Ask The Editors: Mojave Timing Blues
We’ve been in similar situations with modified used machines. It can oftentimes be difficult to determine whether or not all of the parts the previous owner installed are behaving nicely. The first step, were it ours, would be to measure the piston with a micrometer to ensure that it is indeed the correct size for the bored cylinder. There is the remote possibility that aftermarket cams, when mixed with the incorrect piston, aren’t offering enough valve clearance.
If all of that checks out (you can also discover more by googling up the number stamped on the piston), the next step we’d take would be to check on the condition of the lower crank assembly, particularly the solitary gear responsible for driving the cam chain. In rare cases these gears fail (either missing teeth or entirely by spinning freely on their shaft). Check out the following schematic for a more detailed look:
