Fuel Starved...

I have a 84' VF500F and I've been having an issue with it starting and staying running. I have narrowed it down to a fuel issue and I think it's the petcock. If I disconnect the main fuel line and put gas straight into the line (not attached to the tank) it will start and run. As soon as I hook the line to the petcock, I have a 30% chance the bike will start/run. It also helps if I shake the bike back and forth. If it is running and I shake the bike back and forth it will boge down as well.

My question is; If I pull the tank and set the petcock to ON, should gas start to flow out the main port of the petcock? Or does it require some type of vacuum to suck the gas out?

Currently I can set the petcock to ON or RES and nothing will come out of the tank.

 

Most reserve petcocks have a vacuum line that goes to one of the carb boots at the head. It operates a small diaphragm inside the petcock that helps to regulate the fuel. I would wager that your vacuum line is either not connected to the carb boot (look for a nipple without a cap), or missing all together.

OR, the petcock needs to be re-built.

Petcock rebuild kits generally include this diaphragm (which degrades over time), as well as new o-rings.

 

There is three ports on the petcock. The large main and two smaller ones... There is a line that goes from one of the small ports to the side of the carb and the other small port is left open.

Thank you for your response although I'm not sure if I understand. So without vacuum, fuel will not flow freely out of the petcock when it is set to ON or RES?

 

I'd like to see pictures of this petcock to be sure, but I don't think there should be anything that is "open".

In order to get the fuel to flow freely, I needed to set the petcock to "P", which in Yamaha-Land means "Prime", essentially a wide open, no vacuum required setting that would flood your motor if left on. I also had "RES." and "ON" settings on that particualr petcock.

On my '82 Yamaha XS400R, the petcock screwed directly to the bottom of the tank. It had one outlet line that ran to the bowls of the carbs, and one vacuum line that ran to the left carburetor boot. If I did not hook up the vacuum line, the bike ran like doo-doo until I did.

I would wager that you have a fair amount of crap inside your petcock, in addition to a failed diaphragm. Buy a rebuild kit, and while you wait for it to arrive, take your petcock apart and clean it up.:smile:

 

Thank you for the info, it is much appreciated.

Not sure if this matters, but the bike was originally from California and made it's way to Chicago about 4 months ago. It used to have a canister (smog) on it, but the last owner took it off when he arrived in Chicago.

 

Well - If the charcoal canister that most CA bikes have was somehow connected to the petcock, I'd say it DOES matter. If for some reason it WAS connected to that "open" port on the petcock, I'd say that needs to be capped off somehow.

Still, I'd like to see a picture of the petcock if you could take it and post it. Hell, you can even send it to me at krazykarguy at hotmail dot com.