My Carburetor Stumbles in the Turn
We are starting this out assuming your carburetor is in good shape; power valves are good; floats are set correctly and jetting is reasonably close.
First, ask your driver if it stumbles on a restart. If no, there is probably nothing wrong with your carburetor and the problem is fuel slosh or lack of fuel in the bowls.
If the answer is yes, your problem is the carburetor. In this case, just move the squirter size up or down a couple of sizes to see if it gets better or worse. For example, a 31 squirter to a 35 to richen the bottom end up or a 31 to a 28 to lean the bottom out. A general rule, the bigger the manifold or intake ports on the head, the bigger squirter it needs.
If your carburetor does not stumble on a restart and only stumbles in the turn, the first thing to do is lower the front float level. Look at the flats on the float adjusting nut, loosen the jam screw and turn the nut clockwise 4-5 flats. Do this on the front only. Go out on the track again - if this makes it better the problem is fuel slosh in the bowls.
If it makes it worse, you are running out of fuel in the bowls. This could be caused by too small a fuel line; restriction in the fuel line; plugged filter; vent in the tank not working; or pick-up in the tank.
The easiest way to diagnose this is to pull the fuel line off at the inlet side of the pump. Hold the line at pump height and turn the fuel on and let it gravity feed into a container. Drain out a gallon or two. It should flow a good solid stream and not taper off after the first gallon. If it passed this test, you need a fuel pump. I would recommend a CV Products billet pump for gas and a belt drive pump for alcohol.
If your fuel system failed the gravity flow test, start at the pump and remove and inspect one fuel line at a time, the whole way back to the tank until you find the restriction.
Let's get back to the fuel slosh problem...
As you roll into the turn, the fuel stands at a 45 degree angle in the bowls. As you accelerate in the middle of the turn, the fuel runs toward the jets in the front bowl, making the front rich. At the same time, it runs away from the jets in the rear bowl, making it lean. One of the Cup Teams we dealt with a few years ago actually had video shots of the fuel inside the bowl at Daytona - standing at a 45 degree angle and then lifting 1/4 inch off the floor of the fuel bowl.
Entering the turn, your car gets over on the right rear, then under acceleration, the left front starts to lift. At that point, fuel starts to dribble in the right front booster of the carburetor. You are not a full throttle yet, so now the butterfly is directing globs of unatomized fuel to one end of the manifold. Lowering the front float helps this condition. They also make a carburetor spacer that tilts to the left front of the car. If you have a car that gets over real hard on the right rear, lifts the left front tire and has good forward bite, we can go inside the carburetor and put a cut off phenolic float in the front, angled toward the sight plug and a cut off phenolic float in the rear, angled toward the sight plug with notches for jet extensions. Use the Holley screw in jet extensions in the metering plate with the jets on the end of the extensions.
This information should help you fix your problem quickly and in a systematic manner. I would also recommend keeping notes on what your driver said after each change to avoid making the wrong adjustments twice.
Even if you aren't experiencing any stumbling problems, experimenting with squirter size up and down may tweak the carburetor enough to make a driver more comfortable with better throttle response. Or it can be used to take the edge off a radical engine combination that is hard to hook up on a slick track. The same can be done with power valves. Just up or down 1 size at a time until you hit the magic spot.
Most of this type of tuning requires good driver feed back to be beneficial. That being said, I have run across drivers in 35 years of building carburetors, that had no feeling in their ass and can't tell you anything - so sometimes it's best to have a stop watch just to back up what your driver tells you.
DISCLAIMER
The preceding is what I would do in these situations. Your experience may be different from mine. If you are not knowledgeable in spark plug reading, carburetor basics, or engine tuning, maybe it would be better to let an experienced person help you. There is more than one way to skin a cat. This is what works for me and hopefully it will provide you with some help with what can otherwise be a very frustrating problem.
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