350. IMHO.

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I don't believe you have excessive intake you just have the wrong combo of intake and carb. Just rejet the carb to the performance specs of the non smog carb and start from there. I used that same intake on a stock 330 and it worked better than the stock manifold and carb.
MUCH more compression than a 72 350. Again, you guys are comparing apples to oranges. Compression makes a HUGE difference in port velocity, IMHO.[/QUOTE]

I'm not following you sir. You're first post seems to say I shouldn't use the RPM intake, the second says to rejet/recalibrate the carb. Which one should I do? If compression makes that much of a difference(I don't know I'm asking), and the 330 was 10:1 or more, I probably need a different intake.
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Old April 4th, 2012, 08:34 PM   #31
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The issue is volume . In simpler tearms the engine is taking in too much air much more than what it needs. In your case jetting the carb to make it less lean is required since your 1406 is much leaner than the 1405 carb you have to add more fuel to make it run right . You have to make up for that diffrence. Jim a manifold with shorter runners will have better throttle response that is a fact i learned when we swapped the performer rpm air gap on my friends elcamino to a regular dual plane stealth weiand intake . The rpm intake in your stock app. is overkill to an extent. It will work but it will lack that throttle response everyone wants. Now if you have a big cam and stall speed converter more compression yes it will need more air flow to feed the engine . when you have higher compression the like lets say 10 to 1 thats ten parts air to one part fuel so it will need more air. The rpm will work but if you ran it and compared to the original the original would have better response of the line with the o.e. but will cut off much sooner than the rpm. since your set up is stock there really is no gain because you engine wont produce enough power up top to take advantage of the rpm intake longer runners .

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Old April 5th, 2012, 03:17 AM   #32
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Originally Posted by jpc647 View Post
MUCH more compression than a 72 350. Again, you guys are comparing apples to oranges. Compression makes a HUGE difference in port velocity, IMHO.
I'm not following you sir. You're first post seems to say I shouldn't use the RPM intake, the second says to rejet/recalibrate the carb. Which one should I do? If compression makes that much of a difference(I don't know I'm asking), and the 330 was 10:1 or more, I probably need a different intake.[/QUOTE]


Go up a few posts, that was jag, somehow the 'quote" thing did not work right.
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Old April 5th, 2012, 04:03 AM   #33
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Sorry, Copper, but I think you misspoke a bit.

It's a good excuse for me to inject my 2.

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The issue is volume . In simpler tearms the engine is taking in too much air much more than what it needs.
Yes and no.
The issue is volume when going to an engine with more need for air/fuel from one with less need, but not when going in the opposite direction.
The three things that determine the amount of air that the engine can take in (which we generally think of at wide open throttle, but which, when not racing, more often occur with the throttle plates partly closed) are:
  • Displacement
  • Engine speed
  • Cam overlap.
More displacement means larger cylinders and longer strokes, which pull in more mixture to fill the larger volume.
More engine speed means that that volume has to get refilled more often in a given period of time.
More cam overlap means that the cylinder spends less time sealed, which is to say that at some engine speeds it is not filling as much, which acts like less volume (or displacement), as stated above.

Compression has nothing directly to do with the amount of air that an engine can take in at a given speed, but cam changes that may result from having greater compression to play with can change the volume taken in.

As far as the engine taking in too much air - it really can't do that, unless the carburetor is set badly and doesn't meter out enough fuel.

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when you have higher compression the like lets say 10 to 1 thats ten parts air to one part fuel so it will need more air.
10:1 is not 10 parts air to 1 part fuel.

The stoichiometric ratio for gasoline is 14.7:1 (if I recall), and will not change based on compression.
There are a certain number of carbon, hydrogen, oxygen, and other molecules available, and they can only combine in one ideal way.
The compression ratio means that the mixture is compressed ten times if it is 10:1, eight times if it is 8:1.
That is to say that if the cylinder sucks in 10 cubic inches of air, then seals itself, the piston moves up until there is 1 cubic inch of air in the combustion chamber before reversing and moving back down. The same amount of air is sucked in - it is just squashed into a larger or smaller space after being sucked in.

