MGB idling problems – posting to a British car forum

Posted By on September 27, 2011

I’m posting my MGB idling problem to the MG Experience forums in hopes to gain a little help … if you’re “just a car guru” and have some advice I don’t mind the extra thinking – either leave me an email or Disqus comment below. Thanks and additional details below.

From forum:

First, a year ago I replaced the points with an electronic module and replace the Zenith carb with a Weber downdraft. I put in a new gas tank, fuel pump and fuel filter as well as spark plugs and wires. Upon starting (with starting fluid), the engine ran rough and would not idle … eventually stalling; after stalling, the engine would not start while hot. After cooling I was able to start again but no matter how long I worked at tweaking the new carburetor settings, I could not get the little beast to idle below 1200 … an even then it would run lumpy. Definitely not road worthy.

Fast forward to this month. I took the engine apart and installed a new cam, timing chain, oil pump, water pump, alternator and checked the compression again (all 4 cylinders equal – about 90) and made sure there were no air or vacuum leaks from carb/intake manifold. Upon starting, the engine ran exactly the same … lumpy and it will not idle below 1200RPMs. I cleaned all the electrical connections and checked voltages. Coil had resistance of about 4.5 (needed to be over 3) and each plug demonstrated spark. I’ve moved the distributor (advanced/retarded) until the engine stalled but could not get the engine to run smoothly at low rpms. I put in fresh gasoline and carb cleaner (in case it had gummed). I’ve been tweaking the Weber idle screw, but suspect the idle never gets low enough to run without the primary feeding the engine. I’ve used the propane trick (feed unlit propane near intake manifold gaskets, etc) while the engine was fast idling to see if there was a change of idle … the only time it rev-ed a little was when the raw propane neared the choke flap.

I have not taken the head off to check for piston damage or deeply seated or damaged valves, but am reluctant to go that deep again (should have when the engine was apart) unless I hear from the gurus here. Before I go much further, what else should I be looking for OR is there a better way to set the Weber 32/34 downdraft carb?

EDIT: For the “firing order” suggesting … all is correct.



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