When you click on links to various merchants on this site and make a purchase, this can result in this site earning a commission. Affiliate programs and affiliations include, but are not limited to, the eBay Partner Network.
Hi guys, still playing around with the 350 in my 1971 Cutlass and while it is running so much better, there is still this flat spot around 1000-1100 RPM. It also from cold will start, run for a few second then die. It usually fires back up and runs after that, but I think when it starts it is in that flat spot.
Everything about it seemed like there might still be a vacuum leak. I tried the propane thing again and came up good, but thought what the heck, let's try the brake cleaner. To my surprise i found something near the back around the distributor would raise the rpm when hit. It looked to be the manifold fittings for the brake booster and vacuum connections so i pulled the off and checked them, then sealed them up and reinstalled. The leak still seemed to be from there so I tried being more precise with the spray.
It actually looks like hitting the back of the carb does it. I snuggled up the 4 bolts but it still seems to be there. RTV on the gasket seems like a bad idea. Are the lots of gasket designs and materials, and if so, any recommendations? Unless a thin layer of RTV is ok.... I just dont want to keep tightening the bolts trying to seal it and crack something.
Here is a picture of the two gaskets I have. The one on the left I think came with the carb, and the one on the right I bought because it had the notches on the small holes like the one that was on there before.
What are those notches, and would I be better off with the other, or something else entirely?
I changed the gasket just to see if there was any difference and there was not. I think tomorrow I am going to clean up the old carb and throw it on just to see if there is any difference. I am not sure if there is something on the back of the carb that can cause a vacuum leak or in the carb itself, but for the 30 minutes it will take to try I might as well just to see. If not I am not sure what is back there that is causing this. The fittings for the brake booster and the fitting I am now using for the vacuum advance were taken out, cleaned up and reinstalled with no difference, same was done with the fitting on the front that the trans modulator is connected too, and I even did this to a plug that was under the old coil.
Maybe I will find one of those straws to use with the spray to try and be as precise and possible while spraying back there. At least I think I am in the ball park now....
I don't like the idea of RTV either but you might try some Permatex 3H brush on sealant if you happen to have some laying around. I wouldn't rush out and buy any if your to investigate it much more or put on a new gasket.
Indian Head gasket shellac is a similar product but the 3H is much better in my opinion.
Good to know, I will keep that in mind, but after putting a different gasket on and not only having the problem not go away, but it appears to still be in the same spot to me seems like it must not be the gasket, but some thing else back there. I will try the old carb and see if I can think of anything else, but I would like to stay away from any type of sealer for the time being.
I am wondering if there is something better to use in terms of a fluid to check for leaks, The brake or carb cleaners evaporate so fast which is nice to keep things clean, but it would be nice for something to stay on a little longer to help pin point leaks. It also sprays a larger area so I just want to be sure that it isn't just mist from the spray getting into the carb itself. I try to shield it with my hand and spray as accurately as possible but it probably wouldn't take much to give a false reading.
No RTV ever on a carb base or any other carb gasket for that matter.
Flip it over and run a straight edge across the throttle plate. It should not be warped. Many a heavy-handed wrench over torque a QJ.
Install it with a new gasket. Torque to spec. Recheck the torque after a few hundred miles.
I too like the gasket with plastic supports. Or a heat dissipating gasket is even better.
If all else is well with the carb, accelerator pump etc...its likely a minor adjustment.
It sounds like a tip-in flat spot AKA the infamous Quadra Bog. Locate the secondary door clock spring and give it a tweak tighter. youtube it or buy Dog Roe and Cliff Ruggles books. As QJs age, the secondary air valve flap AKA "AVS" door spring loses its giddy-up. It needs to be tightened up...in very small adjustments. The AVS is basically opening up too soon allowing the secondary metering rods to unseat from their jets to early, so too rich too soon.
If you see the AVS door doing anything other than just cracking open while actuating the throttle to WOT with no load(not driving the car) then its too lose.
I tune all analog carb'ed cars with a vac gauge.
