Bad petcock or problem with carburetor?

[samburger]

My '96 GS500E has a problem with fuel flow. It has what I think is called a vacuum valve or some such, which means that instead of turning the fuel valve off & on every time you start & stop the bike, the fuel gets sucked out of the tank when the engine is starting/running. This would be a nice feature, but it doesn't seem to want to work on my bike. I leave the valve in the "On" position (3 positions are On, Res, & PRI) & it runs for about 5 miles & then craps out. The only way I can get the bike to run consistently is if I run it on PRI (prime). Is it more likely a problem with the carburetor or the petcock itself? A friend of mine tried to explain that it's probably a problem with the float bowl, but he lost me pretty quickly into the explanation. I just need to know how much money to expect to spend getting this problem fixed. If it's a problem with the carbs, I can at least go to a mechanic expecting to pay $100+ instead of thinking it's gonna be a cheap fix.

[NiceGuysFinishLast]

Petcock. They're known to go bad. The PRI setting bypasses the vacuum line and just lets gas flow under gravity power. The downside is that if your floats stick, and you don't change from PRI to ON, theoretically, gas could flow through your carbs and flood your engine. It won't hurt your bike to bypass the ON position and run from PRI, but you should remember to switch it back to ON every time you get off the bike. Some people have taken apart their stock petcock and modified it so that ON was actually an OFF, and PRI was the new ON. Also keep in mind that the PRI setting draws fuel from the lower pickup in the tank, so when you use PRI, you are already pulling from the RES pickup. So basically, don't let yourself run out of gas, cuz when the bike starts sputtering and you reach down to flip it to RES.. it won't do you any good.

[samburger]

My god, that explains every problem I've been having with the bike so far! Ah, thank you so much! I ran out of gas a week ago & had to call AAA to bring me some gas because the reserve wasn't doing anything. And I had been wondering how I had been getting 80+mpg (did my calculations assuming I still had ~1 gallon in the reserve) when my bike is rated around 60. So now I know I just need to order a new petcock. Any idea how hard it is to remove/install? Seems like it would be worth attempting myself, but my bike is my daily driver so I don't want to get in over my head & be stranded.

[NiceGuysFinishLast]

It's literally 30 seconds of actual work, once you get to the petcock. I haven't owned my GS in a couple years, but I think you can see/move the petcock from the side of the bike. It's held to the frame with a screw, IIRC. You remove the screw, remove your hoses (there's 3.. one for on, one for res, and one for the vacuum, again.. IIRC). Just match like to like, and screw the new petcock to the frame. Should take a screwdriver for the bolt, and a pair of pliers for the hose clamps, if you have weak fingers. You can do it yourself very easily. Just do your hoses one at a time, so you can't mix them up.

[samburger]

Alright good deal. I'll head down to the bike shop Monday & get them to order a new petcock for me. Hopefully have this done by Wednesday. If I can get this done & put a new choke cable on before November, I should be in good shape for the cold weather.

[wallypiper]

And I had been wondering how I had been getting 80+mpg (did my calculations assuming I still had ~1 gallon in the reserve)

Calculating fuel mileage.

Fill tank. Fill to a repeatable point, typically, the bottom of the fuel filler neck.
Note mileage on odometer. Call this mileage A.
Ride some.
Fill tank to same point used in step 1.
Note mileage on odometer. Call this mileage B.
Note gallons added to tank from gas pump display.
Subtract mileage A from mileage B.
Divide result by gallons added.
Answer is miles per gallon for the fuel burned between fill up 1 and fill up 2.
Note that the capacity of the fuel tank, reserve or otherwise, is not part of the calculation.

[MX Tuner]

You're overlooking a more basic issue. If the vaccum line is unhooked or split, it'll give the faulty petcock symptoms. First thing to do is check and make sure the vacuum hos e is connected and getting a good vacuum signal. You can even unhook the fuel outlet, suck on the vacuum haose and verify if the petcock is working correctly or not *without* having to guess the petcock is the problem. Petcocks do go bad but they can't work if they don't have the correct vavuum signal.

[NiceGuysFinishLast]

You're overlooking a more basic issue. If the vaccum line is unhooked or split, it'll give the faulty petcock symptoms. First thing to do is check and make sure the vacuum hos e is connected and getting a good vacuum signal. You can even unhook the fuel outlet, suck on the vacuum haose and verify if the petcock is working correctly or not *without* having to guess the petcock is the problem. Petcocks do go bad but they can't work if they don't have the correct vavuum signal.


Mark's right. And just FYI, you won't suck gas into your mouth if you suck on the vacuum line, but gas will flow from the gas outlet (assuming the petcock is working properly)... I just assumed the petcock was bad because the rubber diaphragms are known to crumble with age, and yours is 14+ years old.

[Sanchez]

The petcock might also just be gummed up inside. I had that problem on an older ninja 250. I fixed it simply by dismantling the petcock and cleaning it out.

[samburger]

Well I've already got a new petcock on the way but I'll test the old one just to make sure it's the problem. If not I'll just return the new one & go from there.