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Extract from December's MGCC publication - Safety Fast

Unusual fuel leak found on 'H' Type Carburetters

When entering the garage, do you get that acrid smell of petrol instead of that nostalgic aroma of older cars so beloved of classic car magazines?

This has happened to me on far too many occasions; so a determined effort was made to find the cause and then cure it.
I knew that it was not the petrol tank, everything above and below it was dry and there was no sign of seepage from the newly installed tank float unit. The pipe from the tank via pump to the carburettors was all petrol tight, but a wipe of the hand along the bottom of the carbs showed that petrol had collected under one of the jet tubes, where the lever linkages are connected.
The carburettors had been refurbished during the car's last rebuild about four years previous. All the gaskets and seals had been renewed at that time, a new set of jet tubes and needles were installed recently.
The carbs had been on and off the engine more times than a teenagers romance in attempts to get the engine to run properly and each time checked for leaks, as the smell had gradually increased over the past few months. As on a standard 'B' series engine, the carbs are just above the exhaust. Excess petrol in this area is not to be encouraged.
One of the weak points of the 'H' type carburettor is the joint between float chamber and the jet body. Originally this used to be a solid coupling (1959) then B.M.C. modified it to a flexible joint using neoprene grommets which is now quite successful. These joints were checked and found to be petrol tight. The cork washers under the jet centring nut were again renewed together with the two olive shaped seals at the top and bottom of the jet tubes.
The carbs re-assembled, jets re-centred and placed on the work bench in the same attitude that they would be mounted on the inlet manifold, the float chambers filled with fluid and left to await results.
Fluid could be seen flowing from the base of one of the jet tubes. This jet tube was then pulled down a small amount, as if the choke cable was in operation on the car. The tube was dry on the outside so the two olive seals were doing their job but fluid still weeped past the choke lever yoke. The other carb of the pair showed no signs of leakage.
The errant jet tube was removed and looked at more closely. There was a slight looseness where the copper coloured yoke meets the brass jet tube base. I had always taken this joint to be riveted nut in actual fact it is a screwed joint in this case came apart easily. The other jet was found to be very solid in this respect.
There is a recess at the start of the threads of the jet tube which will take a fillet of liquid Instant Gasket, a dab of this fluid was applied to the threads of the set screw and a fillet of the same fluid run into the above mentioned recess, care being taken not to allow this sealant to protrude into the jet tube which might foul the needle when it is in the fully depressed position.
All was re-assembled and tested; dry as a baby's bottom when using new improved "Pampers".
The car has now covered over two thousand miles since and now smells nostalgically as a classic car should.
It is possible to carry out a check if a leak in this area is suspected without having to dismantle the carbs or even remove them from the engine. Simply disconnect the choke linkages and withdraw the jet tube down and out of the carb body, of course the remaining contents of the float chamber will deposit itself over your hand, so be warned.

Clayton Spear