I was just at GT Thunders web site and there was a critical note about the 400EX. It says, "If you advance your timing at the flywheel or install an aftermarket cdi, you will have head gasket and head bolt problems." Has anyone experienced any problems with the Sparks key or an aftermarket cdi? Why would either cause this?

 

There is a email forum that i belong to. We have went over this issue before. The theory is that only applies if you put in a big bore kit with high comp piston, like 89mm or so.