1987 Johnson 150GT J150STLCUR

[Kcampbell3]

Hey guys, I have been working on this issue for over a year with motor and cannot for the life of me figure out whats going on. This engine is on a 18ft fiberglass bass boat. Only when in use on the lake (under pressure) and after getting on plane the RPMs jump up and down very dramatically from 4000rpm to 2500 - 3000 to 4500 rpm and so on. Its as if its bogging down the jumps back up to where the I have the throttle. If i then stop, come to complete stop it boggs down running at low RPM and will not go back onto plane until it randomly decides to start working again... When in low RPMs during the issue they dont sound bad like its choking or sputtering it just wont continue its revs to increase the RPMs. somedays its very bad, other days it does it for 30 seconds and is fine the rest of the trip.

Here is what i have done so far:


- replace bulb and full gas line
- replace fuel filter
- fully removed fuel filter out of the equation for a moment. (no change)
- cleaned all 3 carbs (not professionally or replace the jets.. just cleaned)
- took the boat for someone to check out the VRO jump
- he replaced a membrane inside that was torn
- this did not resolve so i disconnected the oil completely and pre-mixed gas. No result change
-took off exhaust manifold, cleaned and replaced gasket to make sure no water was getting into cyliders
- replaced a bad temp gauge
- compression tested all cylinders ~90 PSI on all (within normal)
- replaced and tested all spark plugs


any ideas would be helpful thank you in advance.

 

[flyingscott]

Check for spark to jump 7/16" on all cylinders. You may have a shift interrupt switch on that motor that could be failing.

 

[Faztbullet]

If it has the T pack look at stator and rectifier outputs...these are the worst ignitions to troubleshoot.

 

[clanton]

Using a mulitimeter set on 3/400AC with long leads or helper. Connect +lead to input to ignition coil, - lead to ground. Run boat in water until misfires. You should have same and steady readings for good working ignition system. Low readings on any cylinder, move to 2 sets of brown wires from stator. Each set should read same as other, Low readings, replace stator, stator reading good replace pack. This engine loves stators, keeps CDI working. Flywheel magnets have to be checked, they come unglued and have to be reglued.

 

[Faztbullet]

The is a TSB on this engine as the spark plug wire will burn off in the boots.....

 

[boobie]

Fazt, what's a TSB ??

 

[racerone]

Technical Service Bulletin ?

 

[1983 ercoa 21']

I had the same issue on a 85 150xp.
It was the timer base.

 

[daselbee]

Symptoms sound like it is running out of gas, fuel pump catches up with a bit of gas, then engine runs out again.
I would shut engine down when this is happening, and pull a couple of the carb bowl drain screws to see if there was gas in the bowls.

Another way to check, is to pump the primer bulb when the engine is acting up. You are then providing a manual fuel pumping action, in the event the engine's fuel pumps are not working to full potential.

Or maybe as simple as water in the gas.