Only Runs in Prime Despite New Petcock - Any Ideas?

[Carbon]

Hi folks:  I just purchased a 2004 GS500F - my first bike.  Previous owner let bike it sit for three years.  After I purchased it I took it to a local bike shop to have carbs cleaned and rebuilt.  However, shop owner recommended replacing frame-mounted petcock because it has a slight leak.  I ordered a new Suzuki OEM petcock and installed it myself.  If I run it with petcock in ON or RES it eventually stalls.  It runs fine in PRI.  From what I've read on this site the problem is typically a bad diaphragm in the petcock.  However, I wouldn't expect this with a new valve.  I checked all the vacuuum hoses and found nothing missing or leaking.  Any ideas?

[herennow]

Hi, it's a simple circuit to debug. Firsts make double sure all hoses are correctly connected (note there's an error in the manual, I forget exactly, a search on this forum will bring it up).
You can easily make the petcock open with vacuum pressure from your mouth. Yeah, I know, some people like the taste of Gas  less than I do 😉. Remove vacuum hose from petcock, and attach a new good length of hose to the petcock vacuum point.
Attach a hose to any carb drain and run it into a jar. Open drain to get rid of gas in bowl. Now, when you suck on the hose in "on" fuel should come out the carb drain. If it does, I'd say your problem is probably a blocked vacuum nipple on the carb. If not triple check hoses on tank and petcock.
Good luck.

[Carbon]

Thanks hereennow.  I started the bike today with petcock in Prime and removed the vacuum line from the petcock.  I didn't feel and suction from the hose end. I don't know if this a valid test.

[Kilted1]

Thanks hereennow.  I started the bike today with petcock in Prime and removed the vacuum line from the petcock.  I didn't feel and suction from the hose end. I don't know if this a valid test.

Yes, that's a valid test.  You should have also noticed the bike running poorly as soon as you removed the vacuum hose.  Sounds like you have either a blocked hose, blocked port, or it's not connected in the right place.

You can test the petcock as herenow said but using a squeeze bulb or something instead of your mouth.  It doesn't take much unless it's leaking.

[Carbon]

Thanks Kilted1.  Strangely there is no change in engine performance when I remove the vacuum tube to the petcock with the engine running.  I traced the vacuum lines back under the tank to the carbs hoping to find a missing, disconnected or leaking hose but found nothing unusual.  The only thing unusual was a small plastic inline valve between two vacuum line segments.  it had a tiny arrow on the body of the valve pointing toward the rear of the bike.  I assume this is a one way valve.  It looked clean but I have no way to be certain it's functioning properly.  As you stated the lines could have been improperly routed by the shop that did the carb rebuild. 

[Kilted1]

If you could get a pic of that valve and/or the vacuum line routing it might help.  Or just try twisting the valve around to different positions to see what happens.  Usually the arrow will indicate the direction of flow so I'd say make it point to one of the carbs. 

[Carbon]

I hope I've attached two photos.  The first photo (taken from another GSTwin.com member's post) shows and overhead view of the engine with tank removed.  I've added a red circle to highlight the in-line valve.  The second photo is a close up of the valve.  I'd be curious to know the purpose of the valve and if there is any way to test that it is functioning properly.

https://www.flickr.com/photos/140750338@N08/44743458230/in/dateposted-public/

https://www.flickr.com/photos/140750338@N08/44743458240/in/dateposted-public/

[mr72]

I can't help debug a 2004 but the symptoms suggest you have a gross vacuum leak. Look for things like a vacuum port that normally has a cap on it missing the cap, or otherwise a hose that's totally missing. As always I'm a fan of starting over with 6 ft of vacuum hose and redoing the whole thing according to the factory manual. Don't forget that there are o rings inside the carbs that may go bad and create vacuum leaks.

[Kilted1]

I hope I've attached two photos.  The first photo (taken from another GSTwin.com member's post) shows and overhead view of the engine with tank removed.  I've added a red circle to highlight the in-line valve.  The second photo is a close up of the valve.  I'd be curious to know the purpose of the valve and if there is any way to test that it is functioning properly.

https://www.flickr.com/photos/140750338@N08/44743458230/in/dateposted-public/

https://www.flickr.com/photos/140750338@N08/44743458240/in/dateposted-public/

I think that's just a check valve to keep things moving in the right direction.  If you remove it, you should be able to blow through it in the direction of the arrow, but not the other.  Or just take the tube off the petcock and blow into that.  You should be able to blow air through that with just a bit of resistance.  If you can't, then take one connection at a time off and if you can blow through that, put it back and move on to the next until you work your way to the other end.  If there's a blockage in the hose (and that check valve is a very likely place for it), you'll find it. 

72 and I both have older models where the only vacuum line is from the left carb to the petcock.  So check the diagrams to make sure things are connected where they need to be.  I've heard that there are errors in some of the diagrams so if you find anything out of place, verify with a different source before moving.  It could be that your shop used one of the bad ones too.  Maybe someone else will chime in about that.

[ShowBizWolf]

Here is the wiki link to the corrected diagrams:

http://wiki.gstwins.com/index.php?n=Main.HoseRouting