The engine is in standard spec so do I go by the book? I understand modern fuels require less advance but how much less?
Advice greatly appreciated.
Hugo
Hugo,
I think the most direct answer is: if your engine is stock go by the book. The Factory most likely came to the listed timing figures by testing the engines on their dyno.
As to the second question, in the best of all possible worlds, one would test on the dyno to see what happens to the power and torque when the timing is altered. As Rip mentions, there are lots of machine variable that could affect the outcome. If you ask what does the timing number represents, it will help you make decisions about where you should time your bike. The ideal timing would occur near TDC. The spark would occur and instantaneously push the piston down ATD. This assumes that combustion is started and finished in a degree or two of crank rotation. That would be an incredibly efficient burn. If the Factory reccommended 40 degrees at full advance then they are saying the combustion chamber is quite inefficient. If you fire at 40 you are effectively lowing the compression ratio from 7 to 6 because you only get the pressure from the 7 to one piston at TDC. This lowers the combustion pressure and combustion pressure is power.
Additionally, the longer the combustion, namely the more advanced, the hotter your engine will run because the flame is hanging around so long. More heat means less power. This also happens when you have a high compression piston with a high dome and a deep combustion chamber. This set up could double the surface area exposed to the flame.
If modern fuel burns faster then you can inch your timing closer to TDC. I do not know if that is the case, but as you say, others are suggesting it. The use of dual plugs tends to speed up combustion by creating two flame fronts instead of one, thus, two plug ignition timings tend to be closer to TDC. When dyno timing I move the mag 2 degrees at a time. With dual plugs, I ended up a 19 degrees fully advanced. What I think this told me was that some of the combustion chamber mods worked and I made the burn much quicker.
The reason I mention the above is that Comets need all the power they can muster. It is worth trading some heat and compression loss for BHP, so as long as your jetting is good and the starting procedure still works, I would not hesitate to make a small change toward TDC.
David
