Stinking vacuum fuel tap or bastaard blocked fuel cap?

[terry]

Ok, so my 1981 8 valve (in pic) GS1000S is running like a swiss watch after the recent "el cheapo" top end rebuild, with one major problem, once the tank is a little less than half full, it cuts out.

I doesn't restart unless I: a. take the vacuum hose off at the carbs and suck on it so as to open the diaphram on the petcock, to move some fuel into the 34mm CV carbs, and b. crank/push start the beetch with the choke on until it fires up again.

This has happened to me a couple of times this week, both times (once I got it running again) I rode to the gas station, filled it right up again, and it was fine. I'm currently adapting a manual tap but due to space constraints, I might put it on one of my other bikes that will be running pods.

By the way, my fuel petcock (tap) is the little sucker with what appears to be a screwdriver slot rather than a handle, and regardless of what is cast onto the exterior, there is definately no "Prime" position, as per the earlier models. As usual, any help will be appreciated. Cheers, Terry. :twisted:

[Anonymous]

I had a similar problem with our GS550 last year.
I had replaced the fuel line, leaving the new one a bit longer than the original one. To avoid any kinks in the line, I pushed the extra length underneath the fuel tank, under the "backbone" of the frame, where the fuel tank rests on.
Enough space in there, I figured.
WRONG! This way I had created a loop in the fuel line, that came above the fuel level in the tank, as soon as the fuel tank was about half empty.
This stopped the fuel flow in the line, causing the bike to stop.
I shortened the fuel line to normal length; no more problems.

It might also be a blocked fuel tank cap.
Try taking the fuel cap off, next time it happens, and try to start the bike.
If you hear a hissing noise when you remove the cap, there might have been a vacuum in the tank.

[KEITH KRAUSE]

I agree. It sounds like the gas cap vent needs cleaning and/or the fuel line is not routed perfectly.
I think your petcock does have a prime. The difference in operation with your style and others is the engine must be cranking to initiate fuel flow, even in the prime position. Fuel won't flow just because you turn it to prime.

[argonsagas]

Since you say it has happened twice in only about a week, it appears that it was not happening previously.

Unless you have changed something else on the bike, such as the mentioned routing of the fuel line, then it is almost certainly a plugged cap.

The reason is that, as the fuel level drops, air must be brought in to take its place. If not, a partial vacuum develops above the fuel, and, eventually, the pull of the vacuum balances against gravity, and fuel will not flow.

[terry]

Thanks guys, I'm hoping Jojo is on the money, I removed my petcock and stripped it, but it was as clean as a whistle, so I pulled the fuel cap apart (very interesting, lots of little parts including two ball bearings) and blew everything out with compressed air, but nothing untoward appeared to be nesting in there either, so finally I chopped an inch off my fuel line, which probably was a little long.

I did ride it about 50 miles afterwards but the fuel level is still over half, so I haven't tested Jojo's theory yet. Hopefully though, all is well again. I suppose to answer Argonsagas' question, the bike has been virtually unridden in the last 3 years, in that time I've changed many things including ignition, carbs, pistons, cams, head etc, but with only short rides. I need to drop the needles a notch or so, as it's running quite rich leaving a sooty black layer around my tailpaipe, but I was happy to run it a little rich during the first couple of hundred miles after my "el cheapo" top end rebuild.

The theory of the "prime" position I thought, was to allow fuel through without vacuum pulling the diaphram open? That's certainly how the "prime" position works on my 1979 GS1000S petcock, but I'm happy to stand corrected, after all, if Suzuki went to the trouble of casting the word "prime" onto the petcock, I guess they did intend for it to have a "prime" position? Thanks again guys, Cheers, Terry. :twisted:

[Anonymous]

What would be the reason for a pri position, if you still needed vacuum for the fuel to flow. If you have vacuum, the fuel will flow when in the on position. It's been years, but I've owned 4 "80" model bikes with this style pet cocks, and I sure thought if you took the fuel line off the pet cock & finally found the pri. position, fuel would flow. I do remember the pri position was hard to find.

[KEITH KRAUSE]

What would be the reason for a pri position, if you still needed vacuum for the fuel to flow. If you have vacuum, the fuel will flow when in the on position. It's been years, but I've owned 4 "80" model bikes with this style pet cocks, and I sure thought if you took the fuel line off the pet cock & finally found the pri. position, fuel would flow. I do remember the pri position was hard to find.

