37chief said: (quoted from post at 21:28:09 07/11/08) To save some gas I am putting my 68 Yamaha 250cc back on the road. It seams to have this problem. When I give it gas I can see the fuel mixture blow back through the carb. It runs crappy at low speeds, but cleans out at high speeds, and pulls good at the top end. Any thoughts? Stan
In a 2 stroke engine there is a valve between the carburetor intake boot and the crankcase. This valve is either a reed type of rotary type valve and is usually located inside the crankcase. How it operates is that as the piston goes up there is a partial vacuum created in the crankcase. This vacuum pulls the fuel mix into the crankcase. As the piston comes back down the fuel is supposed to be pressurized and then be pushed up through a transfer port into the cylinder. In your case the valve is not sealing correctly so some of the fuel is being pushed back out through the carburetor.
Clear as mud?
This most often happens with a reed valve type of engine. The reed (which is basically a thin piece of plastic like and overhead transparency) gets brittle over time and does not bend smoothly like it is supposed to. However, it can also happen with a rotary type valve it is just not nearly as common.
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