Hesitation When Accelerating

[eschaider]

OP pay attention to Patrick he is pointing you in the right direction.

When an engine stumbles it is caused by a lean condition not a rich condition. You particular lean condition is not going to be impacted by a main jet. Main jets are for WOT tuning. There are only two items that can affect the off idle / part throttle lean condition, i.e. stumble.

The first thing to check is your idle vacuum. If you have the lumpy idle most Cobra owners desire you are likely idling with a low manifold vacuum. A good manifold idle vacuum would be between 12 and 17 inches of vacuum. If you are down around 6 inches of manifold vacuum this is the beginning of your problem.

Out of the box, Holleys will almost always come with a 6.5 power valve. That means the power valve opens at 6.5 inches of idle vacuum. If your car idles at 6 or 7 inches of vacuum that means your power valve is open at idle creating a rich idle. You want your power valve to open at roughly ½ your idle vacuum. An engine with 6 inches of idle vacuum wants a pwervalve with a 3 inch opening value.

When power valves are open at idle, owners lean down the idle mixture screws to lean out the mixture so the engine can idle w/o asphyxiating every one within ear shot. This is great for idle but terrible for throttle response. As you open the throttle the engine goes lean and stumbles because the power valve is already being used to idle the engine.

There are two fixes, get a different cam and higher idle vacuum — which is generally unacceptable to Cobra owners. The second fix (also cheaper) is get a different power valve that opens at a lower manifold vacuum (½ your idle vacuum) and then properly set the idle mixture screws on the carburetor.

Now as you open the throttle, both the accelerator pump and the power valve will add fuel as manifold vacuum drops below the lower valve's preset opening point. The fuel addition will either mitigate the stumble or eliminate it. If there are still vestiges of the stumble then you need to take the next steps.

The accelerator pump and power valve work together as a team. The carb uses both to supply adequate fuel to the engine early enough at part throttle acceleration to eliminate stumbling or flat spots. Power valves will also provide additional fuel at WOT well after off idle and low to intermediate speed enrichment.

If we assume your accelerator pump has adequate pumping capacity then the metering of the fuel is insufficient for your engine. The first step would be to make sure you are getting the largest accelerator pump shot your linkage provides for. If you are then you need to go up a step or possibly more in your accelerator pump nozzles. As you do you will increase the volume of fuel the accelerator pump adds to the mixture as the throttle is opening early in the engine rpm increase.

If the stumble persists your accelerator pump diaphragm is too small and you need to go up one step in the Holley accelerator diaphragm hierarchy. Something to consider is the larger accelerator pump diaphragms when used with smaller accelerator pump nozzles spread the addition of the accelerator pump fuel shot over a wider rpm band, which can sometimes be helpful.

BTW I'll bet you can't guess why Holley calls them accelerator pumps ...



Ed