Timing!

[Mike Lenz]

I have questions. Many say to advance the timing on a 100/125 to around 3mm BTDC for maximum engine performance. However, when you look at the factory specs the LARGER displacement bikes have more advance. The smaller the engine, the LESS advance recommended by the factory? If it worked better why wouldnt they recommend at least 2mm BTDC, instead of around 1mm? 1mm is near the spec for KTM 125's from the late 70' thru mid 80's. Its my understanding that as rpm's increase you want less advance because the intake is mixed better and flashes faster. One highly respected tuner says you only want about 1mm advance at 10,000 rpm. He says more advance helps mid range but hurts top end. He says its a needed trade off.  Its my understanding more modern ignitions do this. Giving more advance in the mid range and less advance at upper rpms. Am i correct on that? If so I dont understand the idea of so much advance on a older non advance/retard ignition, as was the idea for better performance in the first half of the 70's. Was that thinking wrong? Am I missing something?

[Rick Bennett]

With larger pistons the flame takes longer to spread.

[Bill Campbell]

I am no expert in this dept but my mates have discussed this topic. We have been talking about from the view of piston speed and the time it takes to arrive at TDC. Logic is it arrives faster at higher rpm therefore needing more advance to fire at the optimum time. The mantra is advanced for top end power retard for low end power. From the above talk there are more variables in head/squish design and I guess high octane fuels also enter the equation. Seeing as I cannot ride and do not wish to ride my 6 Days flat out all the time I am happy with the factory specs. Love all of my motors but they are all way better than my riding ability.

[Mike Lenz]

I also originally thought the same thing. However, the tuning books say just the opposite!  They state at higher rpm's the fuel air mixture is mixed so much better that you need LESS advance. I have been doing some research of timing curves verses horsepower. The simple answer seems to be that more advance does gain peak HP, but over a much narrower spread

[Carl Hill]

My brother worked at a Yamaha dealership for many years. A racer back in the mid 1980s bought a new YZ125 and despite multiple jetting changes could not eliminate a "flat spot" when getting on the throttle. He brought the machine in and my brother retarded the timing a bit and the flat spot went away.