Need help! Mind of its own Johnson gt150😬

[The nut]

Hey all
I’m quite new to this and hopefully this is the right thread to post this on
So I’ve got a Johnson GT 150 (late80s-early90s) not exactly sure..
Anyways see if I can explain my prob
So for awhile the thing used to have what it seemed like 2 WOT positions
From what I can tell WOT is supposed to b like 5300-5500 rpm??
Well it would randomly decide that 4500 rpm is all it wants to do
Still sounded completely fine just wasn’t revving out
I’m not a pro with mechanics but I ordered a carb reseal kit and pulled all the carbs apart and changed gaskets and seals but it didn’t really change anything
Just lately tho I was out again and for the first time for me it only revved to 2500 rpm
I wasn’t sure what’s to think so I kept it pinned for prob 2-3 min and all of a sudden out of the blue he took off like a rocket right up to 5500 rpm and we cruzed across the lake to our first fishing hole
Fished for a few hrs and decided to try somewhere else
Fired it up and back to 4500 rpm wot
That gets me on plane no prob tho so cruzed for a bit then stopped and fished for awhile
Time to move again and fired it up
Same thing, 4500 rpm but then as we were cruzing it started dropping rpm down to 2800-3000 rpm which put me back in the hole
Just to make sure it’s not throttle cable related I went back and open the throttle all the way right on the carb but no change
After about 2-3 min tho of doing that he decided ok time to go and climbed back up to mb 4700 rpm and held it there no prob for the rip back to dock
Every time it lost rpm it just sounds like I’m not giving it throttle tho
Didn’t seem to run rough or have a cylinder not firing
Any ideas??
Thanks in advance

 

[saltchuckmatt]

Wow sounds to me like you're randomly losing a cylinder and possibly other problems. I would not keep running it and I would do a compression check right away. If you're constantly leaning out one cylinder you're going to smoke it.

 

[racerone]

Run with a timing light.----One cylinder at a time.----Will help you determine which cylinders are losing spark.----And do you have a water separating filter?----Could be a plug wire shorting to cowling as well.

 

[dwco5051]

On a Johnson GT 150, the S.L.O.W. (Speed Limiting and Warning) mode activates and drops RPMs to roughly 2,500 by disabling half the cylinders when the engine senses an overheat condition. There are a few other signals from the VRO if I remember right which also could activate SLOW. I sold my boat with a GT175 some years back and gave the buyer the factory manual and don't remember all the things that can cause it. Possible bad shift interrupter switch. If you use a timing light like Raceone suggest and you have lost half your spark to the cylinders now you have to figure out what is turning SLOW on.

 

[racerone]

I am not sure if S.L.O.W. was on that motor.----Model # of this motor is ?

 

[The nut]

OK thanks a lot guys
I’m not at the boat currently but I’ll try get that model # next time I am
So if I’m losing cylinders I won’t necessarily b able to hear that?
Just sounds like less rpm?

 

[saltchuckmatt]

OK thanks a lot guys
I’m not at the boat currently but I’ll try get that model # next time I am
So if I’m losing cylinders I won’t necessarily b able to hear that?
Just sounds like less rpm?

Somewhat the same thing.

 

[The nut]

Here’s the model

 

[The nut]

Ok just did a timer light test and one cylinder the light quite flashin when I wiggled the wire!
May b my prob?
I have never done a compression test yet my self
I googled it and I can prob figure it out but would someone mind sending the best way to do it?

 

[The nut]

And I got it done!
Lowest compression was mb 89 but could argue it was 90
Avg across all of them was 91-92 with 92 or 93 being the highest

 

[dwco5051]

Here’s the model

Appears to be a 1990 model. "ES" That year should have S.L.O.W.

 

[dwco5051]

And I got it done!
Lowest compression was mb 89 but could argue it was 90
Avg across all of them was 91-92 with 92 or 93 being the highest

My numbers were all in the low 90's not to long after a complete rebuild. I was told not uncommon on those old crossflows.

 

[saltchuckmatt]

For me, even if they run with those values...they have no power.

I will say, lots claim though.