1977 gs750 fuel coming out of crank case vent tube

[hdcrossbones09]

I just bought a 77 gs750 that is running horrible. I have never had a suzuki but am familiar with the older bikes i had a 77 kz1000 and a couple of cb1100fs that I did most of my own work on but i cant figure this carb problem out. When I bought the bike it was running good. But come to find out the guy prob just changed the plugs right before I came over that way it would run decent then my dumb butt bought it only to get the shaft lol. It fouls the plugs after about 1 mile of riding. When I first loaded the bike it would leak fuel out of the carb over flow tubes if it sat for a while. I figured the float was just stuck so I didnt worry too much about it. After I took the carbs off and cleaned them everything seemed good or at least it wasn't leaking fuel any more. The next day after it stopped raining I took it for its first ride it ran great for about half a mile then started breaking up over about 2000 rpms. It is running soo rich that raw fuel drips from the valve cover vent tube. The plugs are very very black. The jets are stock and clean. The carbs seemed great and I almost wonder if its some thing else causing it like a petcock or vacuum leak. I searched the forum and found some ideas but nothing has worked yet. Please help!!!! Thanks in advance!!!

[Nessism]

Petcock dumping fuel down vacuum line and into engine?

[hdcrossbones09]

Could be possible is there any way to check with out buying a new petcock?

[Nessism]

How about inspecting the petcock? Pull off the vacuum line and replace with clear tube so you can see if fuel is in the line. Pull fuel hose off petcock and see if gas come out while the engine is off.

[hdcrossbones09]

Ill give it a shot.... could anything be wrong with the carb to pull too much vacuum or normally just a petcock?

[Suzuki_Don]

When you cleaned the carbs did you replace ALL the "O" rings including the intake boot "O" rings. Did you set the float height correctly?

[hdcrossbones09]

all "o" rings replaced i put the floats back exactly how they were....its not just rich its a ton of fuel never seen anything like it... a ton of fuel... i also opened the bowl drain screws and it seemed to poor gas faster than normal bikes ive seen but im not familiar with the vacuum petcock all the bikes i have had had a fuel shut off on the pet cock

[Nessism]

all "o" rings replaced i put the floats back exactly how they were....i

You should have checked the float height. You still don't know where you are at.

[hdcrossbones09]

If I unhook fuel line from carb gas poores out with the bike not running is the petcock my problem then?

[Nessism]

If I unhook fuel line from carb gas poores out with the bike not running is the petcock my problem then?

Is the petcock set to On, Res, or Prime? When set to Prime gas will flow all the time. You should not normally use this position unless the bike has sat for a while. If fuel flows when set to On and Res, the petcock has an internal failure.

I believe that model petcock is very expensive. You might want to investigate rebuilding it.

[hdcrossbones09]

It flows matter what setting it is on... Also the vacuum line for the petcock just t's into the fuel line is this correct?

[mike_of_bbg]

No!

The vacuum line should head to a vacuum tap on one of your carb bodies - #3 I believe on the VM carbs that a 77 should have. If the diaphragms in the petcock develop leaks they'll often send fuel down the vacuum line, which is obviously a problem. That's just further confirmation that your petcock is shot...

It flows matter what setting it is on... Also the vacuum line for the petcock just t's into the fuel line is this correct?

[hdcrossbones09]

Okay well if that's the case my petcock is shot. Also some thing is up with my carbs causing it to run that rich? I think I'm in over my head on this one

[TheCafeKid]

Okay well if that's the case my petcock is shot. Also some thing is up with my carbs causing it to run that rich? I think I'm in over my head on this one

If it ran well prior, try replacing the petcock, and see what happens. If you're still fouling plugs, then we can help you address the carbs... But the petcock MUST be dealt with or it can RUIN your motor. That much fuel can wash the bearings and cylinder walls, and create metal on metal contact, which if you've worked on anything before, you know is bad.

On the petcock, REPLACE, dont bother with a rebuild, success rate on rebuilds are pretty low... Petcock may be a little pricey, but replace it now, and dont worry about it again for 20 years...

[mike_of_bbg]

Your vacuum petcock should have two lines - one fuel line, which is thicker, running to the fuel rail on your carb rack. The smaller line is the vacuum line which should be running to the vacuum tee on carb #3.

If your petcock is sending fuel down your vacuum line, you're going to be running rich on #3 (look at your plugs and compare #3). If it's overflowing the bowls because it's not shutting off properly, then you're going to be rich on any number of cylinders until the excess fuel gets a chance to burn off. Some of that fuel can also get into the cylinders and eventually into the crank case. Check for signs of fuel infestation in the oil (overfull, thin, smells strongly of gasoline).

Until you can verify the correct operation of your petcock, further troubleshooting is useless. Do you have a hand vacuum pump?