I had a Chonda engine on a go-cart in a while ago that wouldn't rev under load and produced black smoke like a diesel engine. When the wheels were lifted off the ground it revved happily without any sign of over fuelling. I checked the spark plug gap and tried it running it without the airfilter. Then I put on a spare carby to see if there was a blocked jet. I also removed the muffler as I have experienced them choking up with carbon but none of this made any difference. I decided to replace the spark plug even though it looked new just in case before I went any further. This turned out to be the problem! It can be so easy to overlook the plug even if it is brand new as they can fail under load but still produce spark. This turned out to be a valuable learning experience!

Last edited by LRT; 14/08/17 09:38 PM.