Make sure your choke is fully off and the Air/fuel setting is spot on as well as the timing to include the vacuum and centrifugal timing events. Give it as much timing as it will tolerate before spark knock.
Adjust the A/F screws to get the highest vac reading possible. If your running points make sure they are clean the bumper is lightly greased and the gap is set at 30* of dwell or .016-.017" using a clean feeler gauge.
It needs to be tightened up...in very small adjustments.
For perspective of what that means, I found that a 1/8 turn of the adjusting screw made a significant difference in operation. Be sure that you use a screwdriver to hold the adjusting screw in place as you loosen the Allen set screw so that the spring tension doesn't retract and mess up the adjustment.
Good point Kenneth. Yes, don't let that spring unwrap.
I recommend watching youtube if one can be found.
You have to mark your starting point and keep track of how many tweaks you do to get it right.
Practice with an old carb with the air horn off and upside down.
A road test is the best test. It has to see engine load to get the whole operation right.
Over the years I've learned how it feels to get it into the ballpark on the bench then do a final tune on the road. It also depends on your cam. If you have a large cam some other specific tuning is required to get a QJ to respond to lower vacuum signals on the idle circuit, off idle and other circuits. Shouldn't be the case here thou.
Also, note the AVS should open smoothly(test when static engine off). If it doesn't the rod hanger or its cam needs close examination.
A QJ is a complex device but very tunable and responds well with knowledge and lots of patience.
Thanks for the tip about that adjustment. It is amazing how many adjustments and ways to fine tune these things there are. The carb is basically brand new rebuilt by Ken at Everyday Performance. I was reluctant to start messing with things like that just yet. I wanted to throw the old carburetor on and then do the spray test again to rule out something leaking with the carb itself. I wanted to use the new carb that was on there first to get the base number for a good comparison so I used it to get up to operating temperature. To my surprise the issue seems to be gone. I am spraying in the same spot but not getting the same result. Maybe I am just doing a better job shielding the carb preventing it from getting in the top and giving false results - either way it seemed better so I opted not to switch carbs.
Instead I went back to playing with the tune settings. I know before when I was advancing the timing it did seem to be happier with a lot more advance than the 16 I had it at, but when watching the vacuum gauge and then checking where it ended up I was at 36* of base timing which unless the mechanical timing was locked out and vacuum disabled I didn't see that ending well and back it down to 16* base, 10* vacuum off manifold and the 20* mechanical that the distributor offers which gave me that flat spot.
I figured I would try to find a middle ground that the car liked but wasn't so far advanced that it could cause issues. Right now it has 20* base timing, about 7* vacuum advance and the 20* mechanical advance that with the heavier springs doesn't come in until around 2500 which looks to be about 40*. I can now slowly raise the RPM through the 1000-1100 RPM range so it appears the flat spot is gone. I will see how the cold start goes tomorrow morning but hopefully with that flat spot seemingly gone it goes well. Then just a quick road test to confirm that there is no pinging and hopefully it will be happy where it is.
The only strange thing I noticed today was after the above noted tuning, it sat for about 3 hours while I watched the Indy 500, then I went to move it to give it a wash, and while it started it ran pretty rough. I moved it and proceeded to wash it. Then when I started it a little bit later to move it back it ran nice and smooth. I think the car just likes screwin with me, lol. Not sure what that could mean but I figured I will leave it and put some actual miles on it before messing with it some more. I might try playing with the vacuum advance set screw a little more but over all I am happy with it.
Thanks for all the tips and suggestions, it is greatly appreciated. Car is cleaned and ready to go... hopefully, lol.
From a cold start today it fired up, runs for a few seconds, then dies. It takes a few starts to get it running on its own and never really seems to settle into a good high idle. There is no chance of driving it until it is fully warmed up. If put in gear it will keep running, but touching the gas makes it struggle really bad and almost stall. Even after it is warmed up it still feels really sluggish and to me seems like it is really chugging.
Could this be an issue with the choke? How would I confirm proper operation of the choke?
