My fuel petcock ON setting is malfunctioning - WHY?!

[TimTalkShow]

I'll try to keep this short, concise, and to the point as I tend to get rather wordy when I post issues on forums.

- Bought a 2001 GS500, with a little under 7,000 miles back in April.  Rode it for 1-2 weeks, both conservatively and somewhat aggressive.  No fuel issues.

- Went to a friends place, stayed late one night, hopped on the bike and let it warm up for maybe a minute or so.  Prob not long enough, but I took off b/c I wanted to get home.  nights in SF are rather chilly and lots of moisture in the air, prob at 50 or so at night.

- On my way home, I was gradually losing power.  When I pulled the clutch, the engine abruptly died.  Efforts to resuscitate it eventually succeeded, but died few minutes later.  Luckily, I got home.

- Rode it a bit the new few days, trying to figure out what the problem was, even after I had warmed up the bike considerably, and it was idling at around 1,200, bike would die in the upper RPMs under load.  Even when I had it set to PRI, riding around for 15-20 min., no issues.  I then immediately switched to ON, and the bike would die under load at upper RPM's.

- I let it alone for a while, and just had my bike on PRI b/c I didn't have any time to deal w/ it or research it.  Bike was running fine, local streets, or highways, no issues whatsoever.  This was prob a period of 2-3 weeks on PRI.

- This past Sun, I replaced my gas tank fuel filter, air filter, as well as took apart my fuel petcock to try to fix the ON problem.  Didn't see any issues, I blew/sucked on the vacuum tube as well as the vacuum head on the petcock, and there was airflow, no problem.  Afterwards, I fired it up, and the ON setting worked fine.  Took it for spirited ride, no issues under load.

- Next day, Monday, went w/ some buddies and went to ride around.  we did local roads, some highway, all sorts of riding, and the ON worked like a charm, just like it should be.  at one point I was hitting 6, 7,000 rpm under load on the highway.  Great.

- Stopped at a friends house afterwards, chilled for an hour or two, time to go home (12 midnight).  I hopped on the bike, didn't QUITE let it warm all the way back up, rode off.  I got on the highway, and immediately noticed loss of power even under full throttle.  Luckily, I reached down switched to PRI, and it was ok enough for me to ride 10 miles back home, no issues.

- This morning before work, I tried it on ON again, and even under conservative riding, it would die immediately afterwards at a light.  HAd to switch to PRI for it to work.

Any suggestions?!?  HELP!  I'm thikning it has somethign to do w/ the vacuum portion of the engine, as my fuel filter is new, fuel lines are clear, and the bike works fine under PRI.  And as far as I knew, it was fine on ON before I jacked it up again.  I can safely deduce that it has something to do w/ allowing it to warm up properly, but now it won't let me run ON at all even after it's warmed up.

Is there a lot of build up in the vacuum portion of the engine after I don't let it warm up properly, and I need to let to dissipate over the course of a few days before trying again? 

Thanks in advance, and plz excuse the lack of technical terminology, lol.

[ATLRIDER]

Seems to be a common problem with the earlier style petcocks.  Not sure which one yours has.  Do a search "CRF250" and some info on this topic will pop up.  I just ordered a petcock from a Honda CRF250X to cure similar issue you're having.  I'm sure others will chime in soon.

Good luck

[TimTalkShow]

I have the stock fuel petcock, as listed on Bikebandit.com

http://www.bikebandit.com/suzuki-motorcycle-gs500k1-2001-fuel-chicken/o/m6062sch238704

Thanks for the tip tho.  So it's an issue w/ the actual petcock, not w/ the vacuum portion of my engine?

[Danny500]

If it was something to do with the vacuum in the engine you would notice it right away. If you pop the vacuum line off the carb and start the bike you should hear it sucking, almost whistling. It should have enough suction to hold your finger to the end of the fuel line that goes into the fuel valve assembly.

It could be condensation.. but that would allude to a vacuum leak... Which you'd be able to tell right away... (bike won't idle correctly if there's a vac leak).

It sounds like you have a vacuum problem in the TANK itself. Your gas cap may not be letting air into the tank fast enough to compensate for the amount of fuel you're using...

When you set it to the PRI position you're allowing BOTH RES and ON settings to open up so that the tank will actually vent through the ON hose instead of the tank vent itself.

Check your gas cap for any problems... once the bike starts to display symptoms of bogging out, pull over, pop the gas cap, close it, and see if the bike will start and drive any better in the ON position.

I had this same problem happen to me on my old Honda CM200 and it pissed me off to find that it was a piece of dirt stuck in the cap vent. 

Dan

[TimTalkShow]

Actually, Danny500, that may very well be the problem.  I just got back from a friends place, and rode PRI the entire way there.  On my way back, decided to get some gas.  As I was hanging strapping back up, I decided to check GSTwins mobile forum website when I saw your post about it may be a vacuum issue in my gas tank.  Since I just filled up and depressurized the tank, I decided to switch it to ON.  I then drive around SF for the next 20-30 min, and it seems to work flawlessly now.

Thanks!

Now begs the question, where exactly does the pressure equalize in the fuel tank?  Where does the air come in?  Would this be the curious L shaped outlet on the bottom of my tank, that goes to the vacuum canister?  If so, where does it let the air in, near the gas cap?  Any diagrams would be bonus as well.

Man, I feel so much better with a solution being so close.  I can taste it, and it tastes DELICIOUS!

[Danny500]

Nope, that tube that comes out of the bottom of the tank is the over-flow. The cap breathes through the key hole. It's quite simple to take the cap apart and clean it.

http://gstwins.com/gsboard/index.php?topic=41061.0

That should help ya.

Dan

[TimTalkShow]

Awesome!  I will definitely do that tomorrow, maybe make a trip to Kragen to get the parts.

When I removed my tank this past weekend, I noticed that there were 3 openings on the bottom of my gas tank.  the first one being the fuel petcock, the 2nd being the overflow, and the third was an L shaped output tube, that was connected to a component of the engine.

I'm absolutely positive of this third tube, b/c I had to disconnect all three in order to remove my gas tank.  any idea what it is for?  the return line for the vacuum system you've mentioned that utilizes the gas cap?

[tripleb]

the three outputs on the bottom of the tank are the on fuel line, the reserve fuel line and the overflow.  The on and reserve fuel lines are immediately next to each other and towards the left side of the tank and the overflow is in the middle.  Both of the on and reserve hoses should go into the fuel petcock unless you have some kind of jimmy-rigged fuel routing system like my PO did.