Question for Lars or tuning experts

Eastltd

Eastltd

Member
Joined
May 26, 2008
Messages
22
Location
Northern Ontario
Please help me solve my off idle stumble.

I have a 73 454 with the Edelbrock performer heads, intake and 750 carb (1411). The car has an MSD 8572 Pro-Billet distributor set with 12 degrees of initial timing and the centrifugal advance set at 21 degrees. (one light silver spring, one light blue spring and the blue advance stop)

With this set-up the centriugal advance curve starts at 1200 RPM and is fully advanced at 2400 RPM.

I have the carb now set at the base setting (.110" main jet & .075" X .047" rod).

If I let the clutch out at idle 800 RPM I get a stumble for a few seconds.

Should I be chasing carburation or timing?

Help from any of you fellows would be greatly appreciated.

Thanks in advance.

BTW, I measure 14" of vacuum at idle.

Ian

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If I just let the clutch out on my 383 at idle it will do the same thing. Do you blip the throttle a bit as you let the clutch out? I hate to sound rude but it might just be your driving technique.

BTW - Great looking 'Vette!
 
DeeVeeEight said:
If I just let the clutch out on my 383 at idle it will do the same thing. Do you blip the throttle a bit as you let the clutch out? I hate to sound rude but it might just be your driving technique.

BTW - Great looking 'Vette!
Click to expand...

Yeh, I've tried that. It seems as soon as the motor sees a load, the RPM's drop to 500.

I just dropped the accelerator pump linkage down one hole hoping for a bigger initial squirt. No real change.
 
With your low (not seriously low but low-ish) idle vacuum I would bump the initial timing to 16 or 18 degrees and retry. If it works then change the limit bushing to take out the difference. I run a big solid roller with about 13.5" of idle vacuum. I have 20 degrees initial, 18 mechanical starting at 1,200 all in the by 2,600.

It looks like your vacuum advance is connected to a ported source. Connect it to manifold vacuum before changing the initial timing and retry. I think your engine is lazy at low rpm and throttle openings and a little timing can wake it up. You may need to lower your idle speed and reset your idle mix screws too.
 
pcf_mark said:
With your low (not seriously low but low-ish) idle vacuum I would bump the initial timing to 16 or 18 degrees and retry. If it works then change the limit bushing to take out the difference. I run a big solid roller with about 13.5" of idle vacuum. I have 20 degrees initial, 18 mechanical starting at 1,200 all in the by 2,600.

It looks like your vacuum advance is connected to a ported source. Connect it to manifold vacuum before changing the initial timing and retry. I think your engine is lazy at low rpm and throttle openings and a little timing can wake it up. You may need to lower your idle speed and reset your idle mix screws too.
Click to expand...

I agree with this. I just re-set my timing as per Lars tech article to 16* base and 34* total timing @ 2500 rpm (36* was too much for my motor), it runs real good! 12* base timing may be too low....
 
pcf_mark said:
With your low (not seriously low but low-ish) idle vacuum I would bump the initial timing to 16 or 18 degrees and retry. If it works then change the limit bushing to take out the difference. I run a big solid roller with about 13.5" of idle vacuum. I have 20 degrees initial, 18 mechanical starting at 1,200 all in the by 2,600.

It looks like your vacuum advance is connected to a ported source. Connect it to manifold vacuum before changing the initial timing and retry. I think your engine is lazy at low rpm and throttle openings and a little timing can wake it up. You may need to lower your idle speed and reset your idle mix screws too.
Click to expand...

All the literature I have from MSD and Edelbrock indicates I should be on the ported vacuum.

I changed the accelerator pump setting for a bigger squirt and eliminated the low bog. Now I have detonation when accelerating through 3000RPM.

Is this too much total, not enough timing or lean-out?
 
From my experience an off idle stumble is caused by a carb lean condition.

Also with a heavy car, such as a corvette it is recommended that your total timing should not come "all in: until around 3000 rpm.......... I believe since you the accelerator pump setting to allow for more fuel you helped your off idle (lean) stumble. But your advance is coming in too quick and causing detonation.......step up your springs in that distributor and give it a try.

Remember to connect your advance to a full vacuum source. The main reason GM switched to using a ported vacuum source in the early 1970's was to help with emissions.

When I set timing, i never check my base.......I only set overall. The guy who builds my engines is a local machine shop here and he said dont worry about base timing....it dont matter. Just make sure you are "ALL IN" by 3000rpm. He must know what he is talking about here is one of his race cars...Check this bad ass car.....

[ame]http://www.youtube.com/watch?v=4ZsewUGKCGo[/ame] world record!!

He has built me a 434 small block stroker and my 427 big block I have now.

He also has a 7 second corvette, but I figured 1 video was enough for this post.
 
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jsssgrl1209 said:
From my experience an off idle stumble is caused by a carb lean condition.

Also with a heavy car, such as a corvette it is recommended that your total timing should not come "all in: until around 3000 rpm.......... I believe since you the accelerator pump setting to allow for more fuel you helped your off idle (lean) stumble. But your advance is coming in too quick and causing detonation.......step up your springs in that distributor and give it a try.

Remember to connect your advance to a full vacuum source. The main reason GM switched to using a ported vacuum source in the early 1970's was to help with emissions.

