CHEERS

My first guess is low octane fuel and or lean jetting causing the transitional lean fuel ratio condition. Gas-Gas recommends a minimum octane fuel of 95 octane which means that if you live in the States a combination of pump and race gas. The highest octane I can get here in the Midwest at the pump is 93. I mix about 30 to 50 per cent 110 octane unleaded race fuel and have had no problems. Be sure to use unleaded race fuel only. The cooler/dryer temperatures this time of year mean denser air mass and therefore leaner jetting. You may want to look at raising the needle (lowering the clip) and if necessary, going to a richer pilot jet, although the needle usually fixes the problem. Run a plug check and look for a milk chocolate brown color on the electrode, which should be safe. If the knock persists, look for a leak at the intake manifold which would also cause a lean condition or in very rare circumstances, the ignition timing needs to be reset. If the knock still is there I'd let your Gas-Gas dealer look at the engine. The knock means that your engine is unhappy in some way as with the correct conditions, your engine will not knock.

Jon

Jon,

Thanks, I have just been out and adjusted the mixture on my bike and sure enough the"knock" has vanished. without your advice I would have been out there at the weekend stripping down the engine looking for a more serious fault. Thanks once again for the expert diagnosis you made and im sure you will be hearing from me again as I spend a week in Scotland in the mountains in June helping to set up a 2 day trial, and at least 1 bike fails during the week due to the peat bogs or overheating up the mountainside. Thanks once more ,

Paul