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Carburetter leaking (Mk V)

Started by chrishoddo, 16 May 2013 at 19:06

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I have a problem with the carbs on my Mk V. They are Amal 274/AJ/4A.

The problem is they drip petrol. It is not the float chamber as I have checked and cleaned this and it functions fine.

The leak appears to be coming from the small hole at the base of the carb where the mixing chamber union nut is attached. You can see this hole as a half round at the joint of the nut.

I cannot get the union nut to move and I am a bit reluctant to give it too much force as I dont want to damage the carb body. I have had the carb in a vice with a socket set on the union nut and it still wont shift.

Has anyone any suggestions about this leak or how to move the nut.




Have you tried gently warming it, perhaps by pouring boiling water over it, or with judicious application of a hot air gun?

How are you holding the body?  I use a suitably sized bar inserted through the centre and grip the bar in the vice, whatever you do don't grip the body in the vice!

When you get the bottom union nut off, there is a very thin washer between the brass jet block and the union, this is often in very poor shape, I generally make them from a piece of thickish paper (or very thin card), good quality computer printer paper is satisfactory in my experience.

When you have extracted the brass block from the main body, pay particular attention to the tiny hole at the bottom of the countersink at the rear of the jet block, which in my experience is almost invariably blocked.  Use a piece of copper wire so as not to damage the brass, and inspect the matching hole in the main body also.

After I cleaned my carbs out, one of the float pots was still flooding when standing still.  after checking the float was properly located, I administered a very light tap to the bottom of the needle, that sorted it.


Thanks for the info.

I have telephoned Amal who were very helpful. They say this slight leak is normal for this type of carb that has a breather hole at the base where the union nut attached. This looks like a half round hole at the screw joint. When the engine is running this should not leak and is one reason why the petrol taps should always be turned off when not running.

Have tried this and sure enough when the engine is running it does not drip fuel.

I am going to abandon trying to seperate the union nut for fear of irreversible breakage.

Amal have also sent me FOC a very good catalogue with loads of reference meterial

Thanks everyone



         Be careful if you apply any heat to the bottom of the carb body - the brass choke block is made in 2 parts and is soft soldered together - excess heat may damage the joint. My preferred way of loosening the base nut is to apply a good dose of WD40 and then repeatedly gently squeeze it in the vice, turning it a bit each time. This usually disturbs the corrosion that is locking it, and it can then be undone quite easily.
         Regarding the weep of petrol from the carb - check that the hole in the bottom of the float chamber is free of sediment - the hole sometimes gets caked up to the extent that it forces the needle off centre so that it drags and doesn't seat correctly.