One of the most valuable lessons to be learned when playing with go karts is engine repair. Also, like any big problem to solve, some of the engine problems can be very elusive and frustrating. I had this engine that just would not run right. It would run, but not fast, and it was very frustrating.
What would occur is that the engine would start and run just fine, but when the throttle was opened up the engine would just back fire through the carburetor, out the exhaust, blow black smoke and just would not get past really anything but idle. You can imagine my frustration. I have this elaborate strait tube intake (which really is a high rise manifold) and this oversize carburetor mounted to this 4 horsepower Briggs.
I had that engine torn down probably twice. Removed the head, checked the valve clearances, tore apart the carburetor, added more sealant to the intake runner and the carburetor interface…all to NO avail. The engine still ran like junk. Having just exhausted all remedies, I took off the engine and tore it apart completely looking for the elusive problem.
And after I tore it apart I had to laugh, and be educated. The points on the engine had a spring that keeps the points coming back into position. Trouble was, the spring was on sloppy, and allowed the points to literally float as the rpms increased. So what occurred was the engine was literally losing spark at the higher rpms. The floating points acted like a governor. Old “his and pop” engines actually use this method for slowing their engines down. Instead of playing with the throttle, they play with the ignition to slow the engine down.
Once the engine was reassembled the engine ran full throttle no problem. Understanding the reasons why engines behave the way they do is very important in diagnosing the problem. One of the elusive things about this problem though is the assumption that the points are fine, that the spark is consistent. The trouble shooting motto is question everything and exhaust it at the least cost.