So, to go back to "taking in too much air," the problem with a "racing-type" intake, such as the RPM is not that it allows too much air (or mixture) into the engine.
It can't do that.
It can only let in as much air as the engine can suck in with the pistons.
If it let in more air than that, it would be a turbocharger, and that would be good, not bad.
The problem is the size of the intake tracts (or tubes - the passages through which the mixture travels to get from the carb to the cylinder) - on a manifold like the RPM the intake tracts are larger, and on a stock manifold they are smaller. "Well, larger is better," one might say, and that is true if your goal is to flow as much mixture as possible, such as in a racing application, where you are using lots of wide open throttle at high RPMs.
HOWEVER, at partial throttle, and at lower RPMs (like when you actually drive the car on the street), the large, wide-open passages prove to be a problem, because, as we learned in high school physics, the same volume of a fluid (air, water, whatever) will flow more slowly through a larger pipe and faster through a smaller pipe.
So, in the RPM-type manifold, you have lots of air crawling through these giant passages on its way from the partially open throttle plates to the slowly moving engine, which gives fuel time to condense out of the mixture (making the delivered mixture too lean, thus requiring richening the mixture [and reducing fuel economy], as has been noted above), and makes the slow-moving mixture (which, being made up of actual physical matter has mass and inertia) sluggish getting into the cylinders.
In a manifold with smaller passages, under the same conditions, the mixture would be moving faster, perhaps making a bit of turbulence to keep the fuel "stirred up" with the air, and blowing against the valves as they open, rushing, with its inertia, into the cylinders, pushing extra exhaust gasses out during overlap, and "pushing in" extra mixture before the valve closes, thus leading to better performance and faster throttle response.

The trick is to choose a passage size that does not restrict flow at the highest anticipated displacement / RPM / throttle opening combination, but still provides good velocity for the mixture at more moderate condition under which it will be commonly used. Thus, every manifold is designed as a compromise, but, in general, bigger passages will favor higher displacement / RPM / throttle openings, while smaller passages favor smaller displacement / RPM / throttle opening.
In modern times, engineers have gotten around this problem by designing intake tracts with valves in them operated by the engine computer, which run the fuel through different passages depending on RPM, thus creating two manifolds in one and optimizing performance throughout the rev range.
We in the cast iron V8 crowd do not have such options, though.

- Eric
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Old April 5th, 2012, 04:26 AM   #34
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"Sorry, Copper, but I think you misspoke a bit. " Hey i tried to explain it the best i knew . The part about the engine taking in too much air i think i could have explained better i think i chose the wrong words like "engine taking in too much air "since the carb restricts and controls what comes in. You hit it dead on .
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Old April 6th, 2012, 04:57 PM   #35
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Well after some tinkering around today, I've made some improvement. The timing is at about 16* give or take a little. It's past the end of the timing mark, but not by very much. A couple people here have stated they run in the mark too, so I might not be off point. With the vacuum advance connected, it's reading way up there. Gotta be around 30*+. I know someone asked that.

I bumped the accelerator pump up to the top notch, and after playing with the idle screws it runs good.

If I'm cruising and hit the gas, it's right there. If I'm even rolling with my foot on the accelerator, and I nail it, the car goes, and goes well. It'll usually chirp second gear without much of a problem. I live at the bottom of a large hill, thats probably 3/4 of a mile long and it makes for a decent testing area.

But if I start from a dead stop, foot on the brake to WOT, the car doesn't go. It hesitates. There is still a flat spot on idle. I tried switching over from the ported/unported vacuum on the carb, no real change. But if I'm going 2-3mph, just barely touching the gas, and I nail it, it seems fine.

This has got to lead to the accelerator pump now, right? I don't have the rebuild kit or the calibration kit yet, but I have the accelerator pump and the thermo gasket. I'll probably try to change the accelerator pump tomorrow, and maybe do some polishing and such on the carb.

Is there an easy way to tell if this carb has already been calibrated? When I take the top cover off to get the pump out, what should I look for?

Still not sure what to do about my intake situation. I know it "can be used", but I almost think I might have a better performing car if I swapped out the RPM to the OEM one. That I just don't know. It's a lot of work to change and intake to lose any performance.
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Old April 6th, 2012, 07:07 PM   #36
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This has got to lead to the accelerator pump now, right?
Nope. Probably a problem with your transition circuit.

Either a blocked transition port or your mixture screws are turned in too lean, depriving the motor of a bit of extra fuel when you first begin to crack the throttle plates.

Other possibilities include vacuum leaks causing a similar lean state, such as from a worn throttle shaft.

