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petcock question

Discussion in 'The Garage' started by codyxs400, Oct 31, 2012.

  1. ok i was at the yamaha shop and the guy told me there should be a hose conecting to the back of my petcock and to some where else (he didnt know where) and i personaly dont care for this yamaha shop and dont know what hes talking about i got my fuel line but he said there was another line as well
  2. jmd_forest

    jmd_forest Bldg 1980 XS400 Cafe Bike

    The OEM petcock is vacuum actuated. With a properly operating petcock, unless the engine is producing vacuum the petcock will not flow gasoline while in the ON or RES positions. When in the PRI position gas will flow even when the engine is not runing. The tube the shop is talking about is a vacuum tube running from the left carb intake boot to the petcock. It is not unusual for the vacuum actuated portion of the petcock to fail and the owner to clip and plug the vacuum line going to the intake boot to eliminate the associated vacuum leak.
    JPaganel likes this.
  3. 226dark

    226dark 78 2E

    SW MN
    Tons of info if you search




    Each carb boot has a 'barb' and left is usually plugged and not used. Doesn't matter which is used for the source of vacuum to the petcock but there's a tab holder for tube on the underside of fuel tank, and running vacuum line to right carb boot less kink (not so kinky :thumbsup: )

    Do a search and read away.
  4. thanks 226dark i was looking threw the stuff you posted and i seen on the last one i think a picture of the hose that answered pretty much all my questions except where does it attach on to the back of the petcock
  5. 226dark

    226dark 78 2E

    SW MN
  6. jonjkim12

    jonjkim12 '77 XS360 Owner


    i can't figure out how to delete a post
    Last edited: Dec 11, 2014
  7. Unkle Crusty

    Unkle Crusty XS400 Enthusiast

    Sweet, I just looked at mine. Has the ON at the bottom, RES at the top, and PRI at 3 o'clock. At 9 o'clock it says Fuel. Probably no OFF. But now the dumb question.
    Does the skinny end of the lever point to the desired position, or the fat end of the lever?
    If we get it wrong, then we run on reserve and out of gas.
    I get fuel to free flow with the lever aimed at 3 and 9 o'clock, which should be prime.
    So now I need to know if the fat part points up or down for ON.
    I guess in that position, I will find out if the vacuum system is working.

    These modern systems are exciting.

    Unkle Crusty*
  8. jonjkim12

    jonjkim12 '77 XS360 Owner

    for me the side that "points" is longer and has a little tab at the end so its easy to turn. is you're equal on both sides from the center?

    if "PRI" is indicated any where on the petcock and you know which direction allows the free flow then you should be able to figure out which one is pointing, maybe dab it with some paint.
  9. Unkle Crusty

    Unkle Crusty XS400 Enthusiast

    Just had another look. There is an arrow on the lever. It is the skinny end that points. Both ends are equal distance from the pivot point. It is short and can not be any longer, as it would hit the tank.
    So now I will try running the bike, with the lever in what I thought was the off position.
    I simply checked to see if fuel flowed, while the engine was not running. Who knew?

    I like the paint idea. But I will also make some notes on paper for a quick review. And throw away what I had previously written.

    Your logic for the prime is good, but it worked with the lever in both directions, as I said in my previous post.

    Unkle Crusty*
  10. kebabs

    kebabs XS400 Enthusiast

    hi I know this is an old thread. I've just bought a xs 400 1981.i found the vaccum pipe has been disconnected. a fuel pipe runs from one carb to the other.bike tiks over perfect, runs ok but struggles to get over 60mph.ive tried capping the left carb and fitting the vacuum pipe to the right carb .this upsets the idle speed and revs quite rough.do I need to change the gaskets in the petcock also bike is running very rich.any help appreciated.
  11. JPaganel

    JPaganel XS400 Addict Top Contributor

    Most likely you have a leak in the petcock and it's sucking gas through the vacuum pipe when you connect it.

    You don't say what setting you are on. With pipe disconnected, you should get nothing at all when it's ON, and get normal flow on PRI and RES.

