Intake types and ignition timing

[Deleted member 12676]

I'd like to share about ignition timing and intake systems. I've known that the more "obstructions", twists and turns the air/fuel mixture has to make to get to the combustion chamber, the more "mixed" it will be. But I never thought of a reed valve as also being a turbulence causing obstruction before now. I was reading info about the RSA125 GP racer that had a special rotary valve located right where a reed valve would be instead of on the right side of the crankcase. It's ignition timing was noticably more advanced than what a racing 125 normally has. That intake system would present the least turbulence of all available systems. So I put two and two together since the better mixed the fuel and air is, the quicker it burns, and vice versa. Now how does that relate to the chinese engines? They have a piston port only intake system that presents just as little an obstruction as a rotary valve located there. And their ignition timing is fairly well "advanced". Most race bikes have 20-24 degrees BTDC as the most advanced timing and then it retards down to around 10 degrees at 10,000 RPM. The RSA125 has 30 degrees maximum advance and then it don't go down to 10 degrees till almost 14,000 RPM. The Grubee has 30 degrees advance at 4000 RPM, but then instead of retarding with higher RPMs it continues to advance to 33 degrees at 6000 and above, which is typical for a 4 stroke ignition. So the increase to 30 is OK for this stock engine (with low compression) but it fails to retard with increasing RPM. I think the factory uses this 4 stroke CDI because of low cost and because the extra timing advance above 4000 RPM serves as a rev limiter. Before now I never considered switching to a reed valve as a reason to get an aftermarket CDI with more retarded timing at high RPM. My performance CDI has enough adjustability to match either type of engine. None of its competitors are adjustable. Go to www.dragonfly75.com/motorbike/CDI.html to read about it and how to get it. Warning- it is the most expensive but it is the best.