OK, I have absolutely had it with my bike!
A Stock 1992 GS500E, 16200 miles, stock exhaust and rejetted according to the recommended jetting.
Most of the time it runs great, but during periods of high speed it starts stumbling and if you keep hammering it eventually it will slowly stall to a stop.
My commute to and from work has some long, straight stretches where I tr to run 85-90, sometimes running 95 to 100.
Like this evening coming home, I was trying to move along at 90 and it started acting up, it bucks and acts like it's starving for fuel.
It starts hiccups like it's fine for a split-second, then shuts down.
Usually I have to pull over, as everyone I just passed laughs as they go by and my pride is hurt the most.
After a few minutes it will restart and act like everything is fine.
I have replaced all of the fuel and vacuum lines and a brand-new, $80, petcock, but nothing seems to fix it.
Makes me think that the petcock diaphragm is not staying open and that is causing the bowls to empty and causing the catastrophic stalling.

Any ideas before I lose my flippin' mind?
Fuel starvation?
Electrical issues?
Vacuum issues?

Look the truth is that a Subaru WRX wanted to play and I figured that I could lose him in traffic and then hit the open stretch, go as fast as i could to get a lead on him and by the time he got through and caught up to me (let's face the facts a Subaru WRX Turbo will outrun a GS500) I could get to the one lane bridge first. But instead I lost him in traffic and then when he caught up to me I am doing 60 nursing a P.O.S. bike that is bucking and sputtering. My pride took a savage hit tonight and I can't let this go on!!!!!

Please, please, please give me something, anything.
Thanks,
Jay




 

I've heard that the bike's stock fuel system is a little too restrictive for high speed WOT runs. The carbs empty out, and the bike stalls. Letting it sit will fill the carbs enough to keep going.

I race my GS, and spend a huge amount of time WFO. Never run the stock fuel lines and petcock, so I can't confirm or deny the fuel starvation problems.

I can't help with the stock lines (except I heard to run the bike in the prime position), but your tank vent may be clogged. Don't have an inline fuel filter, do you?

Maybe I am running it too hard for the stock fuel system, would larger diameter fuel lines do the trick or will I have to replace the petcock?
Is it just a fuel flow issue or does the vacuum get weak at higher rpms?
Sometimes I have to run into 4th gear to get the speed up or else it slows down.

What kind of petcock do you run on your race bike?

Yes, I use 2 auto filters, one on the ON line and one on the Reserve line.

Maybe I should turn the petcock to Prime before I start fooling around. I'll try anything.

Can the petcock off of a larger bike, SV1000, Bandit 1200 or such bolt into the GS tank?

Thanks for the replies!

I did think that maybe it was the tank vent so I ran it one day with the gas cap open, yes I was desperate, and it didn't change anything.
It's driving me nuts, it's my only therapy.

I use the stock lines. I've replaced the stock petcock with a simple on-off valve... Which means I have no reserve. Fine for the track, not so good for the street.

Yes, On/Off petcocks work good, just don't forget to turn them on when you leave and off when you get there.

When the carbys were jetted, were they rebuilt? If the needle & seat are starting to gum up then this could be causing a fuel restriction - you would be emptying the fuel bowl quicker than it could be refilled. With the amount of people on this forum running jetted bikes, I would have expected the issue of a restrictive petcock to have come up at least once or twice before! Hope this helps and good luck fixing it!

Hmm. Simple way to check would be to temporarily bypass the vacuum-operated petcock and run straight from tank to carbs.

My vacuum petcock leaked, so I disabled it and turned it into a simple on-off switch. You can do this by pulling it apart and flipping around the internals.

The wiki has a great write-up in the "Upgrades" section on petcocks.
http://wiki.gstwins.com/index.php?n=Upgrades.Petcock

Also, member Flash did the install and it looks like he now has the new petcock with one line running strait to the carbs.  His install thread (however painful to read) is below.   
http://gstwins.com/gsboard/index.php?topic=17129.0

"My vacuum petcock leaked, so I disabled it and turned it into a simple on-off switch. You can do this by pulling it apart and flipping around the internals. "

is there a write up anywhere for doing this?

