Accelerator pump tuning

[*13*]

You are down two sizes on your main jets "may" mean that you are dumping too much, too fast from the nozzles into the carb causing a slight bog. I agree that you may need more fuel but like I said, it's tough to say without knowing everything. Try bigger first. If that does nothing positive, go smaller.
I didn't notice that you have a vacuum secondary carb. That also may be a cause. If you can't get the secondary to open correctly with the proper spring change try a double pumper :0)

[Gunner]

Nah, I very deliberately passed on a mechanical secondary, dual pumps or not. Other than instant WOT response, vacuums are generally easier to tune and live with. I'd have fixed that problem days ago but my local speed shop (the only one in quite a radius) was out of the tuning spring kit. So I have a whole care package coming from JEGS.

[eschaider]

Gunner,

My experience with the stumble you are describing has always been caused by a lack of dino juice. I would consider a step up in nozzle diameter and then a step up in pump volume if the diameter does not fix the problem.

When you hit the throttle not all the fuel goes into the cylinder. A small amount of it attaches itself to the manifold and port walls and evaporates from both because of airflow passing over it and engine heat. Your job as the tuner is to reach a happy balance where the liquid on the manifold and port walls continues to evaporate in sufficient quantity (without drying up) to fulfill the engine's acceleration enrichment requirements.

When you literally inject from the accelerator pump an insufficient quantity of fuel to properly wet the manifold and intake port walls for your engine displacement you will experience stumble. When you overdo it you will see a black smoke 'cloud' briefly out the exhaust and loud but lazy throttle response. When you are just about right you will see a quick puff of black smoke and the engine rpm will jump in a fashion that sounds like it is instantaneous. Your engine will sound crisp and responsive.

By the way the Detroit guys call that thin film of fuel on the manifold and intake port walls Tau(t). The greater the volume of air passing through the intake the faster this film will evaporate. As the tuner you need to maintain this film for proper throttle response. More air volume (larger displacement) needs more fuel (larger nozzle diameter and larger accelerator pump shot) to maintain the fuel film during the engine acceleration event.

Ed

[*13*]

Cool, Carbs are fun to fiddle with, especially when you get them dialed in correctly!

Originally Posted by Gunner

Nah, I very deliberately passed on a mechanical secondary, dual pumps or not. Other than instant WOT response, vacuums are generally easier to tune and live with. I'd have fixed that problem days ago but my local speed shop (the only one in quite a radius) was out of the tuning spring kit. So I have a whole care package coming from JEGS.