1985 Atc250r rpm's will increase when on reserve

[96motorhead]

I have a stock 1985 atc250r that will start to rev up when the clutch is not engaged like an air leak but only when it is on the reserve fuel selector.  It will not rev up with the fuel selector in the on position.  Does anyone know what would cause this?  I've checked the petcock flow on reserve and it flows freely.

[2ndmoto]

That is odd, but you could see if it is getting a similar flow rate. Use a stop watch and a ratio rite or similar measuring device.

[Hawaiiysr]

so it runs fine when the petcock is in the "on" position?

im thinking you are sucking in air while you are in reserve causing a lean condition.

[The norm]

Check out the fuel lines between the fuel petcock and the tank