Originally Posted by Tony Newland

Hello guys, I can shed some light on your pinch vs compression ?'s

Pinch is bad.... well bad for performance anyway. Pinch is there only when the engine is "seating" the piston and liner. Once those parts are ready for racing, after breakin, there should be minimal resistance with the plug out. Now compression is a whole different animal, not to be confused with pinch, but almost always is. Compression is how effective the down stroke and compression stroke are sealing within the liner. You can take a look through the exhaust port while rotating the crankshaft very slowly towards TDC and watch for leakdown (bubbles) exiting between the piston and sleeve. If the engine holds pressure inside the combustion chamber for 5-10+ seconds your piston is sealing very well. if it only takes a few seconds for the pressure to bubble out, the piston is not selaing and the engine is not running with optimal compression. Of course other parts also play a role in what kind of shape the engine is in. The rod and crankshaft are assumed to be in good condition.

The biggest thing you will notice when the piston is worn and developing alot of leakdown is the idle will continually drop throughout a run, and the tune will require a leaner mixture (making the piston expand further to seal on the liner)

In the GRP engines I would recommend the H6-H7 plugs (never use the "C" plugs in offroad), GRP plugs are made to be much hotter than any OS, RB etc plug. We have ran OS P3s before but always sacrificed considerable amount of power and idle quality over the GRP plugs.

BTW carbon on the piston top indicates the mixture is too rich on bottom and too lean on top. If you lean it on the HSN and the idle goes up, your too rich on the LSN