Why is my Mercruiser engine misfiring?

Question

I have a 1987 3.0 Mercruiser. Engine started hard and would finally run, but when shifted into gear, sounded like it was missing 3 out of 4 cylinders. cleaned out gas filter under gas pump- very dirty; worked for 15 min., then full throttle, 10 mph max. I replaced the following: coil, plug wires, spark plugs, cap, rotar, points, condenser. Cleaned out upper gas filter- was not very dirty. Now, at high speed, engine misfires occasionally. Had mechanic check compression- 100-105 each cylinder; he also squirted gas into carb at high speed, still misfiring. Mechanic can not figure out why it is still misfiring- told me to drive it until it gets worse. Any ideas what might be wrong? Thank you. Lee

Answer 1

First get a new mechanic, this guy sounds like an auto mechanic, not a marine mechanic.  I say you probably have to remove and check or clean the manifolds and risers.  You may have a blown head gasket or a leaking manifold gasket at the riser.   Your manifolds may be clogged up with sand.  Remove, inspect clean and or replace the manifold and riser. A good marine mechanic can test everything and let you know what the problem is.  Don't give anymore money to the mechanic who worked on the boat before.  

Answer 2

Try this When you take the boat out and run it the bow has an up attitude . Now to the possible problem the pickup in your gas tank is located towards the back of the tank so when going on plane and boat gets that upward attitude at front water in your gas rolls to the picup and sends to motor which cant burn it and give you what feels like a miss . I have seen this happen before. get your guy toraise boat up in front and take out sender in tank and have him do a fuel check. if you put it in a clear bottle you will see the water at the bottom of a full bottle of gas .

At this point he will know what to do

Answer 3

Is the distributor lock tightened down fully?  Maybe the dist is slightly moving under acceleration?

Answer 4

Sounds like vacuum advance may not be operating, but that is so simple , you have probably checked that already. Hmmm.