1990 Johnson 150 gt one plug carbon fouled

[Gkm70526]

I recently got a Johnson 150 gt I ran a decarb and then changed plugs. After running a little while I pulled plugs and noticed one was full of dry carbon buildup. Motor has a miss so I did a compression check also and readings on all cylinders were between 90-93psi.

 

[Bosunsmate]

Hows the piston head look for that cylinder?
Looks like you might need to take the bypass cover gasket off and have a look at its rings

 

[Gkm70526]

I took a light and looked in it as best I could the head looks the same as the rest just with the carbon buildup

 

[Gkm70526]

do compression numbers seem low for this motor

 

[Bosunsmate]

do compression numbers seem low for this motor

A little bit low, but they are all quite even. Hows the cylinder directly opposite it look?
Which cylinder did that fouled plug come from?

 

[racerone]

Remove the bypass cover for the cylinder with the fouled plug.----Just 6 wee screws on the side of the block.----Will perhaps require a new $1.99 gasket.------Remove carburetor high speed jet for that cylinder also.------Test for proper spark on each lead.-----System was engineered to be able to jump up to a 1/2" gap , so that is the test to do.

 

[Gkm70526]

It is cylinder #2 that is carboned up the #1cylinder is good. I put an inline spark tester and ran motor and tester was showing good spark

 

[racerone]

Some folks are reluctant to answer questions and do some simple trouble shooting.--Not sure why that is.-----Have you looked at the reed valves on this motor ?

 

[Bosunsmate]

Ok, i think on that motor it has three carbs and the top feeds both cylinders. So seems like cant blame it on the carb at this point.
The only other things i can think of apart from inspecting the rings is a problem with the reeds for that cylinder or an air leak in from somewhere. Is there any little hoses sticking out the side of the bypass covers? Sometimes they can get holes in them and then itl run bad

 

[dwco5051]

I recently got a Johnson 150 gt I ran a decarb and then changed plugs. After running a little while I pulled plugs and noticed one was full of dry carbon buildup. Motor has a miss so I did a compression check also and readings on all cylinders were between 90-93psi.

I have a 1990 175GT with only a few hours since complete rebuild. My gauge shows between 90 and 92 on all cylinders. Seems reasonable for your motor.

 

[V153]

I doubt it's broken rings or reeds.Things'd be chewed up'n you'd have low compression in that cylinder.

Spose it could be a air leak but I'd take a closer look at that top carb. #2 side could well be plugged up with crud. I agree on pulling the main jet(s) and blowing some fuel thru. Might help.

You got spark and even compression. I'd lean more toward a fuel delivery problem.

Fwiw at first I was terrified of working on carburetors. Due to early traumatic experiences with automotive versions ... But these 2 stroke boat ones ain't so bad.

 

[Gkm70526]

thanks for the help guys I will clean the carbs and see if I can get the carbon problem solved and Ensign I have read that Johnson had some lower compression v6 motors but I wasn't sure if mine was one of them.

 

[Gkm70526]

Bosunsmate it is three carbs two barrel style. It is a cross flow motor and one side of the carb feeds opposite side cylinder

 

[Bosunsmate]

Bosunsmate it is three carbs two barrel style. It is a cross flow motor and one side of the carb feeds opposite side cylinder

ok got it, know the ones, have a look in at the reeds for that cylinder while you have it off

 

[racerone]

Problems with reed valves do not show up on a cylinder compression test.---And are you sure on the fact that the carburetor bore feeds the opposite side cylinder !

 

[Gkm70526]

Thanks bosons mate and racer one I will rebuild the carbs also how difficult is it to change the reed valves I have never went that far before

 

[Gkm70526]

Thanks bosunsmate and racerone I will rebuild the carbs also how difficult is it to change the reed valves I have not gone that deep into a motor before

 

[racerone]

If I recall some GT motors may have come with composite reeds from the factory.-Said to have limited life at prolonged full throttle running.---And are you sure about the carburetor bore on the left feeding the cylinder on the right side ?

 

[V153]

are you sure on the fact that the carburetor bore feeds the opposite side cylinder

Must be a Cris-Crossflow ... (Sorry.)

 

[V153]

Problems with reed valves do not show up on a cylinder compression test

Agreed. But the biggest "problem" with steel reeds is breakage. And if they break they tend to gravitate toward the combustion chamber. Resulting in death and destruction in that cylinder. I don't think that's the case here ...?