to much can blow fuel out the exhaust excessivly when you let off the throttle after a hard wide open throttle and may stall the engine because the tuned pipe is loaded with fuel and it gets blown back into the engine by the tuned pipe when you open the throttle again. depends on how your tank is vented. This was a big issue on the first NTC3 tanks before they weht to the cap pressure fitting. To much will also make the engine finiky to tune and needle sensitive. to little is rarely a problem but usually a sign is that the engine will rev up hard and then lay over or die at wide open throttle. acts the same as having the top end needle to lean