'78 KZ400 Petcocks...

I'm kinda under the impression that my petcock on my '78 400B is either malfunctioning or I dont understand its positions. I've never run across "PRI". Usually its always been "On", "Off", and "Res" for Reserve.

My understanding is that this petcock operates off a vacuum. It has two line, one that seems to be a vacuum to the carb and one that feeds fuel. With the lines unattached, I've checked that in fact when the dial is turned to "On" and "Res", NO fuel comes through. When on "PRI" or Prime, it dumps fuel through. However when I'm running my bike, I"ll prime to start it then turn the dial to "On" once its up and running. and after say a few minutes on a ride, when I come to a stop, the bike just dies. I've also experienced that after those fews mins on the road the upper end of throttle started to almost starve. Then when I stop it dies.

HOWEVER, it seems to love running almost all the way dialed to "PRI". I turn the dial a bit towards the "On" but mostly to Pri and it seems to love it.

Can anyone explain to me the way these petcocks are supposed to work? Can I replace with a regular gravity fed petcock or will that spill fuel into the bowls constantly therefore getting into the airbox? only reason I say that is that I left it on PRI and walked away today only to return with a gas puddle near the back, and I noticed it was leaking out of the airbox. :huh:

Any help of suggestions?

PS my gen plan when I got this bike was to get it running which I have, then I'd actually like to get some Mikuni VM32's for it. Do those need a vacuum petcock?

I had the same setup on my 1979 Z400B , but had to change the petcock after fitting MV32s as they do not have provisions for the vacuum pipe.
You are right about how they SHOULD work, it sounds like yours is faulty.

A vacuum petcock has three lines; fuel in, fuel out, and a small vacuum tubing from the engine intake.

In a vacuum petcock, ON is for normal running and vacuum from engine running would open it. It should close when not running....and connects to a taller filter on top of the petcock that allows only access to about the top 2/3 of the tank

PRI is open always. It is not affected by Vacuum

RES is like ON, operated by vacuum, but opens a lower filter on top of the petcock to allow fuel to flow from the very bottom. In reality there is not "RESERVE", more like slurping from a glass with a short and long straw and if you count miles, or have a fuel gauge, you can leave in ON all the time.

....you have two problems, the petcock is leaking, but also the carburetors are leaking at the foat valves. Either rebuild the Petcock with a so called Kit (diaphragms), or buy a new one, but carbs would need cleaning, and/or float valves changed also.

It is normal in very old bikes to have these rubber leaking problems. We are talking 40 years here.

You can use a manual fuel valve, but you have to remember to switch it to OFF when the engine is not running & the bike is parked. no big deal. Also, the fuel valve doesn't care what carburetors are on the motorcycle. The bolt-hole spacing on your petcock is probably 44mm, but you should measure it to verify.

The stock vacuum petcock is an automatic fuel valve, but it's still gravity fed >> same as a manual fuel valve. The automatic feature consists of a vacuum-actuated diaphragm assy & vacuum line. Engine vacuum actuates the diaphragm to allow fuel to flow by gravity. when the engine is stopped, a spring returns the diaphragm to the closed position. PRI bypasses the diaphragm assy, allowing fuel to flow by gravity >> engine running or engine stopped.

The diaphragm assy has a small o-ring that can harden over time. sometimes the surface where the o-ring contacts the petcock body can be pitted or scratched. I buffed the surface w bronze wool & installed a rebuild kit. Also, the carburetors may need cleaning >> crud can get between a float needle valve & seat.
...

-The engine runs better at large throttle openings when the petcock is on PRI.
-With the engine off, the carburetor only leaks when the petcock is on PRI.
These suggest that the petcock is sealing just fine, but is not opening fully. This is likely caused by a problem with the vacuum system. It is probably due to a leak in the vacuum hose, due to cracks or hardness. It can also be caused by a faulty diaphragm that is torn or has holes. Not likely, but possible, is that somebody stretched the spring or installed one that is too strong for the vacuum to overcome.

A carburetor leaking when the engine is off and the petcock on PRI, suggests a float valve malfunction. Usually, a good cleaning and polish of the valve will take care of it. It is also possible that the fuel level is a bit too high in that carb.
When replacing the rubber bits in a petcock, I always polish the sealing surfaces, including the face of the lever, with a fine aluminum polish. This helps insure a good seal and smooth operation.

wow lots of helpful info here. I did clean the carbs super well, cleaned the jets with a carb cleaning wire, lots of cleaner and compressed air. The needles I was told were "new" and tbh they look it. But how new? No idea.

I did take apart the petcock initially bc when I got the bike it was leaking fuel. Perhaps I didnt see the little spring you all are talking about.

I even got one of those carb bowl tools that connects to the drain screw and do a manual "wet service" check I think its called? Anyway it has a clear tube that you put up against the side of the carb to check the float height. I believe I did them to spec (although there really insnt one) I think it was 1-2mm below the bowl cup line?

Anyways, I will be going the route of the new carbs, tank, and petcock anyway, but I would like to get this figured out first.

Thanks for all your help! I have oil seeping out the front sprocket so I've ordered the oil seal and am waiting to repair that now.

[strike]All 4 should be 3.0mm below the bottom edge of the carburetor body (+/- 1.0 mm).[/strike]

edit: got my topics mixed up. doh. I have no idea what the service fuel level of a KZ400 is.
....

Actually Martin, wouldn't it be all TWO of them? :whistle:

Are you certain that you have an oil leak there, or is some gasoline from the overflowing carburetor loosening up some of the chain spooge caked up under the cover, and then dripping out like an oil leak?

Why do you want to replace things that may be perfectly serviceable with a little cleaning and maintenance?

I bet the fuel level on the '78 is the same as earlier ones. It looks like the 1978 400s had both style carbs, depending on which model it was.

www.kzrider.com/modules/ServiceManuals/7...chablefullmanual.pdf

when it did leak really bad, once, I took the clutch cover off and saw, smelled and felt it was oil. funny part is I kinda messed with the current seal and it seemed to take care of the major leak, and just now seems to seep a bit. tbh, i dont know what it takes to replace the seal which was only a few bucks, so thats another thing coming up. gotta look up how to do that unless you cant point me in the right direction?

that service model doesnt cover my model or year. mine is a '78 400B. Has the front disk brake and flat top style carbs.