    I would say your petcock is due for a clean and a rebuild with new seals/gaskets.
  12. kebabs

    kebabs XS400 Enthusiast

    thanks for your reply. it's on reserve settings as not much fuel in the tank.i read when it's on the on position or reserve fuel should not flow unless the engine is running if the petcock is ok.fuel is coming out both settings with the bike not running. I've ordered a new gasket set for the petcock .is it the left or right carb that gets blanked offthanks
  13. gpounce32768

    gpounce32768 XS400 Enthusiast

    left carb has the vacuum hose to the petcock, right carb is blanked (if by blanked you mean the vacuum port is blocked with some kind of cap).
  14. kebabs

    kebabs XS400 Enthusiast

    thanks .is the left carb when sitting on the bike ill change the gaskets in petcock then try this out .if this fails what would my nxt moves be .thanks
  15. gpounce32768

    gpounce32768 XS400 Enthusiast

    if you want to exclude the petcock vacuum as a problem, then cap off both the left and right vacuum ports. Something as simple as a short length of hose with a screw in the end is sufficient. Then to get the gas flowing turn the petcock to prime- the other 2 positions will not (should not) flow gas in this case. With the 2 carb vacuum ports blocked off then you can start to get at the running rich situation. At that point its probably time to open up the carbs, look at the float valves, diaghragm springs, enrichener mechanism, fuel passages.
  16. kebabs

    kebabs XS400 Enthusiast

    hi there.thanks for your reply. I'll try not to rant on.just bought this bike as a project. wish I hadntI didn't know anything about petcocks.when I bought it the owner told me to run it using super unleaded and keep the switch to prime.drove the bike home .never missed a beat but struggled to get over 60mph.also noticed if working around the bike when idling the fumes would kill you.ends of exhaust are suity black.i noticed a pipe coming from the petcock. previous owner said it was a breather pipe.hes done what you have said .he bridged each port with a piece of fuel line to cut off the vacum .bike idles great with a slight pop from the left exhaust but minimal.ive taken the tank off cleaned it with tank cleaner.dismantled the petcock and cleaned.but now I only get a flow of fuel from reserve ,a dribble from on and a dribble from prime.could I run the bike on reserve position with the 2 carbs cappedshould I buy carb repair kits and do both carbs.to tackle the rich mixture.
  17. gpounce32768

    gpounce32768 XS400 Enthusiast

    bridging the ports is bad- thats basically a vacuum leak, the ports should be individually capped. If its that choking rich it points to carb issues- likely dirty. Bogging down is probably related to running too rich also. Mine runs clean and makes it into the 70's before running out of horsepower- its trying but just can't go a lot faster. OTOH mine has (I think) a somewhat faulty ecu, which I'm setting up to do some more diagnostics on... it runs great doing for-real riding until some threshold is reached after 15mins or so and the ignition cuts out. Sit by the roadside for another 10-15mins and it will start right up like nothing happened.

    The petcock is quite easy to replace seals in. I was dreading mine a bit having had PTSD over it on my old Bandit but the xs's was straightforward. With no vacuum on the inlet then there should be no flow on main or reserve, only from prime. Might be the petcock gasket. You can certainly run by manually putting the petcock on reserve, then off when you shut down. But with the petcock properly serviced then the vacuum will work nicely- just hook a short length of tubing from the petcock's vacuum port to the nipple on the left-hand intake manifold.

    Before you start buying stuff better get the carbs apart. If they've not been overhauled then you'll likely need bowl gaskets and float valves, possibly new floats also. The good news is all that stuff is easily found on ebay et al, and easily replaced. The carbs are not hard to service or get on/off the engine.
  18. kebabs

    kebabs XS400 Enthusiast

    thanks for your advicethis may sound stupid but maybe I'm looking at the switch the wrong way . it dosnt deliver fuel at on or reserve but flows at prime.does that tell me the petcock is working?also what carb does the vacum pipe go to.is it the carb furthest away from the petcock the vacuum pipe connects to?I could connect the vacuum pipe bk up and put the switch to on or reserve and try this out first.
  19. kebabs

    kebabs XS400 Enthusiast

    are u in the uk.can I give you a call ?
  20. gpounce32768

    gpounce32768 XS400 Enthusiast

    In the US. With no vacuum on the inlet port, then the petcock will not flow in in the on or reserve positions. It will flow in prime position regardless of vacuum. Carbs are identified as left or right when sitting on the seat. Both carb inlet boots have vacuum nipples. The nipple on the right-hand carb's boot should be capped. The left-hand carb's nipple should have a short length of hose connected to it leading to the petcock's vacuum inlet.

    I found it helpful to replace the fuel hose leading from the petcock down to the carb inlet with the transparent yellow tygon tubing, then you can see when fuel starts flowing out of the tank.

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