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.

Could be a short in the kill switch.. I don't have much experience with these things, but the only thing I can think of is an electrical connection somewhere. Plug? Plug wire? CDI?
Something in this area I would say..

Sounds like your float is set a little too low and it's running out of fuel momentarily.

By going to 100:1 your FUEL TO AIR MIXTURE should be richer, not that it would cause or exaggerate the problem.

By changing to 100:1 you may be revving the engine a bit higher and it's just running out of fuel in the float chamber.

could also be vibration, causing fuel foaming. how smooth is it running at the speed it cuts out at?

You need to take the bowl off of the carb for two reasons. One is there is a needle valve in there that you need to clean or inspect for debree. Second you need to take the bowl off to adjust the float. There is a couple of metal prongs the float pushes on. You need to bend them slightly to adjust the float.

I use opti also but 100 to 1 was letting my motor run too hot. You might try 75 to 1. My new top end has run on 75 to 1 since day one and I have great compression. How much viberation are you getting? The jet in the carb sometimes loosens up. You can also have fuel foaming. But I think you have a vacuum leak. Try short shots of wd-40 around the intake, carb mount, cylinder base, also check the head bolts. If you hit a leak the motor will slow for a moment. The WD temp. plugs the leak. If the motor races then it's almost for sure vacuum. Esp check the cylinder base. The motor gets hot the head bolts expand and a leak that wasn't there before suddenly is. Good Luck