I also noticed that the plates on the top over the secondaries of the old one have solid plates, but the rebuild has notches in them. Does this matter at all? Should these ever open? Even once warmed up they are closed solid. I can't even open them by pushing on them.
I decided to look into that secondary door clock spring. To my surprise, it actually looked like it was already loose. If it is that likely means that spring inside unwound itself and could be causing the issue. I have so far turned it about a half turn in 1/8 turn increments and have not really felt a difference, still bogging down hard on light throttle. If the spring was indeed unwound, how far should I expect to turn this thing? I don't want to snap the spring or something...
Get a vacuum gauge or use a tach/dwell meter to adjust your carb. Adjust for the highest rpm and then turn the screws in a 1/8 turn. The biggest problem sounds like your choke is out of whack. There is a separate idle adjustment for it.
If the spring was indeed unwound, how far should I expect to turn this thing? I don't want to snap the spring or something...
You won't snap the spring, it will start turning easily, then get harder to turn then stop turning when fully wound. If you don't feel any tension, the hook on the spring may have come loose.
I used to pop open the "secondary air valve" blade full open, then slowly turn the slotted screw (clockwise, I think) til the blade closes, then turn 1/4 turn more. That should give you a good starting point. If the secondary air valve blade stays open, then there is no spring pressure or spring has become detached.
After hearing that it was safe to keep turning on it I went back out and tried. I felt the spring start to load up, then get easier, load up, then easy and so forth. Using my old carb for reference in terms of how much pressure it took to open the blades, I found that the new ones took no pressure at all to open, and wouldn't come back on their own. I took the carb off so I could see what was going on. The spring was in bad shape and not connected. It was visibly damaged (assuming my fault) so I took it out and used the one out of my old carb. They now feel like the old one did at least. I will try that trick of opening the blades and tightening until it comes back then going 1/4 turn more and see what happens.
There was no change after I installed the new spring, but at least now I know there is one in there that works. I will play around with it some more tomorrow and see what happens.
I am wondering, does covering the primary on the carb and having the engine speed up and smooth out always point to a vacuum leak or can it just depend on where the timing and fuel mixture is?
Well, now I am thoroughly confused. I gave up yesterday and parked it running like crap. Yesterday took about 4 start attempts from cold, never really hit high idle, and regardless of operating temp could barely drive because it was bogging down so bad it felt like it was going to stall or back fire. Today, it only needed one restart, went into high idle, at about 130F I kicked it down to normal idle and it seemed smooth. I took it for a quick run around the lane and back. To my surprise it felt good. So I came back, threw the air cleaner on and hit the road. It was GREAT! No hesitation, sounded nice, for the first time was able to kick it down from third and hear it. All was good... for about 5 minutes. I slowed down at the end of my street at one of those round about things, went around it and then tried accelerating to head home and right back to bog city. I limped home and put it away. It wasn't an operating temp issue because it was running great around 130F and after operating temp after the thermostat had opened. I did check the distributor and it was still tight. I put the timing light on it to make sure it hadn't changed and it was fine too.
I don't know what to do now. Apparently the settings that it has now are at least decent, but something is able to change without my assistance so I am reluctant to keep messing with timing and fuel mixture now. I checked the secondary clock spring that I changed yesterday and it hasn't moved, still has tension. No vacuum caps have fallen off.... anyone seen anything like this before?
If nothing else I am happy that my rebuilt transmission works really nice, lol....
Last edited by 71OldsCut; May 28th, 2019 at 05:03 PM.
Based on the above info, I am now wondering could it be the new distributor or possibly plug wires? I don't know if they can intermittently work or they just fail outright.
Will it idle fine once its warmed up? Have you rechecked your timing (with a timing light), performed the choke adjustments, and the idle air fuel mixtures with a meter or vacuum gauge?
It will idle but typically is rough. I have played with the timing using the dial back type light and adjusted fuel mixture with the vacuum gauge. No mater what is done, vacuum advanced connected to ported or manifold vacuum, nothing really made it run smooth, but it had decent vacuum through it all between a steady 16-17.2 depending on where the timing was. There always seemed to be that flat spot when looking for 1100 RPM to check the timing, which is about where it seems to stumble really bad trying to drive it. It goes into gear and RPM drops to about 600 but then touching the gas even gently makes it bog and feel like stalling or back firing.
