Had the boat out a couple months ago and it ran fine, 36 MPH at 5500 RPMs  1975 85HP Johnson on a 1987 18.5' Skeeter Bass boat. Friday the boat would not go over 3600 RPMs and not plane. I picked up a couple of carb kits and a friend was going to rebuild them, he done some adjustments first, and removed a screw from the bottom of carb, ran the motor while spraying carb cleaner and lots of dirt came out, after that the motor would rav like some kind of race boat in neutral. Took if out to the river and no power, took compression test, almost 125 in each cylinder. Any idea what to look at next? Sorry for long post.

Thanks

Joe

Well I'm a little confused. Your friend took a screw out of the carb, and squirted  some carb cleaner into the carb some where, and dirt came out from somewhere. Then it reved like a race boat in neutral. But when you returned to the river you had no power.  

I will assume the screw was at the bottom of the float bowl and he squirted the cleaner into the float bowl (from below), and dirt came out of the float bowl. If it is a 85 hp motor you probably have 3 carbs and 3 float bowls and each would have received the same treatment.

Since your compression numbers look good, the problem seems to be either ignition or fuel to the motor. An inexpensive step is to replace the spark plugs. Then make sure the fuel delivery to motor is good. The motor probably has a fuel filter, see that it is clear. It also has a fuel pump, check that the fuel line to the carbs is providing lots of fuel, at least a pint every 2 minutes at 5000 rpm. It is possible that since dirt was flushed out of the carbs, that one or more high speed jets in the carbs are partially clogged and will have to be cleared. If you are not mechanically talented, you will probably need help. Good luck. :-/

If junk came out of the carbs when your friend sprayed cleaner in them, both carbs need to come off and be cleaned properly.

Most likely the motor is down a cylinder is the reason for the loss of rpm.  As mentioned, this can be a clogged jet in the carbs or it could not be fiing on one.  The not firing can be any of a number of problems.   Depending on the brand, it could be a spark plug, plug wire, coil, power pack/trigger module, timer base/stator, or bad connection in one of the wires.  To trouble shoot ignition problems you're going to need the right test equipment, a book and know how or it can get very, very expensive.

Since you said the compression is 125 on each cylinder the motor is not down a cylinder. Most of the V-6 Mercury motors that our shop rebuilds come in with damage due to a fuel delivery issue.

Start with the fuel line and all filters. Be sure to see if your motor has a filter mounted up on the motor. This is the most over looked filter as most people do not know it is there. Since we only work on Mercurys I cannot be sure if that motor has one. I bet it does though.

Are you running an ethynol mix in your fuel? If so that is probably the problem. The earlier floats in the carbs are not designed to work with that mixture. I think you will find that the floats have absorbed the mixture and will no longer float. they will need to be replaced. Good luck and let me know what you find or if I can help you.