[jrlastofthebreed]

I've been running this bike with a stage 2 hot cam and a wiesco 12.5 -1 high comp piston for a few years. Over 200 hours so i desided to replace the piston and cylinder. Ordered the same wiesco piston and a cylinder works cylinder. Both measure identical to the ones removed.

Now when i do timing i align the gear on the right side with the mark on the case. Making sure only to turn in the clockwise direction. then install the sprocket, set the tensioner then turn the engine over once or twice and check again to verify.

On first assembly the timing chain could be on one tooth or another (depending on the angle you looked at it). So i picked one and tried to start. Nothing. just an occasional backfire with a short flame out the exhaust.

So I tear it apart again and change the timing 1 tooth. Turn the engine by hand and the exhaust valves hit the piston. Hard. Ok so i think maybe my new gaskets are to thin. So i install the old ones. (even though they measured the same). It does the same thing. Then just for kicks i install both gaskets at once. Now the timing chain isn't long enough to get onto the camshaft.

Now in starting to get annoyed. With the camshaft removed (valves closed) I hooked up my compression gauge and turn the engine over and get about 30 psi. with the camshaft installed and timing set to the first setting i get 5 psi. (my other 450x also gets 30 psi with everything installed)

Heres what else i've checked.
The piston is installed with the arrow forward and the large cutouts to the intake side,
Piston rings are gaped and aligned 180 as per manual
When i hook up the camshaft sprocket to the camshaft the lobes are aft.
I have good spark and have also swapped plugs,

It appears the exhaust valves are opening to soon? relieving pressure and eventually hitting the piston. Not sure how this could happen as it has been running fine for years and i haven't been into the crankcase. Could the internal timing be off somehow?

[JimDirt]

Something is not right with the valves ... or the timing is definitely off ...... you should have 58 psi at 360 rpm , (so turning it over by hand should yield a good compression reading) if it does not , then you have a bent valve somehow , and/or the timing is off enough that its holding a valve open ... only other option for low compression , is the rings are either aligned for some reason and its not sealing , or a ring is broken (doubtful) , but something is holding open and allowing compression to bypass somewhere ....

Did you check the compression when you first assembled ???? ..... Can you do a Leak Down Test ???? , this would tell you if the issue is a ring or valve leak .... or if it is a timing issue ...... The thing with the timing is , if the cam notches are level , and the dot/arrow are aligned on the clutch side and the line is visible in the small plug hole on the ignition side , and the cam lobes are facing 2 O-clock (towards the rear) you should be in time .... if any of the 3 are not , then you need to relook at the timing as something is off ......

When you set the timing , and the valves hit , how were the marks aligned ??? ... 1 tooth off you should not be hitting valves and pistons .... so something else is wrong ... Go back and recheck the timing , if its on , you should be able to rotate the engine several times , and get it back to aligning the marks on the cam , then the rest should still be aligned .... if they do not , then pull the gear or the cam , hold the chain up where there is tension on it .... , and get the piston (with the plug out) to TDC , then look at the clutch and ignition marks if they are aligned (with the timing chain tensioner loose or out) , then install the tensioner , the marks should not move .....

[Back2-2]

Timing is off. That is why the valves are hitting.