1996 Johnson 90 SPL won't plane intermittently

[mr300z87]

My friend has a 1996 Johnson 90 SPL on a Aquasport 17 center console that is having and intermittent problem running above 3000 RPM. The symptoms are a slight miss at idle barely noticeable if not used to the engine, when throttled up it will only get to 3000 RPMs and will not get on plane. After a minute it will start to rev up and will run 5100 RPMs. On today's run we ran for half hour slowed for a no wake under a bridge and then back to normal cruise RPMs. This was repeated again for a longer no wake and back to normal. Just before we returned to the marina we slowed and then tried to resume speed and it would not go above 3000 RPMs the entire rest of the way. This has been the case for the last few uses.

So far the mechanic at the marina has done a compression test and said the results are within spec, but I do not know exact numbers. A new OE fuel pump was installed. Today my friend the owner and I took apart the carbs (they were spotless inside) cleaned them blew air through all openings re-installed carbs, the results being the ride described above. There had been some discussion with the mechanic regarding a bad coil, which had been ruled out some how. We also tried opening the fuel fill, priming the bulb and applying choke while bogging none of which helped. It kind of seems electrical to me but do not know enough about these engines to go about diagnosing it further.

A little about me, my boats have been I/Os and even though it is in a boat they are easy to work on GM engines. The OMC 2 stroke outboard is foreign to me. I am hoping you guys can get us pointed in the right direction and as always any advice is appreciated. Have a great night

 

[Bosunsmate]

Slowing down for under a bridge seems a very strict condition, here people speed up to make a scene for car users.
How do the sparkplug electrodes look? Any different?
I would do a drop test on all cylinders when its struggling and not accelerating properly. You can do that rather quickly. When you/if you can trace it down to a cylinder not providing any power, then do a spark test on that one by checking spark jumps a 7/16 gap.
If it doesnt you could try changing a coil over from a good cylinder and seeing if the problem follows the coil or stays on that cylinder.

 

[jimmbo]

It could be a heat sensitive component in the ignition system acting up, a intermittent short/open, a Float in one of the carbs sticking, a plug fouling by a piece of carbon flaking off the head/piston. Another possible cause could be a leaky Crank Shaft Seal. Try running a 24:1 mix, the extra oil often makes for a false seal and the engine performs better. If it does, pull the Flywheel and look for Oil on the top of the bearing carrier

 

[racerone]

Test run with the hood / cowling off..

 

[racerone]

Run with a timing light hooked.up------Will help in finding intermittent spark.-----Check on all 4 plugs.-----Post the compression test numbers.

 

[fireman57]

Agree with racerone. Start there.

 

[mr300z87]

Thanks for all the thoughts, I forwarded it to my friend. Unfortunately he is not that mechanical and since I am not there he is relying on the mechanics at his marina. Head to my boat today for some 4th of July celebration and fireworks on the water.

He is also considering a repower with a brand new 90hp etec 3 cylinder as he has teen age kids that will be learning to operate the boat and wants it to be reliable.

BTW in New Jersey all bridges are no wake. It a safety thing.

happy and safe 4th to all