1981 yamaha 650 what position should the petcok b in? pri res oo on
Fuel petcock position depends entirely on what you're doing with your bike at the time.
In normal use, the fuel petcock should be set to the "on" position and left there. In that position, gasoline will be drawn from the tank only when the engine is running and generating vacuum. This prevents overfilling the carbs when the bike sits.
When you're riding the bike and it starts to sputter, it's time to move the petcock to the "reserve" position so that you can use the last few drops in the tank (on a 650 special, this was either .3 or .5 gallons--can't remember which off the top of my head) to nurse your bike to a gas station. Once you've filled the tank, move the petcock back to "on." The bike will still run fine if you keep the petcock in the reserve position, but when it starts to run low on gas--well, you won't have that emergency reserve to get yourself to the gas station.
If you've managed to run the bike out of gas completely, or if you are getting the bike ready for service after having drained the carbs for the winter, you'll need to move the petcock to the "prime" position for a few minutes. This by-passes the vacuum line cutoff mechanism and lets gasoline flow until the carb floats shut off the fuel valve (this is also how all older motorcycle fuel petcocks operated). A sticky float or poorly sealing float valve in the carb can, however, cause far too much gasoline to drain into the carbs (and into the crankcase, if you're particularly unlucky), which is why you should not leave the petcock in the prime position for an extended period of time.
Some Yamaha XS650 motorcycles (can't tell if that's the model you have) have a tank with two fuel petcocks, one on each side, to fuel each carb. If you have a tank with this setup, be aware that setting one fuel petcock to "prime" will affect both cylinders, as there's a crossover tube to equalize fuel levels between the carbs. Also, for reasons I could never figure out, on one XS650 I owned, moving either petcock to "prime" permitted fuel to flow on a gravity feed out of both petcocks simultaneously. I could never figure out exactly why it was doing that.
Jun 06, 2011 |
Yamaha XS 650 G Motorcycles