Vintage Dirt Bike Q & A

Hi, I have a 1976 MR175 that is equipped with a Mikuni VM roundslide carburetor (the kind with the 2 seperate floats). The fuel keeps running out of the overflow tube and it is flooding out the motor. Ive rebuilt the carburetor several times and i have adjusted the float level several times but with no luck. I have also tried several needle valves and seats in the carburetor and had no luck. I think it is the floats but when I shake the floats I do not hear any fuel sloshing around inside the floats. Does any one know anything about these carburetors or this problem? Also, does anyone know how much it would cost to put new seals in this motor. Any help would be appreciated, thanks.

Is the carb sitting level when bolted on the bike?See if the floats sink when put in a container with gas. Try lowering the float level slightly and see what happens.

the carb sets at a slight angle. I have adjusted the float level all the way down but it only slows it down, it does not stop the fuel flow. Ill try to put the floats in gas tomorrow and see what happens. thanks

Some basic stuff is to take the float bowl off completely and with the gas on manually close the float valve. Obviously if gas leaks by when it is held closed you have a faulty valve. Then check that the floats actually float as described.

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Michael Stiles
1979 Honda CR250R Elsinore | 2006 Husqvarna SM510R | 2007 Service Honda CR500R-AF

i have checked the floats and the needle valve. The carb still spits fuel out of the overflow tube. Can it be the float bowl seal/gasket? Ive never in my life had this much trouble with a carburetor. Ive had more problems with this carburetor than with a British Amal Carburetor. Amal carburetors are notorious for their high maintenance and general problems, but ive never had problems with a Mikuni, except for now.

Any help would be appreciated, Thanks

Unless the carb is at an angle, fuel will always come out of the overflow tube before it comes out of the junction between the float bowl and the carb body. This is because the fuel level reaches the top of the overflow spigot before the bowl fills completely up. If you are sure that the float valve is working correctly, one needs ask where is the extra fuel coming from to keep the leak going once the level in the bowl falls below the height of the spigot. Maybe you have a leak out the bottom of the bowl?

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Michael Stiles
1979 Honda CR250R Elsinore | 2006 Husqvarna SM510R | 2007 Service Honda CR500R-AF

The carburetor is not at an angle like you think. If i hook it up to a auxillary tank on the bench, it still overflows, even when the carburetor is level. This carburetor must be possessed. Ive tried 3 different needle valves/seats with this carburetor and still have this problem. Has anyone had this problem before? Does anyone have the original carburetor to a MR175 for sale.......

Is the vent tube clogged? Maybe its not venting and pressure is pushing the fuel out. Its just a thought.

the vent tube is not clogged, i cleaned the carburetor throughly and blew compressed air through the the vent in the carburetor and all the other holes and passages in the carburetor.

Call Mikuni American at 818-885-1242 and ask for Steve in carburation dept. You will most likely get a voice mail. Leave your name and number.He will call you back. He is a super nice guy who helped me out tremendiously. See what he has to say. Let me know how you make out.