yep, the wrong grade of plug could make a nice burnt hole right through a piston!! In an extreme case! second question....You should be running, (as a "standard"plug) one of the following; if its a "champion" plug it should be AMY 8C, if its an NGK it should be D8ES,
If its a Bosch it should be X300T2, and if its a autolite it should be HG1........There are a couple either side of that range permissable for certain exceptions, but allthough folk will argue ( just for the sake of it sometimes!) my money has allways been on NGK, I have never had a problem with them!
anyways to your jet size.......that all depends on what state of tune your motor is in, for example is it stock bore (736cc)? and is it running its factory four pipes or has it got a four into one system? this will make a difference to choosing the correct jets....
