Now for the rest of you time-by-ear guys. The timing requested by the engine manufactorer is a number... Lets say 6 btdc.. In case you do not understand this it means the spark will occur 6 degrees before the piston is at the top of it's stroke.. Now the crankshaft and rod being in the normal arc at 6 deg the piston is ALMOST to the maximum compression..ALMOST! So the fire is lit before the maximum compression is reached and by the TIME the max compression IS reached the mixture is completely engulfed in the flame. This is where the maximum pressure is about to be reached.. AS the piston travels downward from the extreme pressure everything works just fine..
NOW change the timing a few degrees... Well the degrees are not symetrical degrees in so much as the mechanical turning goes.
One must understand that the degrees are the degrees of a 360 deg circle of the crankshaft.. The time the spark occurs can be too soon or too late if only 2 or 3 degrees from expections. If too soon the pressure rises from the flame BEFORE the piston gets to the top of it's stroke...effectively SLOWING the engine! If too late the maximum pressure is realized when the exhaust valve is opening and causes loss of power and a great deal of heat in the exhaust..So a few degrees at the timing mark are CRITICAL! Even though the crankshaft is traveling evenly in a circle the piston STOPS twice in each circle!!! Please get this in your minds! It STOPS DEAD IN IT"S TRACKS! Few have considered this when setting the timing. Ok so armed with this tiny tidbit some of you just MIGHT be running out to the garage to reset the timing...anyway I hope so.
Good! That is why I spent the time.
Big Jim

Ps.. PISTON STOPS DEAD IN IT'S TRACKS!! Didja ever think of this before? <img src="/forums/images/graemlins/pfft.gif" alt="" />