First of all, I'd think it EXTREMELY rare (almost never?) that the timing ring hub slips from the factory setting, causing a change in the ignition timing. Even if it is suspected, one should accurately check to see where the timing is, BEFORE moving the ring and changing the timing to something else.

Having said that, I first suspected a flooded engine, caused by flipping the prop with the choke closed, then turning on the ignition and flipping the prop to start it. This is solved by first turning the ignition on, then closing the choke, flipping till it pops, then open choke and flip 3 or 4 more times till it runs. Krayzc was doing that, and still getting kickback, so my next suspect would have been the ignition itself not properly retarding the ignition for starting. As I mentioned before, the CH ignition is timed at full advance, and it retards the ignition timing when you are trying to start it. Krayzc said it ran NORMALLY once it started, which also would verify that the timing was still at the factory setting. The fact that it kicked back pointed to the possibility of the ignition itself malfunctioning, and not properly retarding the ignition during startup.

The fact is, the fuel mixture doesn't burn instantly, rather it takes time for the flame front to travel from the spark plug and completely burn the mixture. Because of this, it is necessary to light the mixture earlier (28 degrees before the piston reaches top dead center) so that the pressure is building and by the time the piston just passes top dead center, the expanding gases exert maximum pressure to drive the piston down the cylinder. Ignite the mix too early (advanced) and pressure builds too fast and actually fights against the piston while it is still going up. Heat builds, detonation and pre-ignition enter the mix and work to destroy the engine. Ignite the mixture too late, and pressure builds too late to effectively drive the piston down. The fire is still burning when the exhaust port opens, and energy is wasted out the exhaust pipe.

That's the reason that tkg wondered about the ACTUAL ignition timing here. Since the CH is set a maximum advance, if the timing was actually set at 5 deg BTDC much of the energy would be going out the exhaust pipe rather than pushing the piston down, and the engine wouldn't have nearly the normal power since the spark is being lit over 20 degrees too late.

Before one can get accurate readings with a degree wheel, the setup has to be correct so that when the needle reads zero degrees, we know that the piston is EXACTLY at top dead center. If one simply sticks something in the spark plug hole to feel the piston coming up, the zero reading can fluctuate up to 15 degrees either side of top dead center, which is a 30 degree swing! This is because turning the crankshaft doesn't move the piston much at top or bottom dead center. One needs to use either a dial indicator or a positive stop to accurately zero top dead canter. Then, all other readings will be just as accurate.

I'm still suspecting the ignition itself isn't retarding the spark at startup like it should, rather, is staying at full advance. That's why it can rev up with normal power, but kicks back on startup. At prop flipping speeds, if the mixture is ignited while the piston is still going up (28 degrees BTDC) the pressure drives the piston back down and the prop goes in the reverse direction (kick back, and yepper, it BITES!). Retard the ignition so the piston is at top dead center when the mixture ignites, and it can't kick back.


PS - Trying to do all this thinking has got my head hurting!!!