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MG MGA - Back fire

Yesterday as I was driving mt 56 1500 the engine started to misfire. Then backfire, that seems to be coming from the carb area. The engine shakes as if only three cylinders are working, and the backfire is steady and increases with rpms. The car limped two blocks home but lacked power. Any ideas as to what is wrong?
Terry Marcus

You should ckeck the usual things for a misfire/backfire.

(1) With the engine running remove one spark plug lead at a time. If the engine slows down or stops that plug is firing. If no change that plug is not firing. See if the lead from a non-firing plug will spark against the engine block.

(2) Check the distributor timing.

(3) Check compression in each cylinder.

(4) Check spark plug colour for excessive fuel richness, especially on one plug or a pair of adjacent plugs.

(4) Check water in radiator, and the dipstick, for the dreaded "mayo" emulsion.

Mick
Mick Anderson

Terry. The very first thing to do is run a compression check. Your symptoms indicate a bad intake valve, hence, the back firing through the carb.

Les
Les Bengtson

Terry, Les is right, it sounds like you have a sticking intake valve. If you cannot easily check compression, you can see by removing the rocker cover. Turn the engine by hand (2nd or 3rd gear - rock the car forward/back) and check that the valves are returning to the closed position (check the tappet clearance). everything may appear OK if the fault is intermittent i.e. a valve sticks open occassionally or that it closes too slowly. It may also be a damaged valve or valve seat, then you need to check compression or remove the head.

Hope that helps,

Neil
Neil McGurk

Terry, don't discount a vacuum leak. Check out the inlet manifold and carburettor mountings to see if air is being drawn into the system. The same effect could be caused by the jet adjusting nut on the carb. coming loose against the spring. That happened on my car recently and caused missfire and backfire.
Barry
BM Gannon

Sounds like the first drive in my first MGA 35 years ago. The PO had milled the head and had a valve job done. Unfortunately one exhaust valve now barely hit the block. On that first long drive the rocker arm broke and the exhaust was forced out the intake. Took out the spark plug and drove home on three cylinders.
R J Brown

Check the simple things first. Mick mentioned check the distributor timing. I would expand on that and check the points gap. The gap closes as the points wear, altering the timing. Could cause an irregular running/misfire.

Steve
Steve Gyles

This thread was discussed between 14/04/2007 and 15/04/2007

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