timing? TDC? BTC?

[woodsracer144]

ok so i would like some help, when or where should the flywheele be when the pison is at TDC? I know people advance it and you can advance it too much but could you reverse it and retard it too much? If some one could go in to great detail it would help me out alot, i could have posted this under my 02 CR set up post but i thought you could use this info on more then just that set up,

[Honda 250r 001]

Well if everything is set up correctly, the flywheel tab will be near the pick up tab when near and just before TDC. On my cr 250 ignition 01. The piston is at top dead center when the flywheel tab is just to the right side of the top of the flywheel. I hope this helps.

[wilkin250r]

The CDI itself will have a timing curve. I'll be honest, I've never bothered to check what it actually is, but here is basically what it SHOULD be.

At low RPMs, you have lots of advance, like 30 degrees or more. This is because your cylinder doesn't get filled efficiently without the pressure waves from the pipe. Low pressure, and lots of exhaust contamination, you need lots of advance to burn properly.

As the motor gets on the pipe, the exhaust is pulled out better, and the returning pressure wave stuffs the cylinder. Cylinder pressures increase (which makes the fuel burn faster), so the timing drops from 30 degrees to around 12-17 or so (spark fires later, closer to actual TDC).

As RPM increases, the piston is traveling faster, which means you need more advance, or else the piston moves beyond that sweet spot where the power is most effective. So to combat this increased piston speed, the spark fires earlier and earlier, so the actual combustion still takes place when the piston is in roughly the same spot for best power. It advances to 30+ degrees again.

Now, it doesn't change instantly. For example, when the motor is getting on the pipe, the timing doesn't drop from 30 to 12 in an instant, it tapers with rpm in a basically linear fashion.

[wilkin250r]

Now, beyond the actual timing curve, here is how timing changes affect your engine.

Advancing your timing causes your spark to fire earlier, obviously. This means your piston is closer to your head when things are burning, which means higher cylinder pressures. This increases power, but it also increases heat.

Too much heat, and you get detonation problems and run the risk of damage to your piston.

At high rpms, the spark is firing SO far before TDC that it's burning before the cylinder even reaches TDC, This increased cylinder pressure makes it harder for the piston to move upwards, which isn't that bad, except that push is power lost at high RPMs. As a general rule, high advance makes it harder to rev past peak, you kill the over-rev.


Retarding the timing is exactly the opposite. Less heat overall, but also less power. If it's retarded too far, then you start losing massive amounts of power, because the fuel is burning too late, it doesn't push the piston efficiently because the piston has moved past it's sweet spot, so that push isn't really effective anymore.

And, also opposite at high RPMs, retarding the timing generally revs out a little further because you don't have as much loss as the piston is moving upwards towards TDC.

[woodsracer144]

ok thanks, I was way to far advanced, i was in a muddy field when i was testing it runing in a HUGE oval and i looked down just at the right time cause my header was starting to get a small spot of blue, I wasnt puken coolant yet but i stopped and turned it back alittle bit but it is like the only time i get power now is when i really rev the **** out of the motor, Im running a early CR cyl but i think my jetting for the low and mid is off cause its really doggy...