1979 55 hp misfires

[jamesan]

'79 55hp evinrude has an occ***ional misfire. By occ***ional I mean that when the throttle is advanced enough to give the small boat some speed the motor starts running very smoothly EXCEPT about every 5-20 seconds it misfires 1 time. The engine sort of goes "BAM", and then it runs perfect for another 5-20 seconds before misfiring again. It goes on constantly regardless if the engine is cold or warm. It certainly seems like a spark problem (replaced the plugs already) that only shows up under higher rpm's.

From the manual I have learned that it is a Type II Capacitor Discharge Ignition. I do not know exactly how to troubleshoot this problem as it is difficult to simulate the high rpm's at home.

Any thoughts?

[boats.net]

Does it seem to misfire during idle? The Bam you speak about, is that a backfire BAM? It sounds like a possible fuel problem. Check the carburetor adjustments, make sure the there are no clogged jets or air p***ages.

[jamesan]

The bam is not a backfire. It is like it didnt fire at all for one cylinder, one time. It almost sounds like the lower unit hit something underwater. Is that what a backfire sounds like on a boat?
I cant tellhow well idles. I think it should idle better. Others tell me it sounds lile a typical two stroke.
The carbs have no adjustments other than float height, and perhaps idle speed..there are no screws for me to adjust fuel or air. I cleaned out all of the carb.p***ages with pieces of wire and some solvent.