Carb leak

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jddevel
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Carb leak

Post by jddevel » Wed Jul 04, 2018 8:44 am

TF 1250 I`m having terrible difficulty stopping a petrol leak from the banjopipe/bolt between the banjo and the face of the carb top on the rear carb. All faces cleaned and flattened- over the years previous owners had scored the faces. The fibre washers with the proper SU rebuild kit and others I`ve got failed. Have resorted to a "O" ring which whilst it works am concerned whether this will have a early failure. Any one else met this problem and if so how did they solve it. I`m not a perfectionist and don`t worry to much about originality. Even swapped the banjo and bolt from another car. Still failed. Advice appreciated.

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Steve Simmons
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Re: Carb leak

Post by Steve Simmons » Wed Jul 04, 2018 9:27 am

As a last resort you could replace the lid. Or try a tiny bit of fuel-compatible sealer on the fiber washer.
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jddevel
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Re: Carb leak

Post by jddevel » Wed Jul 04, 2018 11:44 am

Thanks Steve. Will do. Any thoughts on the use of an "O" ring?

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Marv
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Re: Carb leak

Post by Marv » Wed Jul 04, 2018 7:40 pm

If you use it and I don't see why you shouldn't, make sure it is Viton or PTFE based material. Buna-N or Ethelene Propylene based materials will not hold up well to gasoline.

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Re: Carb leak

Post by jddevel » Wed Jul 04, 2018 10:30 pm

Thank you Marv.

Tom Lange, MGT Repair
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Re: Carb leak

Post by Tom Lange, MGT Repair » Thu Jul 05, 2018 3:01 am

Just so I am sure, you are speaking of a leak between the banjo bolt and the banjo fuel outlet? Am I right in remembering that there are two different-sized washers there? One fits over the flange just below the banjo bolt head, and one smaller one fits into the recess on the outlet? The wrong washers have certainly caused leaks here.

In my experience that banjo bolt needs to be firmly tightened, but the tendency is to stop before it is really tight. In all my years I have only broken one float chamber lid, and that one turned out to have been compromised by corrosion. My advice is original washers, more firmly tightened.

I also do not use the later rubber-mounted float bowl attachment, preferring to use the sandwiched fiber/brass gaskets that don't allow the float chamber to move.

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Steve Simmons
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Re: Carb leak

Post by Steve Simmons » Thu Jul 05, 2018 8:44 am

An o-ring shouldn't be necessary although I'm sure it would hold the fuel in. Personally I would worry that the slight movement an o-ring allows might also eventually allow the fitting to loosen. Also, rubber o-rings will eventually dry and crack from age. I've never had a fiber washer fail from age.
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Duncan M
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Re: Carb leak

Post by Duncan M » Thu Jul 05, 2018 9:26 am

Using the correct type (irregular) washer at that point is critical to the proper functioning of the carb. The float bowl vent, aka overflow tube, must allow for fuel in an overfilled bowl somewhere to go. That special (irregular) fiber washer must also allow air to be pushed past the washer to vent the bowl. A snug fitting O-ring there will defeat the purpose of the vent tube. Correct fiber washer seals fuel in but allows fuel and air movement through the overflow tube.

Real question is why are you even seeing fuel so high up in the float bowl? The fuel level should be well below where you are seeing the leak. The fuel in the bowl is not under any pressure, it is like a toilet tank that fills up until the level is right for the valve to shut it off.

Abingdon Spares catalog shows what that washer looks like.
https://abingdonspares.com/products/fiber-washer2

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Re: Carb leak

Post by Steve Simmons » Thu Jul 05, 2018 9:44 am

I missed that this was the overflow fitting, not the main fitting. Good catch. I agree there is a notched fiber washer there which needs to be in place.
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Re: Carb leak

Post by Tom Lange, MGT Repair » Thu Jul 05, 2018 5:44 pm

I see I was wrong in my reading of the post. Thank you for the correction.

There are two washers, one on either side the overflow pipe - an internally-notched fiber washer under the bolt head, and an aluminum washer under the overflow pipe. But why?.

Duncan is absolutely right - there should be no fuel that high in the float bowl. For fuel to be coming out the top, the float must be far out of adjustment: the needle valve is defective or sticking, or the float has sunk due to a leak.

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Re: Carb leak

Post by jddevel » Fri Jul 06, 2018 1:11 am

I haven`t made myself clear as to the location of the leak. It is not from the overflow connection on the very top of the carb but from the banjo fitting where fuel is supplied from the fuel pump. This fitting is on the side of the carb lid. So sequence is bolt head with lip which presses against a fibre washer recessed into the brass banjo fitting, banjo fitting with inlet from fuel pump and outlet to front carb,(fuel from this fitting goes via the banjo bolt into the carb) THERE IS THEN NORMALLY A FIBRE WASHER BETWEEN THE FACE OF THE BRASS BANJO AND THE ALUMINIUM FACE OF THE CARB LID. IT`S THIS WASHER THAT WAS LEAKING. Anyway I`ve since spoken to Burlem/S.U. Carbs and they have given the following possible explanation. It has been known that if the banjo bolt at some time has been incorrectly inserted it may well have slightly "recut the female thread in the carb lid. Result the two faces-banjo and carb lid -no longer are perfectly flush with each other. An "o" ring whilst not ideal will solve the problem. Fitted to both carbs (front also didn`t really remain totally dry) currently no leaks.

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Duncan M
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Re: Carb leak

Post by Duncan M » Fri Jul 06, 2018 8:13 am

Stripped and cross threaded fuel connectors tend to be a disaster waiting to happen. Get a new lid.

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Re: Carb leak

Post by jddevel » Fri Jul 06, 2018 9:45 am

Will ask SU and see if I need a mortgage!!

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Re: Carb leak

Post by Steve Simmons » Fri Jul 06, 2018 10:15 am

So it was the one I originally thought. In that case, I stand by my first post!
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