83 650 Fuel delivery - next logical step

[BigD_83]

It's basically an open/shut valve. The petcock just serves to fill the float bowls, and the required fuel for combustion is then metered by the jets. Yes, you can run it on PRIME, but do NOT forget to turn it off, especially if you have concerns that your float needles are not sealing properly.

One other note, IIRC, the PRIME orifice on the barrel is a little smaller than the ON...this theoretically could be a problem at extended high speed/high rpm operation, but in practice you're probably not going to run into that.

[Steve]

Once the engine is running, vacuum will open the petcock, assuming it is working properly. You should not have to turn it to PRIme to get the engine to run. The rate of fuel flow is regulated by the floats in the carbs, not the petcock.

If you have vacuum from the port on #2 carb and the petcock flows if you apply vacuum, there is no reason why it should not provide enough fuel for the engine to run.

Where are you? One of us might be close enough to drop by and lend you a hand.

.

[Doug650L]

So .... Funny thing. It seems that if you intentionally kept your fuel tank low for this sort of trial and error testing and taking the tank off often, there may in fact not be enough fuel in the tank for the "prime" setting to function properly. Such is the case with my problem. I turned the petcock to "reserve" after reading other posts and it remains running. I had examined and disassembled my old petcock valve carefully and could not see how the reserve function worked, but someone had pointed out that on reserve, the fuel is drawn from the bottom of the tank. it simply never ocurred to me that there wasnt enough gas for the prime position. reading the manual, i believe there is maybe a gallon in reserve. So chalk that one up to inexperience. Thank you to those that helped educate me on the vacuum system. I'm moving on to other aspects of the performance with a new thread.

[lemonshindig]

Good catch!