Will only stay running when petcock is on primary?

[mds2004]

For some reason I cannot start the bike unless I set it to primary. When I switch it back to on it will die after a little bit. This is all with the choke pulled out too.

Any ideas? Im not too efficient with carb work.

[natemoen]

Something is switched around at your petcocks. Either the diaphragm is not sitting right or the vacuum hoses for the petcocks could be kinked. Where is the petcocks vac line connected? Should be on the intake boots and not on the #2 carb.

[Ken Talbot]

The "on" and "reserve" positions on standard petcocks are vacuum controlled. No vacuum, and they shut off, no fuel flows. Prime is the only position that will flow without vacuum, such as when the motor is not running. Like Natemoen says, check to see thet there is a vacuum line running to each petcock from one of the vacuum ports on the intake manifolds, usually #2 and #3.

[skids]

Tranparent fuel lines help to see what is going on. I think I remember seeing a "flapper valve" in the petcocks (like in fish aquarium air pumps) that make the valves retain vacuum with the pulsing pressures. You can connect a length of fuel line to the petcock outlet and suck on the tube that supplies vacuum to the petcock. The valve of course should then open allowing fuel to flow (have someone catch it in a container). It should immediately shut fuel off when the vacuum is stopped. All of this sounds pretty nasty doesn't it? Use a paper towel to wipe off the grease from your lips.

[cywelchjr]

Tranparent fuel lines help to see what is going on. I think I remember seeing a "flapper valve" in the petcocks (like in fish aquarium air pumps) that make the valves retain vacuum with the pulsing pressures. You can connect a length of fuel line to the petcock outlet and suck on the tube that supplies vacuum to the petcock. The valve of course should then open allowing fuel to flow (have someone catch it in a container). It should immediately shut fuel off when the vacuum is stopped. All of this sounds pretty nasty doesn't it? Use a paper towel to wipe off the grease from your lips.

No flapper valves in mine. But that shouldn't matter as the valve opens each time the cylinder hits the intake stroke which while not open all the time at idle, it should be open long enough to let enough fuel flow to keep it running, and as the RPM's rise, it stays open more and more.

[cywelchjr]

For some reason I cannot start the bike unless I set it to primary. When I switch it back to on it will die after a little bit. This is all with the choke pulled out too.

Any ideas? Im not too efficient with carb work.

Did you make sure that the hose on the front of the petcock goes to the nipple on the carb boot? If those are not hooked up, then the vacuum petcocks don't open and let fuel flow. If those are hooked up, they might just be bad and need a rebuild. It's not too hard to do, but most of the kits need the spring stretched just a little to have them seal when the engine is off.