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Old 09-07-2003, 10:05 AM
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The Stumble Theory

Hey Guys,
I just found this forum and decided I would do a little reading. I am impressed with the courtesy you guys award each other. I intend to administer the same.

Any way..on with the problem .. I just bought a '79 Z28. It has a classic 650DP Holley and an Edelbrock Performer RPM Air Gap intake fresh out of the box bolted on now. Has serpentine belt system and air blows cold.
Alum. Heads 2.02/1.60 64cc
280/480 Cam
3000 Stall (pulls at 2500 rpm)
MSD Ignition
.030 Flat-top Pistons
373 Posi Rearend

The brand new carb is giving a stumble off idle and a little above and spits out a small puff of black smoke. The stumble is present whether you goose to WOT or if you just barely move it This is not a BIG problem, but Im sure if I get this small bug worked out I can pull my 13.30s down to high 12s.

I set the timing on 20 degrees.

The float and the idle mixture screws are still positioned the same as they were out of the box.
Ive got a 28 shooter in the front and Ive tried a 25 with no difference. I think what might be happening is that the float in the front is a little too high and it is running a little rich at idle. Thus, when I goose it, I see smoke from the previously rich idle mixture, puff out of the exhaust, and immediately assume that the shooter is too big, when actually the shooter is not big enough.

I think I should lower the float level, increase the shooter size to 31 and make sure my idle mixture screws are set right.

OK, Im through babbling now

What do you guys think about my theory??

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Old 09-07-2003, 05:39 PM
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I would set the float levels like Holley says; fuel level at bottom of threads. Deviating from this will cause main metering to come on too soon or to late creating a hole or rich spot in your fuel curve.

Verify idle mixture screw settings.

Check total timing at 3000-3500rpm.

Write down where you are starting from jets, squirters, timing, etc.

Then start your expirementing making one change at a time youll figure it out.
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Old 09-07-2003, 06:28 PM
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Thanks for the advice Greg. I was beginning to think I was on my own with this new carb thingy.
I have another problem also. Now that the intake is about 3 inches taller, I have very little vacuum and NO brakes. I wonder if a vacuum canister will be adequate on the starting line long enough for the light to turn Green. I can't even run the engine up to get a good launch because I can't press the brake pedal hard enough and I keep rolling out of the stage and red-lighting!

Do you think a canister will solve this or will I need a vacuum pump of some sort? And is this a situation where I'll have to measure vacuum to be able to know what I need?

Thanks In Advance for your help....

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Old 09-07-2003, 07:31 PM
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If you tried what Greg said and still have trouble's I think it's in the accelerator pump system, below it tell you how to adjust it.
Please read the info on the site for all the problem's. Then when you get a problem you will know where to start looking.
The Knowledge Base has a lot of onfo on this subject.
Copied from Holly site:
The accelerator pump system consists of three main components: the pump diaphragm, the pump cam and the pump nozzle. This is the carburetor system that is most responsible for having good, crisp, off-idle throttle response.
Its purpose is to inject a certain amount of fuel down the throttle bores when the throttle is opened. By accomplishing this purpose it acts to smooth the transition between the idle and main circuits so that no stumble, hesitation or sluggishness will be evident during this transition phase.

A larger pump nozzle will allow this fuel to be delivered much sooner than a smaller pump nozzle. If you need more pump shot sooner, then a larger pump nozzle size is required. During acceleration tests, if you notice that the car first hesitates and then picks up, it's a sure bet that the pump nozzle size should be increased. A backfire (lean condition) on acceleration also calls for a step up in pump nozzle size. Conversely, if off-idle acceleration does not feel crisp or clean, then the pump nozzle size may already be too large. In this case a smaller size is required.

Here is Holly's site, they have a free downloadable manual and trouble shooter's guide.
Luv the smell of NITRO in the morning.
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Old 09-09-2003, 09:56 AM
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Problem Solved...

The float levels on the new Carb were too high straight out of the box and the modulator valve vacuum hose was disconnected down at the tranny causing a vacuum leak.

This solved both the stumble problem and the brake problem.

Thanks for the help guys.....

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