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MG MGA - Leaky SU

Hi All
In search of a little advice please?
I have noticed a small weep of fuel from one of my standard H4 SU carbs. Its really not a lot, I noticed a slight petrol smell when tinkering around under the bonnet and eventually traced it down to a small weep from just above the adjusting nut.
The carbs were completely refurbished by Bob West six or seven years ago, and the other carb is fine. It seems to remain dry while the engine is running.

I am assuming the answer will be to remove both the carbs, and install a new seal kit but was wondering if there was any magic dust or short cuts. The thing is I have only just finished renovating the entire braking system and would dearly like to start using the car while the summer (?) is still here.

Is it possible to install the seals in situ? Doesnt seem much room to do that, or is there something else I can try in the interim that may solve it?

Presumably its easier to remove both carbs than just one?

Thanks, Graham
Graham V

I have tried a couple of times to replace the seals without removing the carbs. While it might ultimately be possibly, I will tell you it will be a lot easier to remove the carbs.

That said, if the leak is on the front carb, it may likely be easy enough to remove only the front carb. It is not easy to remove only the rear carb, so if the rear is leaking, pull them both off together.

Been there, done that, not such a big job once you set your mind to it.
barneymg

Agree it is far easier to remove and it really isn't that difficult or time consuming. I suppose if the leak is very minor you could live with it and enjoy summer first doing job in winter.

Paul
Paul Dean

If you have the right offset ring spanner it is a simple job to remove the cabies, manifold, heat shield and air filters as a single assembly. Just undo the manifold nuts, throttle and choke cables and fuel line.
Mike
Mike Ellsmore

I agree Mike although you miss the one item that I always find the most fiddly, i.e. the vacuum advance. I normally get the whole assembly part of the way out before doing this.

Paul
Paul Dean

Thanks for the advice. I have bitten the bullet and taken the carbs off. Now I have them on the bench, the job looks much more straight forward than from the complicated diagrams!
Only problem is the seal kit from Moss is supposed to include the two rubber grommets for the base of the float chamber, but just includes flat washers. The old ones look ok so I will use them after very carefully spreading a very thin layer of Holymer Blue just to be safe.
I did get the carbs off without too much problem, but intend to grind down an old spanner to help with the re-installation.
And you're right, the vacuum tube is a pain too!
Thanks, Graham
Graham V

Yes Paul I forgot to mention vacuum advance, mine is very easy, rubber hose just slips over a short tube extending out from the carby base.
Graham, there are two types of fixings for the float bowls either fixed which just uses an aluminium washer each side or the flexible type that uses a rubber grommet each side (a later development), also two different fixing studs have been used over the models. You can see these differences if you go to the Burlen site exploded view of the H4 carbies. To change to flexible mounts isn't cheap - as I recall cost me about 58 pounds!
Mike
Mike Ellsmore

Various MGA SU set ups.
http://sucarb.co.uk/cf/vehicle/list/?manufacturer=MG&vehicle=M.G.A
Mike Ellsmore

Mike
Thanks for the explanation. Mine has the bolt, and two rubber grommets but nothing in the gasket kit from Moss (that appears to have come from SU) resembles those rubber grommets (AUC1534) even though the Moss catalogue says it should be included.
I cannot see how any of the various other washers in the AUE801a gasket pack (and it looks like there will be much left over) can possibly due to the job.

I'd like to get the job finished and hence my decision to reuse the old rubber grommets with some Holymer Blue and hope it does the trick. If not, hopefully I will be able to remove the float bowls in situ once I have screamed loud enough at Moss and got the right parts.

Graham
Graham V

Be carefull with the hylomar Graham, you dont want to get any inside the carbs.
It does work really well although, I have only ever had use it sparingly on the fuel sender unit gasket.
Cheers
Colyn
Colyn Firth

Hi Colyn,
Thanks for the word of warning. By coincidence I have also used it on the fuel sender unit, and remember being warned at the time, possibly by you, to be careful.
Her indoors has actually talked some sense into me, so instead of being impetuous, I have delayed the work. I will go to Moss tomorrow and get new rubber grommets and do the job properly.
I was concerned that you can't just keep tightening the bolt until you have a good tight fit, as you would normally, because the bolt hits a stop in the carburettor body. If that's not tight enough for the rubber grommet to seal properly, you're stuffed!

Regards, Graham
Graham V

Graham, don't forget that the washer that hold the rubber seals on the bottom of a float bowl should be slightly dished (concave when viewed from the top) so that when the bolt is tightened they force the seals up onto the mating surface rather than splay them out which a flat washer will do. You should not need any type of sealant.
Best Alan
AR Terry

Adam
Thanks for mentioning it. I do have the dished washers. I have seen from the archives that is an easily missed factor.

Regards, Graham
Graham V

Graham

I think it was me who warned about sealant on sender units. I had major problems several years ago after having been rather generous with it and it got into the tank and kept blocking the pipe. It wasn't though Hylomar AF, which I presume is what you mean by blue, and I have used AF since although very sparingly and it is the only one I know that is supposed to be OK with petrol. Personally, as Colyn said, I wouldn't use it anywhere near the carb which I think is what you have now decided.

Paul
Paul Dean

I have installed new seals on the jets and new grommets on the float bowl connections.
Today I found time to put the carbs back on the car. It started first time and runs nicely. But I was then disappointed to find a minor leak on the other carb (I changed seals on both) from the bottom of the jet, around the adjustment nut.
It only seems to leak when I apply the choke.
So I need to take them off again - maybe the jet seal was not seated correctly.
But on reading archives it appears there is a possibility there is a leak on the join between the fork and the jet tube (hadn't thought of that before).
I would like to get it right next time round but would I be right in thinking that if it only leaks when the choke is applied, that indicates it is more likely the upper and lower small jet seals?

Thanks
Graham V

Sorry don't know why I even posted that. Pretty obvious answer is for just 10 ,I can buy a new one and will eliminate that!
Graham V

This thread was discussed between 28/07/2017 and 07/08/2017

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