Carb vent line fuel leak

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  • #5819
    thepiewalker
    Participant

    Gents,

    This is my first older bike so I’m just finding my way at the moment. Since the 1978 Z750 has been parked in the garage this time (around a week) I’ve noticed some fuel leaking from one of the carb vent / breather lines. I’ve fired the bike up, warmed it up, made sure the fuel line is closed (not sure if this is standard but it’s on my brat scrambler) and made sure the choke is off. I think the choke may have been left open slightly. Would this cause it to fill the left carb (the way she leans on the stand) and leak?

    Hope she ain’t broke already!

    Thanks

    #12298
    philpassingham
    Participant

    its gravity fed so a possability

    #12303
    lonesome
    Spectator

    Nah, not the choke, just a float height/sticky valve snafu. Bit of a bugger they are, too high and fuel pisses out, too low and it starves when you demand whats left of its beans. About 3mm difference in height and no really good way of setting it up. Welcome to the Z750!

    #12305
    thepiewalker
    Participant

    I read about this valve on the other fuel leak query further down the page but if the fuel line tap is closed how can this valve overfill the carb when it’s not running?

    #12299
    Kaptainkwak
    Keymaster

    If fuel is switched off and still leaking I would suggest tap refurb kit required (see NRP in suppliers section) that is assuming standard set up is what you have. It could also be a sticky float valve just emptying out the fuel in the pipes from the tap but nothing to major.

    #12306
    thepiewalker
    Participant

    Thanks KK,

    Am I right in thinking that even if the tap was left fully open there would still need to be an issue with the float valve to allow the float chamber to overfill? I’ve got the fuel line disconnected now and think there is a little bit of fuel passing. Will leave it for an hour and check.

    After this is done I’ll get to work on the float valves. To get to these I can just take the float chamber off without removing the whole assembly I take it? Is there anything to look out for?

    Cheers

    #12300
    Kaptainkwak
    Keymaster

    Yes you are right. The float valve is there to cut off the fuel supply when it reaches a certain level. If they are not shutting off the flow it could be one of two things;

    1) The float valve seat has gone. It takes very little to break the seal of the float valve (minimal wear, speck of dirt, strand of fluff etc). These are cheap and easy to replace.

    2) The float height is set wrongly. This can be done accidentally without realising. One possible way to check if it is the float levels is to see if the problem occurs on both the side stand and centre stands.

    It is possible to remove the carb bowls when fitted to the bike but there isn’t much room to work in. If they are fitted with cross head screws be careful you don’t round them off

    #12307
    thepiewalker
    Participant

    Right, took just the chambers off and there was a bit of dirt but not too bad I don’t reckon. While they were off I opened the fuel tap and lifted / dropped the float to open / close the port to hopefully shift any dirt. I put the chambers back on and opened the tap to refil them. I’ll head out tonight and check if I still have a leak.

    While taking them off I realised one of the screw holes had been helicoiled which didn’t work. I hate finding stuff like that! The chamber seems to be sealing on the 3 so hopefully no issue for the minute.

    I take it the next step is to take the carbs right off the bike and strip the part above the chambers to get to the seats?

    I’ve got the tap seals from NRP on the way anyway.

    Some pics for anyone who hasn’t seen the chambers and floats.

    #12301
    Kaptainkwak
    Keymaster

    Sounding quite good so far. The varnish like coating you can see is the residue left behind from stale petrol. All that needs to be removed and it is likely will be apparent around the carbs including float valves etc. that could be your problem. It would not hurt to remove the carbs and give them a thorough cleaning. See how you go for now

    #12308
    thepiewalker
    Participant

    Right on. Thanks again and I’ll keep you posted if I strip them down.

    #12309
    thepiewalker
    Participant

    Looks like removing float chambers and playing with the floats cleared whatever was stopping them from seating as the leak has stopped from the overflows. To be fair I did find that one of the float chamber screws had been over tightened before stripping the threads in the carb and now have a leak from both of the drain screws but I’m still calling it a win!

    Anyone know where I can get new drain screws for less than the £10 including shipping for one from NRP (£15 for 2).

    Cheers

    #12310
    thepiewalker
    Participant

    Looks like removing float chambers and playing with the floats cleared whatever was stopping them from seating as the leak has stopped from the overflows. To be fair I did find that one of the float chamber screws had been over tightened before stripping the threads in the carb and now have a leak from both of the drain screws but I’m still calling it a win!

    Anyone know where I can get new drain screws for less than the £10 including shipping for one from NRP (£15 for 2).

    Cheers

    #12302
    Kaptainkwak
    Keymaster

    I don’t. Get the 2 from NRP. They are spot on and you know will be problem solved.

    #12304
    AERIAL0
    Participant

    Had the same problem with my carbs and all due to varnish build up from stale fuel.
    Took them of complete and dismantled. Soaked in thinners for a couple of hours then used Halfords carb cleaner in the spray can to blow through all airways. And finally with a airline to clear. Surprising how much shite came out.
    Fitted new float bowl gaskets, refitted carbs, fired up and all ok.

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