Re Pinging in the RM370,
I was reading an old copy of the "Moto-X Fox Guide to Preparing the RM and PE Suzuki for Competition" booklet the other day and noticed a paragraph under Carburetor and Throttle about altering the ignition timing on the RM370 to help stop the "Ping".
It quoted, "On the RM370 you should change the ignition timing from the factory setting of 4.1mm BTDC to 3.8mm BTDC. Retarding the ignition before top dead centre will give the 370 a little more low end power and help prevent detonation which the 370 engine sometimes suffers from".
It also said the carby jetting for the RM370A,B should be 320 main, Q4 needle jet with the clip in the third from top on the needle.
If any or all of these hasn't been done already, they may be worth a shot.
Hope this helps