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SU CARB (HELP!!!!)

Last post 01-24-2008, 8:08 AM by fleshy1. 19 replies.
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  •  01-09-2008, 3:20 PM 12514

    SU CARB (HELP!!!!)

    I replaced my original with a brand new SU from Victoria British (Part No 3-740). It has been running great. This fall it started leaking fuel from the bottom of the anti fun on valve. Figured I should replace that, the hoses, etc. Well come to find out its something with the carb all along. I just turn my key on and I hear the fuel pump working. It usually stops once it builds up pressure. Well now it doesn't. I just keeps pouring gas strait into the carb. Floods out so bad it goes into the crank case. Why is the gas running through the carb. like this? What do I need to do to fix this?
  •  01-09-2008, 3:35 PM 12515 in reply to 12514

    Re: SU CARB (HELP!!!!)

    The first thing that springs to mind is a fault with the float chamber valve. What year is your car, and what type of SU carb (or carbs) do you have fitted?

    Occasionally this sticking can be remedied by tapping the float chamber (with a screwdriver handle for example). However it may need closer investigation to find and clear the fault.

    Cheers,

    Ed


    I want my MGB
  •  01-09-2008, 8:00 PM 12518 in reply to 12515

    Re: SU CARB (HELP!!!!)

    It's a '76 GB w/ an 1800cc single carb. If this helps the original was a Stromberg 175 CD2 and the new one is a direct replacement according to British Victoria.....
  •  01-10-2008, 9:14 AM 12523 in reply to 12518

    Re: SU CARB (HELP!!!!)

    Thanks for the update.

    Yes, a direct replacement should be a Zenith Stromberg unit - your post with SU in the title was a tad confusing! No worries there ;) These carbs have a shallow air chamber cover, unlike the tall "bottle" on the top of an SU carb.

    I still think that a sticking/dirty float valve or stuck float mechanism is the likely cause of your problem. The Haynes manual has plenty of info regarding disassembly and setup. I don't have any direct experience with the ZS unit, so am unable to offer much more advice.

    Cheers,

    Ed


    I want my MGB
  •  01-10-2008, 10:30 AM 12528 in reply to 12523

    Re: SU CARB (HELP!!!!)

    Guys,

    Victoria British and others sell an SU which is designed as a direct replacement for the Stromberg. It is plumbed to accept all the lines the Stromberg had. I think this is what is involved here. Either way, The problem is fuel getting past the float valve. It could be a stuck valve. It could be dirt in the valve. It could even be a sunken float. Any of these will result in the float chamber overflowing into the vent line and hence to the canister, where it becomes a short downhill trip to the anti run-on valve. The only other thing to check is fuel pressure. I gather that this car has had an electronic SU fuel pump installed. Pressure shouldn't be an issue, as these are manufactured to deliver the low pressures which are used in both Stromberg and SU carburetters. However, I'd be tempted to check it anyway. It shouldn't read more than about three and a half pounds, maximum.


    Motorbill
    From Lola to Land Rover, If it's British and has wheels, it's likely I've bloodied me knuckles thereupon
  •  01-10-2008, 6:38 PM 12536 in reply to 12528

    Re: SU CARB (HELP!!!!)

    Well here is the thing. It's ran great for yrs. with no problems till this fall when all this happened
  •  01-10-2008, 6:38 PM 12537 in reply to 12536

    Re: SU CARB (HELP!!!!)

    So I doubt it would be a fuel pressure problem
  •  01-10-2008, 10:23 PM 12539 in reply to 12536

    Re: SU CARB (HELP!!!!)

    H8rzMotivation:
    Well here is the thing. It's ran great for yrs. with no problems till this fall when all this happened

    Parts wear out.  You need to check the fuel bowl.  If the float developed a hole and is filled with fuel, the valve won't close and seal.  If there is debri blocking the valve and seat, it will not seal.  If the valve is worn, it won't seal.

  •  01-12-2008, 11:36 AM 12557 in reply to 12539

    Re: SU CARB (HELP!!!!)

    Alright. I finally got around to testing the fuel pressure. It is sitting at 3 - 3.5 lbs. That is where I was told it should be. So thats all good. Took the carb into my shop and my boss and I took it apart, cleaned everything up, checked it over really well, tested a few things and everything checked out great. Got home, heated up the garage, put the carb back on and its doing the same thing. So I think Im just going to get the original Zenith-Stromberg back on the motor....
  •  01-14-2008, 8:25 AM 12576 in reply to 12557

    Re: SU CARB (HELP!!!!)

    H8,

    I hate to see you give up on the SU. We have installed a number nof these on cars originally equipped with the Stromberg. Not only have they been acceptable, they have been easy to tune and they perform better than the original. Did you check the float valve specifically?


    Motorbill
    From Lola to Land Rover, If it's British and has wheels, it's likely I've bloodied me knuckles thereupon
  •  01-14-2008, 8:40 AM 12578 in reply to 12557

    Re: SU CARB (HELP!!!!)

    I'm with Bill on this.  Don't trash the carb. You could have sucked up some trash from the fuel tank and caused that needle valve to get stuck open. I'd change out the fuel filter. Then, I'd replace the float needle and seat with one from Joe Curto (www.joecurto.com).  This guy will get you reliable quality SU parts.

    You can also try turning on the fuel pump (after changing the filter) and rapping the carburetor bowl with the butt of a screwdriver.  This *might* cause the debris to come loose from the needle and seat.

  •  01-14-2008, 9:24 AM 12580 in reply to 12578

    Re: SU CARB (HELP!!!!)

    ^^^ percussive maintainance. My favourite kind Smile
    I want my MGB
  •  01-18-2008, 6:08 AM 12667 in reply to 12576

    Re: SU CARB (HELP!!!!)

    It's not that I'm giving up I'm just in a time crunch with getting my B running so she can go to the shop
  •  01-18-2008, 6:09 AM 12668 in reply to 12578

    Re: SU CARB (HELP!!!!)

    I have already tried all of that. I already went through the carb with my boss. See, we are both mechanics but are lost when it comes to these British cars....
  •  01-18-2008, 6:11 AM 12669 in reply to 12578

    Re: SU CARB (HELP!!!!)

    I tried the site you gave me and I'm not finding it.
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