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Monbloc leaking issues

Discussion in 'Other Norton Motorcycles' started by Paul W., May 27, 2018.

  1. Paul W.

    Paul W.

    Joined:
    May 12, 2018
    So I got her running last night. Problem is the 389/210 leaks. It leaks bad. I resleeved it and rebuilt it with brand new parts from Amal. I cannot see exactly where it’s leaking, but it’s above the banjo fitting on the bottom of the carb on the main jet. I removed the carb and hooked it back up, and I still cannot get a good bead on where the leak is, but it’s below the pilot jet. Is there a fuel dye I can readily get to help me locate the leak?

    Added: It was leaking before I had it sleeved and rebuilt. I was assuming it was gaskets/washers and that the new parts would remedy the problem.
     
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  2. Bodger

    Bodger VIP MEMBER

    Joined:
    May 9, 2017
    You can try dusting the carb with talcum powder. Just blow it off a sheet of paper and then turn the tap on and watch in a well lit area. Worth double checking that you haven't omitted a fiber washer and that that the metal mating surfaces were totally clean of old gasket gunk that a PO might have used. Also, since the area you are describing is the lowest point on the carb you should double check that float needle is seating, float chamber cover isn't leaking and gas lines (even the fuel tap and tank connection) aren't leaking. A leak from any of those sources will end up at the lowest point on the carb which is the area you are describing.
     
  3. Paul W.

    Paul W.

    Joined:
    May 12, 2018
    Thanks for the reply. The area is free of all gasket material. I soda blasted them and they look like new. The left side was a horrible casting job. It may be that it’s porous and has a very small hole. The machinist that sleeved it said it sounded kinda crunchy when he cleaned up the bore, but he thought it was okay.

    I’m on my way to harbor freight and the autoparts store for UV flashlight and additive.
     
  4. Bodger

    Bodger VIP MEMBER

    Joined:
    May 9, 2017
    Please report back. I have spent many hours trying to solve monobloc issues and i am very curious. I hate petrol leaks. Obviously there is a fire hazard. I hate getting the stuff on my hands and I always do because I am too lazy to put on gloves.
     
    Paul W. likes this.
  5. Paul W.

    Paul W.

    Joined:
    May 12, 2018
    Here's a picture of the casting marks from the offending carburettor.
    [​IMG]
     
  6. Bodger

    Bodger VIP MEMBER

    Joined:
    May 9, 2017
    That's pretty ugly but the question is, how pervasive is it? I guess you will find out.
     
  7. Paul W.

    Paul W.

    Joined:
    May 12, 2018
    I took it apart again to inspect for possible leakage points. I purchased some fuel dye and a UV black light. Upon reassembly, I noticed the surface for the washer at the main jet was uneven. I sanded it and it showed positively where the low spot was, as you can see in the first picture. Then I applied some Yamabond sealant and put the banjo fitting and and jet back in. The leak is now gone. So I decided to take it for a test ride. Now the manual tach cable is squealing like a stuck pig. It doesn't seem like it wants to seat properly in the tach. Down the rabbit hole I go! A 52 year old bike that's been sitting for 6 years is slowly coming back to life.

    [​IMG]

    [​IMG]
     

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