GS500e won't start - feels like fuel starvation.

[Radish Spirit]

Picked up a 1989 GS500E for a thousand bucks - it ran fine for a day and then died suddenly the next day going about 45mph. After some test riding, here's what going on:

The bike runs well on prime, but dies after ~5 minutes or less on the standard petcock position.

It's not an issue with the gas cap not letting air in. Flipping it open does nothing.

I tried removing the petcock and checking the diaphragm. It seems perfect after a suction test. I then re-inserted the petcock and re-secured all hoses. Rode again with the exact same issue - the bike dies with the standard petcock position. Any advice would be great. Thanks!

[Kilted1]

The simple answer is that you're low on fuel.  I presume you've checked that. 

It could be a vacuum issue.  The petcock diaphragm may work fine but it depends on vacuum from the left carb to operate.

The main fuel pickup in the tank could be blocked somehow or the screen plugged.  A kinked hose would do it too.

If you remove the main fuel line, does fuel come pouring out?  Try reversing the hoses.  If it still runs okay on prime but not on normal, it's the petcock.  Let the bike idle and pull the vacuum line off the petcock.  The idle should stumble and you should feel suction at the end of the tube.  Also check the screws on the back of the petcock are tight.

[Watcher]

If it runs on PRI but not ON or RES it is either an issue with the petcock itself or the vacuum system that controls it.

You said you vacuum tested the petcock itself with no issues?  Then I'd replace the vacuum line to the carb and ensure the clamps are good and snug and try again.


If it was a clogged screen or bad fuel filter or bad floats or something it would die on PRI as well, it can only be the petcock or the vacuum to the petcock.