Re: Engine Problem
Back in the day I raced gas powered remote controlled cars. This would happen to us too. I think I know exactly what is happening to your engine. It is at least something to look into.
Sometimes the fuel tank would go empty. When the very last drop of fuel would get to the carb the engine would do the same thing it would rev to redline (even though we would not be giving it any throttle) and then die. I know yours isn't dieing but it is reving really high without you giving it throttle. The reason the rc car would do this is because as the last bit of fuel reached the carb then the mixture would lean out causing it to rev with no input from the throttle. In your case maybe something is causing your engine to momentarily lean out. Maybe something is floating around in your carb and every once in a while it plugs the needle valve and then gets unstuck and your engine goes back to normal.
If you are unfamiliar with the term "lean" or "lean out" it refers to the air to fuel ratio. Lean means there is more air and less fuel. "rich" is the opposite where there is more fuel and less air. Your engine will run its best when it is in proper balance of the two.
Are you using a fuel filter? Maybe take the carb off and take it apart to clean it out. Once I had an issue with a carb on a dirtbike where we used a gasket sealant and a little piece broke off inside the carb and it got stuck in the needle valve.
Hope this helps
BTW I remember this thread from last week and I had mentioned it must be electrical but now with the new info on the engine racing it leads to my new idea.