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ap68

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Discussion starter · #1 ·
just recently re did my top end, milled my 99 vortec heads for more lift and installed a comp 292h cam.

car now has a terrible off idle stumble (or bog? not sure of difference)
carb specs: 66 jets, 31 squirter, 600cfm vacuum sec holley.
motor is a 355 ci flat top pistons, around 9:1 CR

was wondering if going to 69 jets would help. car has 6.5 in of vac at idle
 
#6 ·
Re: holley stumble

ap68 said:
just recently re did my top end, milled my 99 vortec heads for more lift and installed a comp 292h cam.

car now has a terrible off idle stumble (or bog? not sure of difference)
carb specs: 66 jets, 31 squirter, 600cfm vacuum sec holley.
motor is a 355 ci flat top pistons, around 9:1 CR

was wondering if going to 69 jets would help. car has 6.5 in of vac at idle
Click to expand...
You should be running a double-pumper carb, probably a 650. Your cam is big enough to cause the engine to go lean when you stab the throttle.
 
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#7 ·
Re: holley stumble

As have posted so many times before
The sec butterflys set so the idle and mixture screws are in factory spec?
Is the gap at wot on the accelerator pump around 15/1000?
May require a lower value powervalve
An engie vac leak... around the carb or worn butterfly bushes.

As well as jets, what are you running for a distributor and what can is on it?
Click to expand...
When one presses on the throttle, the engine vac drops, the VA can is no longer in use or part of the timing....not an issue.
 
#8 ·
Re: holley stumble

According to what I'm reading around the web, 6.5" of vacuum at idle warrants a smaller power valve in a holley 600. The weird thing is though, that apparently, if you have everything else dialed in right, any power valve will work fine. On this site, and hotrodders, and speedtalk, and team chevelle, and elsewhere, you can find recipes for a good tune on a 600. Some will say stock jets, stock PV, and lightest VS spring in the kit. Others will say that leaned down to 58 jets and a PV that comes in right under cruise vacuum will do it good. Some say use the brown pump cam, others say stick with the stock one. I'm confused.

I am having problems with my holley 600 too. :D
 
#9 · (Edited)
Re: holley stumble

Steptoe said:
As have posted so many times before
The sec butterflys set so the idle and mixture screws are in factory spec?
Is the gap at wot on the accelerator pump around 15/1000?
May require a lower value powervalve
An engie vac leak... around the carb or worn butterfly bushes.


When one presses on the throttle, the engine vac drops, the VA can is no longer in use or part of the timing....not an issue.
Click to expand...
Explain in further detail. As per John Z's advice I changed my can and performance increased. It was my understanding that the objective is to have the can fully-deployed at idle (assuming you have the can connected to a full manifold vacuum source, not to a "ported" vacuum source).
 
#11 ·
Re: holley stumble

So why is it a VA can change improves off idle stumble? This is from John Z's "Timing 101" article:

"Vacuum advance calibrations are different between stock engines and modified engines, especially if you have a lot of cam and have relatively low manifold vacuum at idle. Most stock vacuum advance cans aren’t fully-deployed until they see about 15” Hg. Manifold vacuum, so those cans don’t work very well on a modified engine; with less than 15” Hg. at a rough idle, the stock can will “dither” in and out in response to the rapidly-changing manifold vacuum, constantly varying the amount of vacuum advance, which creates an unstable idle. Modified engines with more cam that generate less than 15” Hg. of vacuum at idle need a vacuum advance can that’s fully-deployed at least 1”, preferably 2” of vacuum less than idle vacuum level so idle advance is solid and stable; the Echlin #VC-1810 advance can (about $10 at NAPA) provides the same amount of advance as the stock can (15 degrees), but is fully-deployed at only 8” of vacuum, so there is no variation in idle timing even with a stout cam.
 
Discussion starter · #13 ·
Re: holley stumble

i am runninga points dizzy that i did a small body hei conversion to. vac advance is attatched to fulltime vacuum. however i am running the stock can, old motor made 15" at idle. Car now has a fluttery unstable idle

so thatd mean i should go ahead and order up a different vacuum can?
 
#14 ·
Re: holley stumble

ap68 said:
i am runninga points dizzy that i did a small body hei conversion to. vac advance is attatched to fulltime vacuum. however i am running the stock can, old motor made 15" at idle. Car now has a fluttery unstable idle

so thatd mean i should go ahead and order up a different vacuum can?
Click to expand...
It would be advised to read John Hinckley's "Timing 101" article. It's invaluable. Do a search, it's on here more than once.
 
#15 · (Edited)
Re: holley stumble

ap68 said:
i am runninga points dizzy that i did a small body hei conversion to. vac advance is attatched to fulltime vacuum. however i am running the stock can, old motor made 15" at idle. Car now has a fluttery unstable idle

so thatd mean i should go ahead and order up a different vacuum can?
Click to expand...

Yes, that'll screw with your idle. If the can isn't pegged at idle due to low vacuum, the vacuum advance will dither in and out so your timing is jumping around.

To test it out, you can just pull the vacuum line off the can and plug the end of the line. Or, you can plug the intake port you have it plugged to and connect the line to the ported nipple if your carb has one just temporarily. It should be on the passenger side above the throttle plates.
 
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Discussion starter · #16 ·
Re: holley stumble

i read that there is a can that would be fully deployed at 8", is there anyone who makes a softer can say a 4"

also, the adjustable vac cans do they adjust the deployment point or the amount of timing advance?
 
#17 ·
Re: holley stumble

Guys..dont mess with VA unles you have an in car vac gauge and know what the engine is doing under different loads at different rpms.....AND you know what the hell you are doing.
Open the throttle up, the vac drops...VA canisters can be adjustable and vairy greatly just in 1 yr of production by GM.
you need to be able to KNOW at what vac itr opens at and stops and how many degs it has in it

Im not going to retype out a full explanation of how to set up a dizzy or VA, nor how to machine you VA and dizzy to customise the curves
There is a bloody search button above.

But unless you actually have real life experiance in setting up dizzies, and know it all works and relates to an engine...please stop confusing ppl...its not complicated..its simple as shooting a running deer acruately with a cross wind.

The issue is a stumble...its not the VA unless its stuffed and causing a vac leak.
 
#18 ·
Re: holley stumble

He has more than one issue that is being addressed.



Steptoe said:
The issue is a stumble...its not the VA unless its stuffed and causing a vac leak.
Click to expand...

ap68 said:
i am runninga points dizzy that i did a small body hei conversion to. vac advance is attatched to fulltime vacuum. however i am running the stock can, old motor made 15" at idle. Car now has a fluttery unstable idle

so thatd mean i should go ahead and order up a different vacuum can?
Click to expand...
 
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#19 ·
Re: holley stumble

"Usually" an off idle stumble or bog is related to the accellerator pump system,what color pump cam do you have in it?Try going up in pump shot size see if that helps.When you crack the throttle is fuel squirting out the instance you hit ?if not accelerator pump needs adjusting.
 
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