I have a modified gx670 engine in 1999 custom EZGO golf cart. My problem is lack of performance. It will not pull over 4200 rpm out on the road. It has a pipe and cold air kit, the heads have been milled 50 and ported, and have 1.3-1 rockers and hd valve springs. The engine will free rev to 5500 RPM plus but will only pull 4200 on the road at 39 mph. I put 102 jets in carb and didnt help. I dont think the carb is big enough after the mods and thinking about a 40 Mikuni. Anybody have any suggestions or experience on rpm issue.