Volkswagen: Engine misses on one side, mechanical advance, left hand side

Question
I have a 1976 Beetle. 1600 engine, with a single Empi Hpmx carby, mechanical advance 009 dizzy with a pertronix ignitor installed.  It has had my local mechanic stumped for the last couple of weeks. It keeps missing on cylinders 3 and 4, ie the left hand side of the engine. When he pulls one of the spark plug leads off and holds it just out of its boot (spark jumping across) it seems to run fine. Put it back in properly, engine starts to miss and flood again as though there is not enough spark to burn all the fuel.

Any ideas.

Answer
Hi Simon,

The most common cause of this situation is an improperly sealed intake manifold.  Check the manifold rubber boot and make sure that the clamps are tight and there are no cracks in it.

Second, have your mechanic spray starting fluid on the intake manifold (where it meets the cylinder head on # 3 -4 ) while the engine is running.  If the engine smoothes out while he sprays... you found your problem.

Best of luck and keep me posted.

Ron