Pull a couple of spark plugs and look for tiny flecks of silver on the white of the porcelain. Those specks are aluminum from the piston-a definite confirmation of a too lean condition. At that point, the driver probably felt the engine "popping" near the end of the straightaway. Raising the high-speed pop-off pressure will cure that. You can be lean without specks; they just tell you you're about to have some serious trouble.
I look for a plug that's just showing a light brown color on the porcelain and a heat, or color line, on the curve of the electrode. From the tip to the curve will be a lighter color than the base. If the lighter color runs around the curve and down toward the threads, you're too lean. If the color doesn't make it to the curve, you're too rich.
These are very basic techniques. I still take plugs to Nelson for an experienced "read." You should work with your engine builder to get a good, solid opinion.
Adjusting The MixI like to run my engine a bit on the rich side. A slightly rich engine makes more torque and lives. A lean engine makes more horsepower and, if you're not careful, dies an unpleasant death.
Let's say you plug the ADR number into Computech's computer. It tells you to install a #70 main bypass pill in place of your base, #64 pill. That'll bypass more fuel and lean the engine. You do it and the engine takes off fine, but gets lazy at the end of the straightaway and blubbers just as you roll into the turn. Or, if the opposite was the case, you change from a #70 to a #64, the car takes off fine again then, halfway down the straight, the engine begins popping and slowing down, but recovers once you burp the throttle or the resistance of turning lowers the engine rpm.
You forgot to change your high-speed to stay in tune with your main. Even though, in the first example, you leaned the main, the engine is still too rich on the top end. That's because by putting less restriction in the main line, (that bigger, #70 pill) you lowered the pressure in the entire fuel system by 6 to 9 pounds. The high-speed bypass you carefully set at, say, 65 pounds, never opened because the system pressure never got that high. What you needed to do was lower the high-speed's pop-off pressure accordingly.
In the second example you raised the system pressure by the same amount and caused the high-speed to open early, leaning the engine on the top end, even though you intended to richen things. I like Kinsler's diaphragm high-speed bypass, because you can simply loosen a lock nut and turn the adjusting screw 11/48 turn for every .002 inch of main pill size you change. When you went from that #64 pill to the #70, you needed to unscrew the Kinsler high speed 31/48 turn. If you go from a #70 down to a #64, tighten it 31/48 turn. If you use one of the can-type high speeds, take it out of the return line, open it up and subtract or add shims to lower or raise the pressure anywhere from 1 to 211/42 pounds for every .002 inch of pill size.
Waterman recommends changing the high-speed by 1 pound for every .002 inch of pill size. I've always used the 211/42 figure. You'll need to do some experimenting to find what works best for your engine. BR Motorsports carries a high-speed checker with a big, easy-to-read gauge that's awfully handy no matter what type of high speed you use.
High-Speed FlowIf the high speed is such a hassle, isn't there a way to get by without one? The answer is a simple, "no." Today's pumps, like the Waterman, put out so much fuel that you absolutely must bypass the excess. And the problem gets worse with every rpm the engine turns.