Today though, after fighting with it yesterday, the last test drive being no different, and parking it back in the garage, as noted today it was completely different. After kicking it down out of high idle, which that in it self was an improvement, it ran noticeably smoother, and the test drive, or at least 5 minutes worth of it confirmed it. I also had the air cleaner off so I could check the choke as per Ken's instructions. He said it should be tightly closed when set prior to starting the car, then open slightly once running and by operating temp be off, all of which happened.
So now I know when whatever the mystery issues is, is not happening, the current timing and fuel setting are at least close where it runs really nice. That is what is making me think maybe it is something with the distributor or plug wires or otherwise ignition related. I just don't know if that is typical or even possible. I will check my wiring to the distributor, maybe take the cap and rotor off and see if there is anything noticeable in there. I suppose if it is running rough I can pull plug wires off one by one to see if there is one that doesn't change anything... don't know what else to do at this point.
Leave the vacuum advance disconnected and plug the line until your get the timing dialed in, the car will run fine without it. Instead of setting your at 1100, set it at what ever your engine rpm is at idle. What is your timing setting at idle? Run a temporary jumper from the battery to the distributor power terminal. If you have a tach hooked to the distributor, disconnect it also for now. Start the engine with the key, note to turn it off the jumper needs to be disconnected. Adjust your idle air screws for the highest vacuum at idle, both screws should be even, they should have approx the same amount of turns from the all the way in to what ever adjustment you made.
Based on the above info, I am now wondering could it be the new distributor or possibly plug wires? I don't know if they can intermittently work or they just fail outright.
What distributor do you have? Different distributors have different failure modes.
It has good reviews but I hear that a lot of HEI distributors are made in china and prone to issues whether it is out of the box, a month down the road or a year. Perhaps I have a bad one...
One more thing that just came to mind - when doing the intake manifold gasket I thought it would be a lot easier with the fuel line to the carb out of the way. But, when I went to loosen it from the pump, it ended up slightly twisting the top of the pump. I don't know what the internals look like, but could this have damaged something inside the pump that is now causing high or low or even just inconsistent fuel pressure? I figured it started and ran so it was fine.
The guy before me did a nice job of rounding a lot of the line nuts off, apparently didn't believe in the right kind of wrench. This makes working with the fuel line to the carb fun, so I figure I will change the line out, and while I am at it throw another new fuel pump on. Just wondered if this might be the issue.
I have never heard of such a part, but a quick google search showed me what it looks like and that it is on the back of the carb. I will have a look when I get home, thanks!
New fuel pump and fuel line to carb should be in next week so hopefully if the hot idle compensator is there and not the issue the new pump will fix it. Ken, who built the carb, confirmed that low or erratic fuel pressure could cause all of these issues, but neither of us is sure if what happened to the fuel pump could cause it to be intermittent like it is. Fingers crossed...
I will check and see if that is the case with mine. Do you just connect the gauge and note the pressure by turning over the engine? What would be a good PSI number to see?
"I am wondering, does covering the primary on the carb and having the engine speed up and smooth out always point to a vacuum leak"...YES!
"or can it just depend on where the timing and fuel mixture is"...If you have it set lean yes. But you have used a vacuum gauge properly to obtain the highest vac reading. What was that reading? You will never get a good carb response or a true A/F tune if you have a vacuum leak.
I'm not sure if I or someone mentioned this...start disconnecting and capping off everything from the carb one thing at a time to see if you can find the leak that way. This will eliminate all the other components.
Stalling is a good indication of a lean mix or a lean choke setting or both.
If its still doing it with both carbs I would start looking at the intake gasket. Chances of both carbs being bad is plausible thou.
How do the plugs read? Black Brown or White?
Last edited by droldsmorland; May 30th, 2019 at 05:52 PM.
