Mark71

I decided to try something new and hooked my vac advance to full manifold vacuum instead of ported vacuum. For some reason, it made a very noticeable difference. The engine seems a lot more responsive. I actually spun the tires in my neighborhood by accident.

I ran into a couple of questions, though. I had to adjust my base idle because it was up around 1000 after just changing the vac hose. I backed out the idle screw all the way and I am still around 750 RPM.

If I back out that screw, doesn't it eventually allow the carb to close completely (and limit the engine speed)?

Also, do I run any risk by leaving it setup this way? My 71 350 has the temp activated vacuum switch installed, but currently completely unused...

Lady72nRob71

See if there is a dashpot or idle control motor that is holding the carb open.
Looks like there was a good reason Lady's vac advance was connected to manifold vac!
She has been hooked up lik that for years - no issues.
Make sure to cap off any unused ports of the temp. vacuum switch (TCS solenoid).

BILL DEMMER

there is a limit to how far the primary throttle valves will close after the curb idle screw disengages. further throttle valve closing is accomplished by bending a certain tang, but you don't wanna mess with that unless you're thoroughly versed in quadrajet adjustment/setup. after you turned your idle speed down, did you readjust the idle mixture screws?


bill

Mark71

Thanks for the responses so far!

I didn't have to adjust the idle mixture at all. It was adjusted when I first installed the carb (which I rebuilt myself, btw. so I am surprised that it works at all ). I adjusted it according to Doug Roe's Rochester Q-Jet book - looking for maximum vacuum. I had to set it waaaay rich, but I think that is a result of the fact that it is a 73 carb on a 71 engine.

Should I have to make mixture adjustments?

Anyhow, I know I could adjust the mixture to slow things down, but then I am getting further away from the optimal ratio. That can't be right...

Thanks,
Mark

oldcutlass

At some point the throttle will not adjust any lower as you have found out. There is no power gain by changing from ported to manifold vacuum, as once it gets off the idle circuit everything is the same again. It will allow an engine to run a bit cooler, and you can probably lean the a/f a bit more at idle.

Mark71

Hmm.... yeah, I wouldn't say there is more power out of this setup. But it seems to get from idle to off-idle a lot faster (responsiveness?). I was used to pushing the pedal a lot harder to get moving then backing off. Now it seems to "jump" into the off-idle with a lot less hesitation. Maybe it is all psychological...

I have made some other changes recently to the carb, so maybe they're having some effect as well? The accelerator pump was sticking a bit and the choke was coming off too slowly - those kind of minor things.

ziff396

I run mine off full manifold vacume as well. When I change it to ported, the idle is really rough.

jag1886

Make certain you don't just need to adjust the throttle linkage, pop it loose and see if the idle goes down. I have also found that my car runs cooler and it does get better mileage around town.

oldcutlass

Again, changing from ported to manifold vacuum on your vacuum advance only effects your idling. After you come off the idle circuit the advance operates as it used to.

RetroRanger

maybe your getting some advance at idle w manifold that you were not getting w ported ?

disconnect, plug and and see if the idle drops.

IIRC you have an HEI what is your base timing at ??

Mark71

Yeah, ported vac is zero at idle, so no advance, just base timing. Ported vac picks up when you are cruising or under part-throttle conditions. Full manifold vac is very high at idle and pulls the vac canister all the way to full advance. That's why I needed to set the idle speed down somewhat. If I pull the advance hose it drops to like 450 RPM.

According to the timing label on the car (), though, I am looking for 650 RPM at idle and I am not getting there with just backing out the idle speed screw. This is with the advance hose plugged into full manifold vac.

Excellent memory - yes, it is an HEI setup.

Timing is:
Base - 12*
Full Mech - 32*

Intragration

Not psychological at all. I noticed the same thing while I was messing around with vacuum advance. It all started when my vacuum solenoid failed. I noticed a distinct lack of low end power/responsiveness, along with a rough idle due to the vacuum leak. I replaced the solenoid, and it was back to it's old tire-burning ways off-idle.

What sort of mechanical advance setup do you have in your distributor? I'm dying to get to 20 degrees centrifugal advance, I'm only getting 10 now, it's driving me nuts.

RetroRanger

I think you could stand 2-4° of base timing if you are not experiencing any pinging issues now.

I only 'remember' your set-up cuz mine is the same

I think the 650 rpm is in drive (auto) IIRC it was 7-800 rpm in park.

As mentioned above check to see if the cable is holding open the carb by disconnnecting the throttle cable, and double check your mixture settings as well

Re Mike: maybe your mech advance mechanism needs cleaning/inspection. I think it was marks set-up that initially only provided a few degrees of advance until he cleaned it up and freed up the mechanism.

MDchanic

Whenever you change your idle speed, you need to readjust your idle mixture, because the final idle mixture is an interplay between the two settings.

- Eric

Intragration

Thanks Retro, I will start a thread on it. Definitely not a binding mechanism in my case, I've tried everything, I'm down to asking people what they're running when they get 20 degrees, maybe SOMETHING will work for me.

