Attached are some responses from e mails that cover the bulk of the questions we receive. I hope these help and feel free to Email or call if you have any further questions.
Q: I am building a new combination and am weighing the options between a carburetor and fuel injection. Which is the best way to go for my application?
A: Obviously the first thing to consider is budget. A carburetor is less expensive on average. The next thing to consider is how often do you plan on changing your combination? A carburetor is more adaptable and easily up-gradable (if any upgrades are needed at all). Lastly, when comparing the performance of an EFI system versus a carburetor, the carburetor wins pretty
much all the time. Now that’s not saying that a custom built EFI system like on a Formula One car or a system built by someone who really knows how to build them (like Harold Martin) can’t compete with carburetors in performance. These systems are highly complex and constantly tuned and refined by the masters of their craft. But the return on investment isn’t as good and the ease of use is still not as simple and predictable as the reliable carburetor.
Q: How do I properly jet my carburetor. I've been moving jets up and down and really don't see any difference in performance unless I go smaller. This
causes it to be lean and the car slows down.
A: The best thing to do is be sure you have a carburetor that has the metering system designed to operate properly at your power level range. If you are trying to draw too much fuel through the metering system you will over work the air emulsions and the fuel curve will excessively lean out or in some cases go extremely rich (depending on the system layout). If you are drawing less air and fuel through the design than its intended operating range, the fuel will be raw, lacking proper emulsification and relatively unburnable at lower rpms. If the proper design is on the car, jetting is simple. You jet up typically 2 numbers at a time until you see the mph or engine performance drop off (depending on the application). Then return back one number. At that point you make another pass/run/lap and begin examining the plugs for corner to corner jetting requirements to balance the engines air to fuel ratios for every cylinder.
Air bleeds are vital tuning tools. But they can get you into trouble if you don't know what you're doing. That's why we operate a 24/7 tech-line.
Q: One of my idle mixture screws has more of an effect on my engines idle quality than the other four. The circuit is not plugged, what's going on?
A: You have to remember, that there could be significantly more cylinder pulse in one section of the intake than the other. This will cause the engine
to operate more off of one section of the carb than the other on both idle and main circuits. Also remember that even small differences in port flow and
orifice size in the circuits makes a big difference in potential flow and fuel delivery. So what you are experiencing is not uncommon in an out of the box carb or even some professionally modified ones. An .002 increase in idle feed diameter is pretty major. Professional modifiers typically tweak the idle feed
diameters a small amount to correct for port flow differences. But it takes some pretty good equipment to measure what you’re doing. So don’t fret it. Be sure the cylinders are balanced within 80 degrees at an idle (use a track temp gun to read the pipes) and enjoy the car. Later on if you’re getting into more competitive style racing send it off to a modifier to get it balanced.
Q: How do I properly set my float levels. I noticed some racers have their float levels set at the bottom of the sight window and some are almost at the top.
What is the preferred method?
A: Picture what is happening in your carburetor when you raise the float levels. The booster can now more easily pull the fuel “over the hump” to allow it to leave the bowl area and enter the venturi area and be ingested by the engine. If you allow the fuel to be pulled in too early the part throttle operation and low speed air to fuel ratio ingested by the engine will typically be rich, ratty and unburnable. If you have your float level too low, the booster will be
delayed a few extra milliseconds when you hit the throttle off an idle and this will create a delay in throttle response and in some cases the car will shut
off. At launch time if the level is too low, the car will be too lean at the launch when air speed/signal is at its lowest point. As a result the car may hesitate or be down significant amounts of torque as a result of being lean but for a moment. If the level is too high, raw fuel may come pouring out of the
boosters (and on Dominator carburetors the intermediate circuit) when idling/part throttle and during hard acceleration. So float levels require tweaking for the application. Start low and work high.