You know, I knew you would ask me that. No, there's not. I did it late at night, to get the bike going, and didn't think to take pictures. It's basically like running the bike in PRI all the time. In fact, you could probably just try that to see if your problem clears up. Warning: You lose your reserve doing this.

Background: The diaphram was blown and leaking gas into the vacuum port... did I mention this bike didn't run very well when I got it? I looked for a new diaphram and they aren't available. Wasn't going to drop the cash for a new petcock if I could make the existing parts work.

What I remember doing next was plugging off the vacuum on both sides (engine and petcock). I removed the petcock lever (it's held on by a clip spring) and looked inside. Some combination of re-indexing something (i.e. the lever or somthing attached to it) allowed fuel to flow through the petcock to the carb, and be shut off by turning it to another position, and kept gas from going out the vacuum line or leaking out around the blown diaphram.

Since lose your reserve doing this, I plugged off one of those inlets to the petcock as well, and the extra port out of the tank... it runs on reserve all the time now. 

It can shut off the fuel to carbs, and back on, which is really all it should do. I turn fuel off if I won't be riding again for a few weeks.

The whole thing sounds more complicated than it is; I drew out the system when I was working on it and understood it well enough at the time. I bench tested it by hooking up hoses to the parts and blowing through them to see where the fluid would go and ensure I didn't get a leak somewhere... didn't want to have a situation where a rubber cap was the only thing keeping the fuel from leaking out... that would be a time bomb.


I've never had issues with high-speed starvation, that otherwise seem to plague these bikes, and seems to link back to the vacuum-operated petcock.

Thanks to all for the good feedback! I tried the PRI trick today when I came upon my long stretch of road and was able to zip along at 100 for about 2-3 minutes and it did fine, I did have to run it in 4th gear. I am starting to think that it is a restrictive fuel system.
I might consider running it with the petcock modified to an ON/Off configuration or maybe try to convert to a GSXR or Bandit 12 pectcock system.
Maybe there is a lack of vacuum at the high rpms and won't hold the diaphragm open.
I should dig out the old petcock and see what I can do with it. 

Quote from: jwgeorge on February 16, 2011, 08:08:30 PMYes, I use 2 auto filters, one on the ON line and one on the Reserve line.

I used a Fram Automotive Filter on my garden tractor and it could barley get out of the driveway....

Remove the fuel filters.

ALso, the fuel line can look fine while it really has a small kink in it. Specially if you've removed the tank to do anything then put it back.

Michael

Update: The bike seems to do better running on the PRI petcock position. I am starting to think that I just don't get enough fuel when I want it.
If I run it at 90 in 5th gear it starts slowing down, if I go to 4th and back off the throttle a little and slowly crank it up it will run 90-95. I am running an auto fuel filter on it, so I wonder if that is hurting it any, I just don't feel comfortable running without a filter. A piece of trash could cause me to have to do a rebuild on the carbs, so I can take the risk of the filter. I also have a huge power hole between 6500 and 7500 rpm, when I shift and the rpms land in there it bogs.

Maybe I am just expecting too much from a "good beginner's bike", made "for girls".   

The GS500 will do amazing things if you show it a little love.

BTW: I run straight lines from the tank to the carbs, with just an on/off switch for rules reasons.

@burning1, I love my GS! I bought my first bike back in 1991 and it was a 1990 Ninja 600, I thought I had to have a racer boy bike. I regret not buying the GS back then. I want to get a big-bore bike for commuting one day, but I have to keep the GS for the pure rideability!

Gravity fed fuel systems don't seem to mix with paper filters, I have had good luck with bronze filters.  Aside from the restriction, the paper filters sometimes shed little bits that stick the float needles open or clog idle passages.