To add some value to this thread, Mark, I would look at maybe going even a little higher than the 12 degrees initial. Your motor should tolerate at least 15 initial, 35 total. I've been struggling with trying to get sufficient centrifugal advance, and I'm limited to 15-16 degrees initial due to higher CR and the fact that I like factory-style starters. Yesterday I was messing with it and a combo I was trying raised the initial to over 20 degrees, at the time I couldn't lower it without restabbing the distributor. I test drove it, with ~20 degrees initial and ~30 total, and it was like a different engine. There's definitely power to be had using more advance, within reason. I'm shooting for 15/35, if I can ever get 20 degrees of centrifugal.

oldcutlass

What happens if you set you total mechanical and initial 34-36* btdc at 3600 rpm with the vacuum can disconnected? It really does not matter where the initial winds up. Then with your vacuum connected it should be around 50+ @ 3600.

Intragration

Initial does matter though, if you're not getting enough centrifugal. I was previously only getting 10 centrifugal, which would have meant I'd have to have been at 26 degrees initial. At 10:1 CR and with a factory-style starter, I was having problems with warm starts. I would agree though that initial doesn't matter as long as you're able to get enough centrifugal for ~35 total and you don't have starting problems where the initial is set.

oldcutlass

What distributor are you running. You should be getting more than 10, closer to 14-18

Tony72Cutlass'S'

I have my vacuum advance connected to the DVCS, (usually its on ported vacuum setting)

Here is a video, i am getting 20 inches of vacuum at idle.

www.youtube.com/watch?v=koEqwZPD0rw

Mark71

You are 100% right - good memory! My dist advance mechanism was stuck something awful. Also, yeah, idle is set in drive (with your wife holding the service brake so you don't run yourself over) to 650 RPM. I'd love to put more advance to the engine - but I'm a little wimpy when it comes to risking detonation and catastrophic engine failure. Either way, my advance is still not right, but slowly getting better.

Would a guy who's only worked on motors with anti-knock sensors even know if the engine was pinging or suffering from pre-ignition / detonation due to over-advancing?

Excellent point as well! Didn't even think of that. I'll go try this tomorrow if the downpours let up...

Mark71

Nice... I see a little over 19" of vac and like you, the gauge is very steady. I am actually surprised because I always thought non-computer controlled engines always bounced around a bit. I guess you have a similar, stock-like cam just like I do...

Mark71

Yeah, I was getting exactly 10* and lots of odd misfiring and so forth. I yanked the distributor, disassembled the whole thing soaked all the metal in carb cleaner. On reassembly, I noticed it moved a LOT better and further. I seriously thought I damaged the setup because it moved so much better. I actually asked someone on here for a pic of how it should work to make sure.

Anyway, a new set of wires, new coil, a set of super-crap advance weights and things are working much better. Gotta replace the crap advance weights and springs though. I get 20* centrifugal now.

I dunno what your problem could be, though. If that advance mechanism is clean and free, it should work.

Intragration

The problem turned out to be crappy weights and too-heavy springs. I got the Moroso kit and used their lightest springs and it's perfect now. 15 degrees initial, 35 degrees total, 10 degrees of vacuum. I went a little beyond until I noticed a small bit of detonation, then backed it off a couple degrees. It's like a new motor now, more power, no lugging, much more responsive. It even stalls noticeably higher.

I would very much recommend the Moroso recurve kit and light springs.

Octania

Excellent story
thanks for sharing, all
I think that info above is exactly what I need now.

Mark71

Is this the kit you got? I got this a short while ago to replace my crappy springs and weights.

How did you notice that small bit of detonation? Did you hear a metallic "slapping" sound or did it lose power or did you use some more advanced method?

Intragration

That is the kit, Moroso 72300, good stuff. It was just the slightest detonation ticking. I was starting from about 12 initial/32 total, then bumped it up to about 16/36 and got the detonation, so I backed down to 15/35 and it was gone. Hitting MILD detonation and then backing off is probably the best way to do it, this way you know you have all you can get. Depending on your CR and other factors, you might be able to go even higher than I did.

Railguy

Can you change the mechanical timing any other way than weights or springs?This is new to me,I've never done anything with the distributor.If I change weights or springs which way do I go.I have an HEI I bought from Mondello's.
railguy
PS should I start a new thread?

Intragration

Springs, weights and centerplate are how you change centrifugal advance on a mechanical distributor. Depending on the weights and centerplate you have, you might get away with just changing springs, some of the factory weights and centerplates provide a good curve with lighter weights.

If you have specific problems or questions, you might want to start a thread, but I'd do a search first, there is a lot of good information already up here. The main concept is that you want to arrive at about 35 degrees of total timing (initial+centrifugal) as early as your engine likes, and you want maybe 10-12 degrees of vacuum advance. (for vacuum I used Napa part number VC1853. This starts at about 5-7 in/hg and provides about 15 degrees total, which I limited with a home made aluminum plate to 10 degrees)

My engine likes a lot of timing early, but if I go over 15 degrees initial, my chosen starter in conjunction with a little bit higher CR has problems with hot starts. 15 being my limit on initial, I wanted to get about 20 centrifugal, and I had a hard time getting this with anything but the Moroso 72300 kit. I don't know how other engines would respond, but as a starting point for a mild 10:1 455, I would recommend 15 degrees initial and the Moroso center plate and weights with the lightest Moroso springs for a total of 35 degrees at about 3,000 RPM.

Mark71

When you heard the detonation ticking, was it immediate or only under heavy load / WOT? Or was it the vac advance that was causing the detonation and only happening under part-throttle?

Intragration

It was only under full throttle over about 3,000 RPM.