Carb Class: Baseline settings for your new carburetor
A question frequently asked here at Quick Fuel Technology is how to properly install and setup a new carburetor. Our focus is more on the setup than installation in this commentary. Some racers are very thorough with a new carburetor and others will just "bolt it on and go". It is essential to go through the basic steps for a pain-free set-up that will save you headaches down the road.
Installing a carburetor is one of the more basic race car procedures so we won't go into detail on it. However, there are a few crucial items needing special attention. A carburetor is designed to precisely meter fuel and air; if the sealing surface is damaged or obstructed the carburetor will essentially have an uncontrolled source of air - the dreaded vacuum leak! A common mistake that can also lead to trouble is over tightening. Moving in a crisscross pattern will evenly distribute the torque of your wrench and ensure that the mounting flange of the carburetor is not damaged. Once the carburetor is properly mounted, take a few moments to move the throttle linkage. For your own safety it is imperative that the throttle returns to the closed position on its own and moves freely.
After the carburetor is mounted on the engine and everything else works properly the initial tune- up can be dialed in. Float level is very important to the overall performance and consistency of your race car; there are ill-side affects if you deviate from this a crucial step. Once you have filled the fuel bowls and primed the engine, start it up. Set the float levels while the engine is running at idle. Normally, a Quick Fuel Technology carburetor will have the fuel level half-way up the sight window with 6.5-7 PSI of fuel pressure. Other brands and some older carburetors will require a sight plug removed to check this. Take precaution make sure that you keep fuel away from any ignition source. The next step will be to set the idle mixture. Most carburetors have an adjustable idle feature whether it is 2 screws or 4. Four barrel race carburetors typically have 4 screws, one for each barrel. The ideal method for setting idle mixture is with a manifold vacuum gauge. Setting the screws to attain the highest reading of vacuum is the standard method but not every engine and combination will follow that setting. Some modern race engines require more fuel at idle. Too lean of a mixture could cause the engine to die once you click it into gear. Idle mixture is a part of the process you will simply have to play with to determine what you prefer, take it slow and go a half turn at a time. Always remember that not all 4 screws need to be turned the same amount out or in.
Once you have set the float level and idle mixture; the last step is adjusting your idle speed. Many carburetors have a transfer slot that aids the carburetor in its transition to wide open. If the throttle plate position is too far up (and into the transition slot) your idle quality will be low and the car might stumble at the starting line. The plate position can be corrected by adjusting your mixture screws (again something you will have to experiment with). The goal of a proper idle speed and mixture is to have the engine have a suitable idle while in neutral or in gear. The main idea is to have an idle that makes the car smooth and predictable.
A properly set-up carburetor will give a solid baseline for fine tuning to achieve maximum performance. If the carburetor is not dialed in correctly when first installed it has the potential to compound other problems and potentially cost more money (and time) to fix.