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MG MGB Technical - Engine Illness Diagnosis

My 1980 GT developed has developed an illness. After running well for a period of 2 weeks after a startermoter overhaul the engine suddenly lost power and could not acccelerate. Limping home the engine decided that all was fine again. Then 3 weeks ago the symptoms returned. The cars idles alright (if a little lumpyly), but on acceleratiion the the revs increase with very little power, making driving up hills problematic. It feels as if the fuel mixture is way too rich. Black smoke comes out of the exhaust, not excessively though. I have tested the cylider head pressure, cleaned the carbs, checked the timing and points, cleaned the plugs and generaklly gone through all the fault finding procedures to the best of my (limited) abilty. The spark is strong but there are black deposits on all plugs. I know that this indicates poor combustion but I cannot work out or fix the root cause. Does anyone have an idea as to what the problem could be?

Thanks

Barrie
barrie

Similar thing happening on mine. I just sold it to somone actually and we are trying to work out what it could be, most probably the fuel pump we think. Does yours get worse when the car is warmed up?

Christine
Christine

It does seem to get slightly worse but I don't believe it is the fuel pump. I would expect fuel starvation from a defective fuel pump rather than what appears to be too much fuel. Too much fuel delivered would come out through the float chamber overflow if the valve were not working.
barrie

Maybe fuel starvation, either bad filter or tank itself. You can have deposited junk in either and it can run ok for a while and when the junk gets in the way the fuel pressure goes to near zero, that would cause poor running, backfiring, black smoke, and all kinds of other problems. You can test by opening up the line near carb and filling a gallon jug up. It should fill up fairly quickly.
John A

Black exhaust and deposits on the plugs sure sounds
like its too rich. But you state all plugs are
the same color?
It seems unlikely that both carbs would have suddenly
started running rich simultaneously.

Your symptoms also sound like a defective mechanical
advance on the distributor. You might try cleaning
out the dist. and making sure the springs are
in good shape and the advance cam isn't sticking.
Maybe replace points,rotor and condensor as likely
ignition culprits too.
Ronald

Barrie. Does it get cold enough to require the use of a choke in South Africa? You should be entering your cold weather season if so. I had a similar problem when the jets did not rise up fully when the choke was released. When I removed the suction chambers and pistons, the jets were quite a bit below the hot idle position. Several others have mentioned similar problems over the years. Easy to check. Les
Les Bengtson

Les and others. It does get cold enough to use the choke in SA (not in Durban though where the winter nighttime temp seldom drops below 12C),but the mechanical choke seems to be working fine. The fuel pump is not the problem - there is plenty of flow and pressure. Is it possible that dirt is somehow effecting the mixture? I had considerd the mechanical advance and tried advancing and retarding the timing and going on test drives. This had no effect on performance - ie the engine still had no power. The points are in fair condiiton and the condenser is new. The rotor seems fine but I'm not sure what to look for here. Other posibilities mentioned by freinds are that the distibutor shaft could be worn or the HT leads are defective. Anyone have an opinion on these as a possible cause?

Thanks

Barrie
barrie

This thread was discussed between 17/06/2003 and 19/06/2003

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