Running intermittingly on bottom two cylinders

[bitsy4643]

50 hp 1984 mercury running fine until last three trips. Motor began running on top two cylinders, intermittingly it would pick up on three and sometimes four cylinders at high speed, but this is very seldom. Has good spark and good compression on all cylinders. Gas consumption tremendous Tried rebuilding carbs, but no help. Could coil be breaking down or possibly bad reed valve? Even tried different gas stations. Any help greatly appreciated.

Bitsy

 

[madgadget]

Re: Running intermittingly on bottom two cylinders

What do the plugs look like?

 

[CharlieB]

Re: Running intermittingly on bottom two cylinders

Many make the mistake of assuming that 'spark' is good just because they can 'see' a spark on each cyl.

It is absolutely necessary to test the spark using a 7/16 inch gapped tester. one can be fabricated using a board and a few nails, bending the heads over so as to get a 7/16 inch gap between the heads. Ground one and rig a jumper wire from the spark plug wire to the other nail, crank and observe for a HOT BLUE "SNAP", repeat test on each plug wire.

Failure to pass this test indicates a weak ignition system that cannot provide spark under full throttle, and the cyl misses.