Johnson GT 150 fouling plugs on port side.

[kmaglothin]

I have a 1987 Johnson GT 150 that dropped approx. 500 RPMs at WOT. Normally runs 5000-5200 RPM WOT and now maximum is 4600-4800 ROM WOT. I pulled the plugs and found the three plugs on the port side appeared either fouled or close to it and the plugs on the other side were fine...dark tan in color. I have read that a fouled plug on one cylinder of a V6 will equal about 500 RPMs so I suspect only one plug is not firing properly. The thing that I am concerned about it that all the plugs on the port side look the same. All of them are oily black and wet. What I want to know is why would all 3 cylinders on one side show signs of fouling? I want to head off a potential problem before it gets worse.

 

[OBJ]

Re: Johnson GT 150 fouling plugs on port side.

Couple things to check Ranger....do a spark test on each hole....spark should jump a 1/2" gap with a strong lighting like spark. <br /><br />Make sure the port side head is coming up to temp while running. Each head has it's own t-stat. The port side stat could be stuck open allowing the head to run cold.

 

[kmaglothin]

Re: Johnson GT 150 fouling plugs on port side.

First, thank you for the response OBJ!<br /><br />I will do the spark test...that is what I am suspecting...perhaps a power pack starting to go. I did check all 6 heads yesterday and all were good according to the 3 second test...could hold finger on each head for the count of 3 before I had to remove it. The temperatures appeared to be very consistent on all 6 heads. The thermostat idea is also interesting...the surface temp yesterday was 47 degrees...if it were going to run cold it would have been yesterday...I know I was!

 

[walleyehed]

Re: Johnson GT 150 fouling plugs on port side.

5000-5200RPM is going to kill that engine, and may be some of your problem now.<br />You need to be a minumum of 5500, slightly above is even better.

 

[walleyehed]

Re: Johnson GT 150 fouling plugs on port side.

Nice.....a double post first-thing....

 

[kmaglothin]

Re: Johnson GT 150 fouling plugs on port side.

Well accoording to my maual the rated RPM is 5000 RPM and that is what it has run for 17 years...which is when I bought it new.

 

[Dhadley]

Re: Johnson GT 150 fouling plugs on port side.

Thats where the motor is rated. It will live much longer if propped out at 5800 with an average load. 5200 is lugging the motor and will lead to excessive combustion temps on todays fuel. Especially if youre using hi test.

 

[kmaglothin]

Re: Johnson GT 150 fouling plugs on port side.

I can fix that. I will shoot for around 5800 RPMs.