Think of timing like this: The more fuel you have to burn, the sooner you will want to light it to make sure all the combustion has taken place BEFORE the exhaust valve opens......more fuel will need more time to completely burn. If you are way overfuelled and don't completely burn all the fuel before the exhaust valve opens, things will get darn hot in a hurry because you can actually start dumping combustion out the exhaust! Obviously, high EGT and a loss of power will follow. You want to dump exhaust gases out of the exhaust - not combustion. There is a delicate balance.....too much fuel too soon will make the cylinder start compressing it to early in the stroke and excessive cylinder pressure can be the result.

 

thanks for the explaining it.