kx250f TDC PROBLEM

[xlay20]

Hi everyone, i need a help with my bike.
I just replaced the valves, piston and rings , now im trying to set the timing. My question is when i turn the magneto wheel to tdc mark the piston is at the top but is tdc after 4th stroke and before 1st stroke or after 2nd before 3rd stroke? Ignition mark is right before the tdc mark so the magneto has to turn almost 360 to ignition so it looks like its after 4s before 1st but the marks on the camshafts are in the direction after 2nd before 3rd. sorry if i got you confused and thanks for help

 

[Rich Rohrich]

It's TDC on the Compression stroke when both the intake and the exhaust valves are closed.

 

[High Lord Gomer]

Isn't it actually both? Since the magneto rotates the same as the crank, it should hit the TDC mark at both the top of the compression stroke as well as at the top of the Exhaust stroke.

Also, doesn't the spark plug actually fire at both times even though it doesn't really matter that it fires near the end of the exhaust stroke?

 

[Rich Rohrich]

There is 720 degrees (2 complete revolutions ) of crank rotation in one "stroke" or power cycle so yes it will hit TDC twice, but only one TDC is correct for timing the cams and adjusting the valves.

TDC on Compression with all the valves closed is the one we are looking for most of the time. Yes, the spark fires at each TDC in this type of system. It's called a "lost spark system" and it's pretty common with modern engines. Some more advanced systems will use a cam position sensor to determine the TDC on the Compression stroke. This type of sensor is most often seen in EFI equipped engines to determine the correct point to fire the fuel injector.