fuel delivery question

[richardgrhett]

so i finally got my 79 gs 550(vm carbs) running seems to do well problem is when i throttle up at like 3500 it dies down.. my problem i think is my fuel pet cock leaking internally therefore i can not hook up my "pump"line ( i believe thats what it is) could this cause fuel starvation to the carbs when needing fuel fast? or is this power loss something entirely different thanks for the help guys!

[Nessism]

The fuel supply is gravity feed, no pump.

To check your petcock pull off both the vacuum line and the fuel supply lines and install separate sections of tubing, about 12” long or so, with the fuel line dumping into a gas can.

With the petcock set to PRI, gas should flow. With petcock set to ON and RES, gas should not flow until you draw a vacuum on the vacuum line going into the petcock. Also, there should not be any vacuum bleed-down – if there is, the vacuum diaphragm in the petcock has failed.

All this said, most likely the problem is not the petcock but rather your carbs. There is a reserve of gas in the floatbowl which the carb draws on – the fuel supply to the carbs is not in real-time such as with a fuel injected engine.

Hope this helps.

[BassCliff]

Hi Mr. richardgrhett,

Our petcocks have two hoses attached. One is the vacuum line and the other is the fuel line. On the bikes with VM carbs, I think the vacuum line attaches to the vacuum port on the #3 carb. The fuel line attaches to the fuel port between the #2 and #3 carbs.

The petcock is vacuum-actuated which means a vacuum must be present on the vacuum hose in order to open the petcock valve and let fuel flow (in the ON or RES positions). In the PRI (prime) position the fuel is free to flow via gravity.

Your symptom could also be related to air leaks in your intake system. Carb boots, O-rings, and airbox sealing usually take care of this problem.

Forgive me if I state the obvious. I'm just trying to help.

Thank you for your indulgence,

BassCliff

[Dave8338]

The fuel supply is gravity feed, no pump.


All this said, most likely the problem is not the petcock but rather your carbs. There is a reserve of gas in the floatbowl which the carb draws on – the fuel supply to the carbs is not in real-time such as with a fuel injected engine.

Hope this helps.

Is everything tight on the intake side? Air leaks?

[richardgrhett]

its got new o rings on intake boots iv done the little test with starter spray no idle change sooo im doubting but definitely not disregarding an intake leak... the vaccum hose leaks no matter where the position is so thats obviously not right, but you guys are sayin that should have nothing to do with it? hmmmm.......

[richardgrhett]

i should probly add if out of gear and in neutral it revs all the way up no problem, its only when under load that it dies out at 3500to4000rpms

[Nessism]

its got new o rings on intake boots iv done the little test with starter spray no idle change sooo im doubting but definitely not disregarding an intake leak... the vaccum hose leaks no matter where the position is so thats obviously not right, but you guys are sayin that should have nothing to do with it? hmmmm.......

What "vacuum hose" line are you refering to? The vacuum tube from the carbs to the petcock? If so, replace it. Or are you talking about the petcock? Either way, replace it.

[Dave8338]

Stock air box and oiled (lightly) foam air filter in place?

[Rock]

May not be the problem, but looking at the intake boots on my bike when I was first going over it they looked good. But having problems jetting it when I had it apart one day I bent the boots a little bit and was very surprised to see big cracks in all the boots but one I believe.

I would have never seen them if I didn't flex the boots to look, because they looked like they were in excellent condition when mounted on the bike.

[richardgrhett]

i have pods on it...

the second port coming out of the pet cock (not the main fuel delivery port) leaks which means the pet cock is bad i believe... and if im understanding correctly this port helps pull gas via a vaccum from carb 3...

but im trying to figure out if by plugging this port for the time being, could starve my carbs for fuel..

sorry im doin a bad job explaining this

[Nessism]

i have pods on it...

the second port coming out of the pet cock (not the main fuel delivery port) leaks which means the pet cock is bad i believe... and if im understanding correctly this port helps pull gas via a vaccum from carb 3...

but im trying to figure out if by plugging this port for the time being, could starve my carbs for fuel..

sorry im doin a bad job explaining this

Sounds like you need a manual; go to repairmanualclub.com and download the GS550 manual for free.

Regarding the petcock, sounds like time to get a new one. Z1 Enterprises sells them for a reasonable price.

Since your bike has pods I hope you understand that it will not run right unless the carbs are rejetted. Do a search on "pods" for more info.

[salty_monk]

Have you re-jetted for the pods? If not that's your problem most likely....

Dan

[tkent02]

Vacuum on the little hose turns on the fuel in the big hose.
Download a manual.

[BassCliff]

R T F M

Vacuum on the little hose turns on the fuel in the big hose.
Download a manual.

True dat. The vacuum port on the petcock needs to be connected to the vacuum port on the #3 carburetor. The carburetor supplies the vacuum that opens the valve in the petcock that lets the fuel flow. Do not plug up the vacuum port on the petcock. If it leaks out of the vacuum port, you probably need a new petcock.

Mr. richardgrhett, it sounds like you are flying blind. I urge you to get a manual for your bike. It makes things so much easier. In the meantime, have you read this?

http://www.thegsresources.com/files/vm_carb_rebuild.pdf

It may shed some light on how things are supposed to work. Keep us informed.

Thank you for your indulgence,

BassCliff