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Old April 6th, 2012, 07:29 PM   #37
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I had a flat spot with the 1405 i ran i switched to a diffrent mettering rod and it went away. There is many diffrent combinations you can achive with the calibration kit. Im gonna guess the initial flat spot is from lack of fuel. Like eric said it could also be a lean idle. Try to run it a litle richer and see it it makes a diffrence if not you may have to jet it up a notch . btw what is you idle set to rpm wise ., initial idle speed can also affect w.o.t performance from a dead stop.
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Old April 7th, 2012, 09:03 AM   #38
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I had a flat spot with the 1405 i ran i switched to a diffrent mettering rod and it went away. There is many diffrent combinations you can achive with the calibration kit. Im gonna guess the initial flat spot is from lack of fuel. Like eric said it could also be a lean idle. Try to run it a litle richer and see it it makes a diffrence if not you may have to jet it up a notch . btw what is you idle set to rpm wise ., initial idle speed can also affect w.o.t performance from a dead stop.

I suppose my next plan of action needs to be to buy the gasket/rebuilt kit, and the calibration kit. I might find the solution to this. When I first bought the car, I don't remember there being a hesitation, not like this anyway. But I've owned the car since 2008 so it's been awhile. If I got WOT from a standstill at about 10-15mph the rear tire will break lose. If it can break the tires loose at 15mph and chirp second gear, it should be able to spin the tires from a standstill. It is, after all, a 3.33 peg leg rear. I don't know what the car is idling at, I don't have anything to measure it(someone asked).

I turned the mixture screws out a full turn and it was better, but i smelled gas. So I turned them back in 1/2 turn, and it's better. I still smell gas once it hits third gear and the secondaries are open. I always test the gar on a large incline, so I can listen for pinging, etc. Now my speedometer is off by about 10mph, but when it shifts into 3rd gear at WOT the speedometer hovers between 85-90 for too long. Now I realize 3rd gear is not the gear to be trying to be accelerating up a hill in, but it could contribute to ideas on what's wrong. This doesn't happen on flat surfaces. After the shift into 3rd, the car doesn't accelerate as fast as in second, but it's decent. Maybe just a shortcoming of the th350.

Now I've been thinking about the intake situation here. The Performer RPM intake is good for a range of 1500-6500 rpms. A stock 350 redlines at what, 5000 rpms? So I'm really not using this intake effectively. The Performer is good for idle-5500 rpms, which is more in line with what I
would need. So wouldn't I need a stall converter to use an RPM intake most effectively?

I gave the last owner a call, and talked to him about the car for a while. He told me he had a problem with hesitation way back, and it ended up being something with the distributor? He also mentioned he looked at the timing chain and it was new when he had the car. Is there anything else I can look at? Base timing is set at just about 16*, when the advance is connected it's at about 35*. (estimate, as it's way off the timing tab). Is there anything else I can check? I don't know much about the old school electronic distributors. To me, it seems it's working okay. But i know there are weights and springs to re-curve a distributor. Could this be what I need?(I'll be searching the forum after this post).

MDchanic: How do I check if the throttle shaft it work?
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Old April 7th, 2012, 01:45 PM   #39
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How do I check if the throttle shaft it work?
Throttle shafts should have essentially no play, so if you disconnect the springs and linkage and try to wiggle the shaft back and forth in the hole, it shouldn't wiggle.

If ti does, it's a vacuum leak, and probably a variable one that changes with throttle position, and will be adding extra air to your idle mixture that you have to account for by adding extra fuel, and the airflow past the transition ports goes out of what the engineers expected, and it all becomes a bit of a mess.

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Old April 8th, 2012, 12:18 PM   #40
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Happy Easter to all!

After tinkering yesterday, checking the time and playing/adjusting the carb, it was almost right. WOT from a stop was still not right.

I took the top of my carb apart this morning, and I changed the accelerator pump. Still have it set up on the top notch. Reset the idle and mixture screws with my father this morning and the car is WORSE. Much worse. I just did it on a hunch, and it's terrible.

There is almost no power from a get go. It hesitates from a stop to WOT, the midrange is down, and the top end right before it shifts is slow. The car was chirping second gear yesterday, and now nothing. My problems are clearly carb related, but I just can't seem to figure it out.

I'm done for the weekend, it'll sit till next weekend when I have some more ideas to run with. It's a disappointment after a weekend of trying, but I'll continue it next weekend. I would just take it to a shop at this point, but I don't know of one near me that I would even trust to leave my car, anycar, so any amount of time. What would any of you do at this point?

Last edited by jpc647; April 8th, 2012 at 12:34 PM.
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