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Discussion Starter · #1 ·
Recently I took my 73 nova up to the drag strip. The car has a 412 Small Block Chevy. It has a 292 Comp Cam, 3k stall torque convertor, 750 vacuum secondary holley with a 4.5 power valve, hi rise intake, no air cleaner, th 400 transmission.
My problem is when i made a few passes the car would take off and immediately stumble. It only seemed to stumble when the car had been running, because it didn't stumble after the car cooled down. Yet it stumbled the next pass after that. I can not stall the converter because we could not hold the car still with front brakes. (I am putting rear brakes on now). My question is what could be causing the motor to stumble when it takes off? Thanks
 

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Stumble is generally caused by an overly rich fuel to air ratio. This could be due to your power valve opening too soon or too much fuel entering too quickly from your accelerator circuit. You tune the accelerator pump with a selection of cams which can be mounted in two different position for fifteen different shot volumes. You control how long the shot takes to discharge by selecting a larger number shooter for a fast shot and a smaller numbered shooter to spread the discharge out over a longer time.

Getting fuel into a motor is never a problem (it is very easy to flood a motor with fuel). The difficulty lies in getting the air to flow into your motor when desired. A high lift long duration cam favors the higher RPM range and could be hurting you at the mid level RPM range as all drag races are won or lost in the first sixty feet.

Two things to consider are a stiffer rear gear (or a tranny with a higher numbered first gear) to get your car rolling. Or a higher RPM stall converter.

Every cam listed in Comp Cams catalog with a 292 advertised duration states Rough Idle; Needs Good Exhaust and Gears. There are no cams with 292 duration at 0.050" of valve lift other than two blower cams, but once you get above 270 degrees of duration at 0.050" of valve lift then the catalog starts calling out for 12.5:1 compression or higher pistons and 5,000 RPM plus stall speed converters with 4.10:1 or better gears, because if you don't the car will stall, or fall flat on it's face off the line.

Big Dave
 

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Discussion Starter · #4 ·
The Cam is a 292H-10. We followed all the guide lines from comp cams with a 3000+ stall and 10:1 compresion. We have 3.73 gears in the rear end too. The fuel system is a holley electric fuel pump and a reg. that is set at 6 psi.

The Holley web site also suggests the accelerator pump as a possible cause. How would I adjust this or know if this is the problem? Thanks.
 

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OK you have gears and fuel pressure when the car is cold (the Holley electric fuel pump recirculates the fuel internally so they are prone to vapor lock when they are heated up by either exhaust pipes or by running dead head pressure for a long time), the two other issues frequently encountered is a cam causing you to set the idle speed so high that you are off the transition slots in the base plate.

The slot when the idle is set too high is nearly fully uncovered.



it should be barely visible at idle.



This slot dumps gas into the engine as the throttle blades move off idle to cover fuel consumption until the main jets start to flow fuel through the boosters.

The other issue with an aftermarket cam is reversion due to overlap (that noise you love when running a big cam) which lowers the intake manifold vacuum to the point where your power valve thinks you have your foot in the throttle with the carb at WOT when you are still just cruising. You select a power valve by using a vacuum gauge at idle. Take the measured value cut it in half and add a half point if it comes up even (power valves are sold with a .5 stuck on the end of the vacuum figure divided by two).

If you have no issue with either of these common problems then we are back to the accelerator pump to cover any flat spots due to running lean (or too much fuel if running a modified Holley carburetor). Generally when a motor runs lean it back fires (anything from popping to a major explosion).

As has been pointed out electrical issues mimic fuel issues. If you have an HEI and you loose full voltage at any time you loose fire. Dropped cylinders will create a major stumble. Cross fire in your ignition can result in backfires or dropped cylinders (wires do not have to be physically crossed as you can induce a current to flow just from an adjacent cylinder firing).

Here is a chart I typed up some time ago to inform you of how to tune your accelerator pump:

Holley Accelerator Pump Cam Capacity measured @ Degrees of Throttle Opening
Cam Position One____________________Cam Position Two
Color__Capacity__Degree__________Color___Capacity_ __Degree
White....17cc..........18°.....................Black.....18 cc..........17°
Blue......18cc.........18.5°...................White.....19.5 cc.......20°
Red...... 18.5 cc.....18.5°...................Red........20 cc.........20°
Orange..19 cc........19°......................Blue.......20 cc.........20.5°
Black.....19 cc........19°......................Orange..24.5 cc.......24.5°
Green.....24 cc.......23°......................Green....30 cc.........29°
Pink.......30 cc........30°.....................Pink.......37.5 cc......37°
Brown....36cc.........37°

These volumes were obtained as the summation of TEN tests with a 50cc accelerator pump lashed to 0.015" clearance (so divide each number by ten for the total volume of one shot).

Big Dave
 

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chris,what dave says is true ,on the power valve.i have run a 292 solid cam and a 4.5 power valve runs well with them.usually you will need a 31 shooter up front, to help on the starting line.set your fuel pressure up to 7 psi.i dont think that has anything to do with your launch,but it will help down track.you may also have to put a piece of fuel line on the breather tubes on the carb.it will help from dumping fuel in the secondarys when leaving hard.also a worn cam or pump lever.after that if you still have it, recheck timing and change the spring in the vacume canister for the secondarys.
 

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Does it only stumble on a hard launch? I had a couple instances where we had to do some fudging to get it to run right on the launch. On my nitrous car, I had a ten inch tunnel ram with dual 600's. It ran perfect, until I had to "launch" it. Then it would stumble as soon as the front end started to raise. A big block guy told me to run 14 psi in the fuel system to the front of the car to overcome inertia. I had a second regulator there to pull the pressure down to 8 psi. (dual quads and nitrous) That solved the problem on my car.

The big block guy had a similar problem and he found out the secondaries where running dry. He had a dominator with bowl extensions on it. The fuel would slosh away from the jets and dry up the jets in the secondaries. He wound up putting jet extensions on the jets to solve his problems. Granted, these cars are heavy hitters with finicky fuel systems, but just another opinion to mull over.
 

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Discussion Starter · #10 ·
these all sound like possible fixes. The only thing that has me dumb founded is that of the 9 passes we made that day, 4 were fine no stumble, 3 had hard stumbles and 2 had a little stumble on launch. It seemed to run without a stumble when the car sat for an hour or so and cooled off. If the Stumble on launch was consistently there, i would be able to try things out.
The previous week before that we had a cross breed tire for street/strip on the car and on launch it would spin the tires for about 10 ft. and then they hooked up. The reason I mention this is because it didn't stumble once in 4 runs. A though i had is that with the slicks on and the 3k stall torque converter, the car launched and as soon as the converter locked up, it would bog the motor down until it could recover? What is anyone's thoughts on this. I am going to try to do some of the easy suggestions, but i really don't want to spend money on buying something till i know it will work. Thanks for your help guys!
 

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Discussion Starter · #11 ·
I have also read in holley's carb book that I should change the shooter size to a bigger one with the set up we have. I know someone mentioned this before. If it was the shooters wouldn't it stumble all the time? As you can tell I dont know everything so I am looking to you guys for help.
 
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