1986 90 hp johnson idle and ramp up issue

[roonie]

I've had this issue since owning the rig.
Starts well but kinda shakes,feels like it's being held back until i start to get the boat moving,then smoothens out.its always done this.

new issue......
The engine also will,at idle, rev up rpms on its own,then cut out like a dead short for a split second, then right back up to higher idle then on its own go down in rpms to where it should be. I've seen a few boats do this but never my boat...until now

 

[roonie]

Bump..,anyone?

 

[racerone]

Post your actual compression values first.

 

[ct1762@gmail.com]

I've had this issue since owning the rig.
Starts well but kinda shakes,feels like it's being held back until i start to get the boat moving,then smoothens out.its always done this.

new issue......
The engine also will,at idle, rev up rpms on its own,then cut out like a dead short for a split second, then right back up to higher idle then on its own go down in rpms to where it should be. I've seen a few boats do this but never my boat...until now

besides testing compression, when where the carbs last serviced? i've had these old crossflows do something similar when the idle circuit was covered in sticky old fuel... of course fresh 93 octane is a must for these

 

[roonie]

The compression value is 120, all 4 cylinders. The engine has been rebored,rebuilt right before I purchased the boat about 10 yrs. Ago
I can see where the engine previous to my owning, got hot (paint peeling off engine block)(some of the plastic protective sleeves that are over the wiring harness, where they've touched the block ,have melted but not through to the wiring anywhere I can see).the previous owner told me the alarm went off but it was too late, the engine overheated and needed to be bored. Even the buzzer itself was melted.
It's ran very well for us over the years but has always had, at lower speeds, this shaking like it's being held back,until I Crack it wide open, she gets up and planes and goes 40mph no problem.the plugs look a nice even tan color, all 4 the same. I've been around cars all my life.i do all my own repairs on my own vehicles etc.father was a stock car racer, uncle was a top fuel AA racer (Jeb allen)(praying mantis).just not a lot of experience with the marine side.
Side note...up here in Canada the larger petro companies have been caught more than once ,selling fuel as premium only to find regular octane in the premium storage tanks.

 

[ct1762@gmail.com]

The compression value is 120, all 4 cylinders. The engine has been rebored,rebuilt right before I purchased the boat about 10 yrs. Ago
I can see where the engine previous to my owning, got hot (paint peeling off engine block)(some of the plastic protective sleeves that are over the wiring harness, where they've touched the block ,have melted but not through to the wiring anywhere I can see).the previous owner told me the alarm went off but it was too late, the engine overheated and needed to be bored. Even the buzzer itself was melted.
It's ran very well for us over the years but has always had, at lower speeds, this shaking like it's being held back,until I Crack it wide open, she gets up and planes and goes 40mph no problem.the plugs look a nice even tan color, all 4 the same. I've been around cars all my life.i do all my own repairs on my own vehicles etc.father was a stock car racer, uncle was a top fuel AA racer (Jeb allen)(praying mantis).just not a lot of experience with the marine side.
Side note...up here in Canada the larger petro companies have been caught more than once ,selling fuel as premium only to find regular octane in the premium storage tanks.

check the 4 check valves on intake manifold. they should only flow out, not in. just get a piece of 1/8 hose and blow into each one. any air escaping back into the intake they must be replaced. does it smoke and load up excessively when applying generous throttle? if the block got hot enough to melt wires, id say thats way over 250f. some loading up is normal for all 2 stroke on the idle to high speed transition.

 

[brodmann]

I'd order carb kits and tear into the carbs. You'll need to order carb kits because there's a plug that you'll need to knock out to get to some passages that need to be cleared out. I've attached a diagram of your carbs. Pay specail attention to #3, #7, #13, and if you don't knock out plug #2, you're wasting your time in there.