Petcock Questions

I have some questions about the vaccum petcock valve on my 1982 XJ750 Maxim. It will not stay running for more than 10 seconds. The only way to start it is by putting gas in the spark plug holes. I think the petcock valve is not getting fuel to the engine. As I understand it, the bronze nipple on the petcock connects to the vaccum hose with suction. The larger silver nipple connects to the larger hose. This is where I am not sure what is wrong. Only the smaller hose has suction and not the larger one. On the bike itself, when I try to blow or suck "ha ha" on the larger hose, neither works. The smaller hose has suction. Is that normal or is something wrong in the carbs or air system? On the petcock valve, when I try to blow or suck on the bronze nipple, neither works. And on the larger silver nipple, it only works when set to prime. I have posted a picture of what the petcock valve looks like. It is almost exactly like the one in the picture. I really need some help, Thank You

 

Pull the vac line off the carb boot, pull the fuel line off the pet cock & put a length of clear tube on there & run it into a jar, turn tap to PRI & fuel should flow, now turn to ON and suck on the vac line, fuel should flow.

 

ok, so when I suck on the vac line, the fuel did not flow. Do I need to rebuild the valve?

 

Yes, you'll have to repair or replace. But, if it flows freely on prime, you should be able to run on that until you replace/rebuild. Just make sure you know how much gas you have as you will not have a reserve when you run on prime. When your are not running, be sure to turn it to 'on' and don't leave it on prime when you park or you might flood your carbs and crankcase if your floats/needles aren't working 100%.

 

Buying a New PETC0CK is the easiest fix.

You can get screwed buying an aftermarket Rebuild Kit.
Some leak worse than the Old Petcock that went bad.

There is a magnificent tutorial that shows you what you need to do in order to rehab a Petcock.
Detailed.
You can devote all that time and energy and still wind up with leaking unit.

The PETC0CK is the Timex Watch of Motorcycling.
The early Timex Watch was an accurate timepiece that was hard to kill.
But, when you looked at your wrist and saw that the thing wasn't keeping good time anymore, ... you didn't waste time and money bringing a 14-Dollar watch to the Jeweler.

You just got yourself a NEW Watch!

For a reliable Part that worked flawlessly for over 20-years before acting up, ... Your Bike deserves a NEW PETC0CK.

 

Not all petcocks ARE rebuildable; it depends on the condition of the valve seat. If the seat isn't pitted beyond all hope, then a rebuild is certainly a viable option.

Petcock rebuild: http://xjbikes.com/Forums/viewtopic/t=25058.html

 

So is fuel suppose to be coming out of the vacuum hose at all because the vacuum nipple is wet. Gas is not flowing out of it though. Now when I suck on vacuum hose alittle gas flows from fuel nipple but not near as much when set to prime. Is this normal? Also when sucking on fuel hose when connected to carbs it seems blocked.

 

You will get some fuel out there if the diaphram is toast.

 

I'm not sure you can suck hard enough to fully open the valve.

I haven't done this with an XJ, but other bikes where I've used a vacuum pump they need about 7" mercury to open. 7" mercury is about 4PSI, that's a lot to hold by mouth.

 

I've found that the most you can usually blow is about 1 PSI (I've tried). Seem like you probably can suck much less than that. But I was able to get mine to open by sucking on it, meaning it's very sensitive when in good shape.

LordSnow, you shouldn't have any moisture at all on the vacuum hose. And you shouldn't smell/taste gas at all. Otherwise, you probably have a bad diaphragm.

 

They can be test-operated by mouth; just be careful not to slobber in it (easier than you think.)

The above replies are correct; if you have fuel in the vacuum line there is a problem with the diaphragm.

 

Petcock fixed but still having carb issues. Thanks for everyones help.