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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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Premium Member
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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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