It is strange because sometimes I can cover the carb and have no change at all, and others it speeds up and smooths out, which goes right along with it driving great for a few minutes then falling on its face. Has anyone ever seen intermittent vacuum leaks? Seems odd, but then again I think so is a fuel pump bouncing back on forth between varying levels of function.
The fitting on the fuel line to the carb is so bad I don't think I am going to be able to loosen and tighten it again so since I have to change that, I will just put a new pump on. If I am lucky it will fix it, if not it is something I can check off the list. Ken that built the carb said that low fuel pressure could cause all these issues so it makes sense, it just seems strange that the problem would come and go. But, at least with a new fuel line I can swap carbs out if need be for testing.
The guys at Summit say it is possible that it is the ignition module. It is more common that when they get to hot or are faulty that they cause the car to stall, then once cooled off will work again, but he said he has seen where they just get weaker. I am going to get it tested and if it fails they will send over a replacement.
Unfortunately the old plugs are no longer around and the new ones have not seen enough run time to really show anything. Depending on timing, the vacuum reading is never less than 16 and as high as about 17.2 which based on the documents posted earlier falls right in the 16-17.5 normal reading.
I am hopeful for the fuel pump. To me it makes the most sense. I always thought having to wind the screws out 5-6 turns seemed like too much but if the fuel pressure is weak that could be why. And if I understand right, those screws only have any effect on the idling circuit and once the gas is applied are no longer relevant, but the weak fuel pressure would cause the same lean problem outside of the idling circuit. It ran so nice for those 5 minutes or so. I can't wait to get it back there. Hopefully one of these two things does it. If not I will try unplugging and capping the carb, and switch back to the other carb if that fails. Eventually something will work.
when I went to loosen it from the pump, it ended up slightly twisting the top of the pump.
You may be on to something here. I don't know what may happen when the top portion of the pump is a bit loose, but it should not be that way. I had one that was very loose and it leaked fuel like crazy, so changing the pump sounds like a good idea to me.
I hope so, luckily they are cheap and not too difficult to change so no harm in trying. It will sure be nice to use a proper wrench to tighten the fuel line finally, lol.
I will be using a Spectre brand pump which I would prefer not to based on an experience with an newer electronic type, but it seemed like they are the only brand in the two connection style. The Carter and AC Delco were both three connections and Supercarsunlimited had one but didn't seem to list a brand but did say it wasn't AC Delco. It should be fine, seems like mechanical pumps are fairly basic.
There are two main Carter fuel pumps for that era Olds: M6108 (w/o return line) and M6109 (w return line). Strange that you didn't see the two connection version. My closest local store carries AirTex / Wells and I got a Carter pump from NAPA a while back. The one you ordered should work; if not, bring it back and try a different brand.
What is the fuel PSI reading with the new pump & line?
Have you inspected the sending unit sock/strainer in the fuel tank? Verify PSI before ripping the tank down. We don't want you chasing your tail. Vacuum leaks are either there or not there.
After verifying fuel pressure I think the next logical step is to back away from where you are now in the troubleshooting process. I want to back you up and start the process(in your head) again.
So if the pump doesn't fix it stop and I'll redirect/regroup you to prevent you from tail chasing.
I think we are overlooking something but Ill get into it once you give me the pump PSI reading.
I am going to try and get a tee-fitting put on the old carb line with the old pump and see what I get. New stuff isn't here until next week. I have not checked the stuff in the tank
Is the size of the tee and hose connections I use going to be critical to the PSI reading? I figured I would just cut 5 or so inches of the old line away leaving a few inches at the carb and install rubber tubing between the hard lines to a tee.
I had the gauge connected from the cold start (had to re-fire once) to operating temp. It was basically at a bouncy 5.5 PSI the whole time. When reving it went up slightly maybe close to 6 PSI and was still bouncing. The bounce was about 0.3 PSI. Not sure if the bouncing is normal or even if the reading itself is decent. It seems to me that the pressure should go up more than 0.5 PSI under load versus operation to keep the bowl full, no?
May 25th, 2019, 12:00 PM
