Triton Thrasher: Slander not towards the engine of course, but towards the professionals designing a basically fine engine. AMC did test their products thoroughly, sometimes day and night, for thousand of miles! There is no evidence of the G15Mk1 being a design failure.
Bernhard: The 750 Matchless hybrids mentioned were G15CSRs (aka G15Mk2), i.e., Norton-engined bikes . G15Mk1 features an AMC engine, and is a completely different bike. To my knowledge the G15Mk1 was never raced in long-distance races. Anyway, there were actually two G15CSR entrants in the 1965 Thruxton 500 mile race. These were unfaired bikes, competing against faired and highly tuned Triumph and BSA 650's. An unven match, I would say.
As for PW's comment, it would be useful to know what his points of criticsm were. I am guessing at the primary drive and the clutch, clearly overstressed by the torque engine and not permitting further tuning measures. Furthermore, the chassis wasn't designed for racing - contemporary tests of the G15CSR mentions slight rear-end weave, which would limit speed on curved tracks.
Regards,
Knut