When I set timing, i never check my base.......I only set overall. The guy who builds my engines is a local machine shop here and he said dont worry about base timing....it dont matter. Just make sure you are "ALL IN" by 3000rpm. He must know what he is talking about here is one of his race cars...Check this bad ass car.....

http://www.youtube.com/watch?v=4ZsewUGKCGo world record!!

He has built me a 434 small block stroker and my 427 big block I have now.

He also has a 7 second corvette, but I figured 1 video was enough for this post.
Click to expand...

I tried the heavier springs which got rid of the 3000RPM stumble, but changing to manifold vacuum brought the off idle bog back in.
 
By switching to manifold vacuum you have done 2 things to your car.....

The first is the following. Your advance canister in your distributor is now getting full vacuum. Which required you to turn down your idle to compensate for the added timing.

The second is because you gave it more advance you now need more fuel to help from swithing from ported vacuum to full vacuum. more timing=more fuel

By changing the springs like I recommened you took care of your 3000 rpm detonation problem, but by changing to full vacuum you leaned out on your low rpm. N youow just need to put a fatter jet or play the edelbrock rod game to make up for the difference.

I dont know what every one else on here thinks about ported distributor vacuum, but any even mild performance engine I believe should run FULL vacuum.

Ported vacuum was an emissions trick that came along about the time of AIR to help the engine cope with leaner mixtures but it resulted in higher temps for the engine and less power or at least throttle response

As far as your overall timing, that is something you have to work out for yourself, each engine is different but a good range is between 32-37 degrees ALL IN.

Dont just take my advice, I am sure there are others on here that know more than I do.

I can say that I am not a fan of edelbrock carbs and usually end up tuning them right out of the box, becuase of " the off idle stumble"
 
I agree with jsssgrl1209 that timing is an "all in" number with the vacuum advance disconnect. I always check the base to be sure something really bizarre is not going on like a rotated balancer ring or f'd timing tab. Go for 34 to avoid getting gready and I like it in fast-ish (so does GM - check the GM Power Book) or all in by around 2,500-2,600. If you do not have a lot of gear this may be too quick for your combination.

Once you connect the vac can to manifold vacuum you will need to re-visit your idle mix. I do not know squat about these Carter/Edelbrock carbs so I can not help you. But if you went to manifold vacuum and did not re-visit your idle mix screws you are probably lean. This will lead you back to the stumble. However going to manifold vacuum will also raise your idle vacuum which is aways a good thing for carb signal. This extra vacuum at the same rpm indicates improved engine effeciency, possibly from better atomization and better burn.

You may have a small carb problem also but with your timing right you will sort it out a lot quicker. And your car will be more drivable.
 
Thank you for all your comments so far guys, they really help.

I'm learning the effects of each change both positive and negative.

I did notice the off idle bog was back as soon as I went to manifold vacuum so tonight I'll richen up the AF screws to see if that helps.

I've also learned to only make one change and test the car. Mulitple changes has me chasing my tail.

I see now why most people run Holley carbs.
 
be careful

Your idle screws are for the most part, just for idle. They do have a small effect on off idle acceleration, but if that don't work you will need a rod or jet change in that edelbrock. Try doing the rod change first, it is simpler. This is where your edelbrock tuning book is helpful.
 
I had a simular happing with my holley set-up. Turned out to be too lean.
 
I think I've got it.

I changed the centrifugal advance spring combination to 1 heavy silver spring and one light silver spring. 21 degrees at 3400 RPM.

Then I changed to 4% rich in the cruise mode and 8% rich in the power mode. (.110" Jet, .073 x .037" Rod)

After several trips around the block, I riched up the AF ratio a tad more and the off idle bog is gone as is the 3000 RPM studder.

My initial advance is still set at 12 degrees so I may bump that a little too.

Thanks for all your help.
Ian
 
BigBlockTank said:
Manifold vacuum will cause the vacuum advance to "come in" at idle.
Click to expand...

And that's exactly what you want. With manifold vacuum at idle, you can run actual timing at idle of about 30-34 degrees (14-16 initial plus the 16 degrees from the vacuum), which is optimum for a cooler running engine and very good off-idle throttle response.

For an explanation and discussion on this subject, feel free to e-mail me for a copy of the "Vacuum Advance Specs and Facts" paper:

[email protected]

Lars
 
lars said:
BigBlockTank said:
Manifold vacuum will cause the vacuum advance to "come in" at idle.
Click to expand...

And that's exactly what you want. With manifold vacuum at idle, you can run actual timing at idle of about 30-34 degrees (14-16 initial plus the 16 degrees from the vacuum), which is optimum for a cooler running engine and very good off-idle throttle response.

For an explanation and discussion on this subject, feel free to e-mail me for a copy of the "Vacuum Advance Specs and Facts" paper:

[email protected]

Lars
Click to expand...

I know from nothing on BBC, but my Pontiacs in the way past daze always had a hard start summer when hot problem with the advance set more then 10* initial....so to widen the mechanical, plus the vac at manifold...

:nuts:
 
Thanks for all the helpful replies. Tomorrow, I'll retry connecting the vacuum advance to the manifold port and reset the carb.

I'll let you know how it all works out.

Lars, I sent you an email requesting your paper to read first.

Thanks again,

Ian
 
Papers sent. As always, feel free to contact me with any questions.

Lars
 
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