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MG MGA - Vapour Trap defective?

I have fuel accumulating on the vacuum side of the diaphragm in my distributor.

What could cause this? Do I have a defective vapour trap that is fitted between the vacuum pipe between the carburettor & distributor?

Richard
R A Evans

Pardon my ignorance...
(I'm new to this forum).I know about vapor separators in other models, but wasn't aware of one on an "A"....Is this a stock item, or an aftermarket part?
On other models, fuel can be so rich, as to load up the canister, and overflow onto the floor...(Or cause a strong fuel smell).
But getting into the vacuum line of the distributor, is a unique problem.
Are your spark plugs wet? Did you recently replace the timing chain?
Also, is it possible that the diaphragm in your advance is leaking...Check that by removing the manifold vacuum hose , (and making sure all the fuel is out), suck on it.
There should be no free movement of air, and you should hear the advance mechanism actuate....
The low pressure is created by the engine through the intake manifold, causing the advance to work....It almost sounds like pressure is going out of the manifold, and into the vacuum line....(Could an intake valve by stuck closed?)....Pull the valve cover, and hand rotate the engine....Look for a valve/valves that aren't moving. This is a new one on me.
Edward
Edward Wesson 52TD

The ignition advance vacuum signal for the MGA is taken from thee venturi area of the carburetor with a small port on the bottom just upstream of the closed throttle plate. Being on the bottom of the venturi and downstream from the main fuel jet, it is possible for liquid fuel to get into the vacuum advance signal pipe (especially when running on full choke with a cold engine).

The fuel separator is a small bulb about the size of two two joints of your pinkie finger in the middle of the vacuum signal pipe. It should have a mounting bracket attached to one of the manifold bolts (originally second stud from the rear). The bulb should be oriented with the carburetor signal end down, and the distributor signal end up.

Inside the small bulb is some wire wool. This should cause any fuel vapor or droplets to coalesce as liquid in the bottom end of the bulb, to be returned to the carburetor on next flow cycle of the vacuum signal. If the bulb is missing or incorrect orientation, liquid fuel could get into the vacuum diaphragm on the distributor (bad, bad, bad).
Barney Gaylord

Barney
Thanks for the information....I have looked over various diagrams of the engine compartment in "A's", but didn't see this detail....Is there a key to correct orientation?
Edward
Edward Wesson 52TD

See here: http://mgaguru.com/mgtech/ignition/ig121.htm
Barney Gaylord

Edward, this was a new one on me also.
I discovered the accumulation of fuel within the diaphragm chamber when removing the distributor to carry out some checks due to engine misfiring after a recent breakdown. I have not carried out any recent mechanical work & I do not suspect intake valve stuck closed. The fuel discovered (about 1 tablespoon) did not smell fresh & I believe that this accumulation may have been built up over a period of time.

Barney, as usual your information has been very helpful, thank you.
On inspection, my bulb had been installed with the distributor signal end down. I guess all the fuel vapour droplets had been working down to the diaphragm chamber by gravity over a period of time. Not sure what affect this would have made to successful operation of ignition advance.

Whilst I await delivery of a replacement HT coil (initial fault) I have cleaned & flushed the unit. When re-installed in the distributor a successful bench check was carried using a 50ml syringe operating the mechanical advance correctly. I suspect in time I will need to replace the vacuum advance unit as I believe that the internal diaphragm will have been damaged.

Had I not carried out this inspection the fuel would have eventually ended up in the electrics with a bad, bad result.

Recommend that all owners check the orientation of their fuel separator bulb.

Regards to all

Richard
R A Evans

This has been extremely educational....My "A" has not arrived yet, (expected within two weeks), but I have owned two "B's" and a "TD"...Neither of these had such a connection to the advance....Makes one wonder why they decided to do the mechanism this way on the "A", but not the others...
The TD had a centrifugal advance, but the "B's' had a straight shot from the manifold. Almost like they started to invent a PCV valve, but didn't quite get it.
Edward
Edward Wesson 52TD

This thread was discussed between 07/05/2013 and 09/05/2013

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