Should I Drill A Bigger Hole In My Slide?

Some jet kits include a drill bit with instructions to enlarge the existing hole or put more holes through the bottom of the vacuum slide. The theoretical idea is that extra holes will cause the slide to respond quicker, make more power, and like all the ads on facebook from women you don't even know, it will "add 6 inches to your length and girth overnight! And magically make a threesome fall into your lap! This is a bad idea and here is why.

Keihin put a hole in the slide that is sized with respect to the venturi diameter to make the carburetor respond in a stable linear manner to your throttle input based upon the load on the engine. In addition to raising the needle to meter the fuel, the slide position directly controls the airflow into the motor, essentially an air flow valve inside the venturi, both of which are a function of engine load. Drilling out the slide changes the flowrate and pressure-balance across the diaphragm that determines the slide height, so the response is no longer linear and may go into an unstable flutter mode.

The result of drilling causes two major problems. 
When the throttle is opened after drilling out the slide, the needle is now pulled too high too fast, which dumps excess fuel that the motor can't use because the airflow and fuel don't meet the engine load requirement. Then to correct this problem a smaller main jet and/or a step-tapered needle is supplied in the “name brand” kit in an attempt to cut back on the excess part-throttle fuel, yet still supply the necessary WOT fuel.

When the throttle is closed the needle now drops way too quickly and the deceleration-enrichment circuit (air cutoff valve) can no longer function, which causes backfiring and lean-pop in the exhaust. So to correct this problem a larger pilot jet is supplied with the kit and the idle mixture screw is turned open to provide the extra fuel needed on overrun. But this dumps excess fuel constantly--which makes the bike run rich at the low end, from idle to ¼ throttle. This fouls the plugs, washes the cylinders and ruins whatever fuel mileage you have and does nothing to make more power.

So one stupid idea, yes, simple but stupid (drilling) has now created two new issues that are patched with band-aids resulting in lower efficiency, and poor driveability and performance.

Unless you are turbocharging, supercharging, changing cams or making some other serious mechanical modification to the motor that increases the maximum airflow requirement to a point that exceeds the venturi’s capability, then the stock carburetor has plenty of adjustment potential to meet the demand without resorting to monkey business using drill bits!