Another fuel starvation thread...

[BockinBboy]

Hey guys,

Putting this up here to get some confirmation or other thoughts before I dive in later this week.

Scenario: I was riding up a moderate incline at about 75mph (been doing this same ride for the past 5 weekends now).  I had just switched to reserve on schedule at about 10-15 miles back and I'm 2 miles from the exit for gas... when the bike starts to sputter and lose power as if I need to switch to reserve - cept I already did that...  Quickly thinking, I set the petcock to prime and it sorted it out in about 5-10 seconds.  This was all while still on the incline - dunno if that part is related or not.  I filled up at the gas station and put it back on the ON position - no trouble the rest of the 210 miles home, haven't switched to reserve yet. 

Thoughts: I'm thinking its the infamous vacuum issue with a problem inside the petcock - probably the diaphragm giving angst...
I'm gonna open it Tuesday and confirm or deny.  If it is the diaphragm, I'm gonna search out the bandit 1200 petcock repair kit.

Extra Info: '04 GS500 with 10.5k miles, valves in spec, clean carbs, rejetted at about 7k miles for aftermarket exhaust, clean air filter (though I am replacing with stock filter once its in because it is original - thoroughly cleaned though), new spark plugs this season = Nice brown color at idle and WOT, fresh oil change at 10k miles

Anyone have anything to add, check, or confirm?  I'm going over the bike in general and re-torqueing everything down anyway.  I had the trip knob come off during some twisties, so it was a nice reminder that I needed to do that again - So, if anyone has a trip meter knob and screw, I'm in the market for one!

- Bboy

[bombsquad83]

Certainly sounds like the petcock vacuum issues to me!  Time to replace the diaphragm.  You could try replacing the vacuum line from the carbs too.  If that tube is going bad, it would cause these problems. 

I really think that this vacuum petcock design is no good with how much ethanol is in today's fuels.  I think the ethanol causes the diaphragm to stiffen and fail over time much quicker than it would have with the good ol' days of pure gasoline.  Think about it...you have a rubber diaphragm that is constantly exposed to the fuel, and has to remain pliable to function.  It's just asking for problems.

[BockinBboy]

Good thought on that vacuum line.  I did replace it with some new line when I rejetted, but I'll check to make sure it is still attached and clamped well.

Its incredibly ironic that I had just commented to a very similar thread about this issue and pointing toward the diaphragm... and the following weekend I have these symptoms.  Hoping that I'm not having tunnel vision as a result, and that it really is as simple as replacing the diaphragm.

- Bboy

[BockinBboy]

Thought I'd update this thread, since I've had some riding time to test it out my fix for what I thought it was...  Turns out, it was the vacuum line to the petcock.  I must have tightened the hose clamp too tight when I replaced it before.  There was a small tear in the line next to the clamp, barely noticeable to the eye.  I had originally cut it an inch or so longer than needed, so I just cut off a half an inch and clamped it back down more carefully.  I have put over 800 miles on it since that fix, and haven't had the problem occur since.



- Bboy