Spark on Compression and Exhaust???

Just finishing up a rebuild on my YZ450. How do you know what stroke the crank/piston is on? Or does this bike fire the ignition on both the exhaust and compression stroke? I have read the manual and searched this forum and have not found anything about this.

I have the I lined up and the piston is at TDC however, there are two TDC's on a four stroke. Just woundering if it mattered.

I believe it does fire on both. Check with FFR, but there has been a recent post regarding this. :thumbsup:

I searched this site for all kinds of key words and did not find another thread regarding this issue. Do you remember the subject line?

TDC == Top Dead Center. Most 4-strokes fire on each revolution, it's just easy that way. I don't know if these engines do or not, but if I had to bet I would bet $1 that they do fire every time the piston gets near TDC.

To tell if it's on compression or exhaust stroke with the plug out, look at the timing marks on the cams (or the cams themselves).

It fires on both compression and exhaust. In order for it to only fire on compression you would need to run the ignition pickup off of something running at 1/2 crank speed such as the cam shafts. Firing on exhaust also helps to clean up emissions by helping to reduce unburned fuel in your exhaust, which in turn keeps the tree huggers happy. :thumbsup:

TZ is correct...fires every revolution. Tdub

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