I agree that the prime on this style of valve seems to defeat its purpose.
The prime position is when the slot is vertical.
Here is what the Suzuki factory manual says for the '80 GS1000E/ST models:
When the engine is not running with the valve in the ON position, the fuel valve is kept in the closed position by applying pressure utilizing a spring so that no fuel will flow to the carburetors. When the engine is engaged, a negative pressure is generated in the diaphragm chamber through the vacuum (negative pressure) pipe which is connected to the carburetors, and builds up a negative pressure which is higher than the spring pressure so that the diaphragm is forced to open the fuel valve and thus allow the fuel to flow to the carburetors.
On the other hand, setting the valve in the ON position keeps the air return orifice open. Negative pressure does not accumulate on the diaphragm at the time of engine stopping, and then the spring pressure actuates the diaphragm to move back to its original position and closes the valve.
However, setting the valve in PRIME position with a screwdriver causes the air return orifice to close, resulting in negative pressure in the chamber under the diaphragm. This negative pressure doesn't allow the fuel valve to close and therefore the fuel keeps flowing to the carburetors. THE ENGINE MUST BE CRANKED OVER TO INITIATE FUEL FLOW, EVEN IN THE PRIME POSITION.

[terry]

Well there you go guys, I've learned something else! I moved the petcock from the hokey "Prime" position to the "on" position and rode the bike about 50 miles this morning, the fuel gauge is now well in the red (I did mention that it is running very rich) and it is still running well, so I'm guessing that Jojo's theory of too much fuel line is correct.

When I get a bit more time I'll drop the needles a notch or two (from memory they're on the highest position, a leftover from when I had the Yoshimura 1085 kit and mild street cams) and hopefully get that pipe a bit cleaner, and even more hopefully, improve my fuel economy somewhat! Cheers, Terry. :twisted:

[Anonymous]

Would the fuel valves for the 2 valve engines be different than the 4 valve engines? My manual is a Haynes, for the 1979 to 1988 GS/GSX 1000, 1100, & 1150 four valve, four cylinders, and it says. The fuel tap design differs slightly between models, but is essentially the same in operation. In all but the pri (prime) position, fuel flow is controlled by engine vacuum by means of a diaphragm assembly connected to the inlet tract. As soon as the engine is started the diaphragm reacts to engine vacuum, opening a plunger valve against spring pressure. When the engine is stopped the valve closes, shutting off the fuel supply. The pri setting allows fuel to flow even with the engine stopped. I can't think of any reason Suzuki would use different fuel valves on the 2 valve & 4 valve engines, but I can't think of any reason Suzuki would re-design their original fuel valve so the "80" models would not have a reserve.

[Anonymous]

Glad you solved it Terry!!!

[terry]

Glad you solved it Terry!!!

Thanks Harry, but I'll give you the credit for solving it, thanks mate!

I took the top off my 34mm CV carbs to drop the needles, but guess what? They are already on the middle position, about where I intended to put them anyway.

Hmmmnnnn, so what now? Well I removed the paper filter and replaced it with a foam "high Flow" item, and I believe that has reduced the rich condition, but has not cured it completely. I took it for another ride of about 70 miles which I would assume would have burned any excess carbon off my pipe if the mixture was right, but it hasn't.

I understand that I need to do a "plug Chop" to get an accurate reading and a base mark for further experimentation, but I was running out of time (and light) tonight, so it'll have to wait for a few days until I get another chance to play with it again. Anyway, thanks again guys, the old girl is running better now than any time in the last 3 years! Cheers, Terry. :twisted:

[focus frenzy]

Would the fuel valves for the 2 valve engines be different than the 4 valve engines? My manual is a Haynes, for the 1979 to 1988 GS/GSX 1000, 1100, & 1150 four valve, four cylinders, and it says. The fuel tap design differs slightly between models, but is essentially the same in operation. In all but the pri (prime) position, fuel flow is controlled by engine vacuum by means of a diaphragm assembly connected to the inlet tract. As soon as the engine is started the diaphragm reacts to engine vacuum, opening a plunger valve against spring pressure. When the engine is stopped the valve closes, shutting off the fuel supply. The pri setting allows fuel to flow even with the engine stopped. I can't think of any reason Suzuki would use different fuel valves on the 2 valve & 4 valve engines, but I can't think of any reason Suzuki would re-design their original fuel valve so the "80" models would not have a reserve.

Suzuki could not make up their minds and used several differant petcocks even during the same year on the differant size and type models ( E/L)