Hi All,

I'm a marine mechanic dealing with a odd engine miss. I'm working on a 1978Mercruiser with a 305 SB stocker. I was asked to do a tune-up because the engine was running rough.

What I have found is the engine has a miss at cylinders #4 and #6 at idle. I checked the compression and all the cylinders are about 150 to 160psi. I changed the spark plugs and check the plug wires for proper firing order,( At plugs and distributor cap) and resistance, all were okay. The old plugs in those cylinders were black. I tried another set of new plugs in those cylinders with no change.

I sprayed around the manifold & carburetor with carb cleaner looking for vacuum leaks, no leaks were found.

It has a 2 bbl Rochester which I took apart and inspected and didnt find any noticeable problems. I flushed with carb cleaner and blew out the passages with compressed air. Everything seems to be working on the carb.

I then test ran the engine on our dyno and the engine is producing the correct power output and reaching maximum rpm.

So other than the miss at idle, the engine seems ok.

Any ideas?

Carlos G