78 Merc 140 hp inline 6 keeps fouling plugs

[AaronIROCZ]

I have a 78 merc 140 hp inline 6 that I can't keep plugs in it. Last year it ran the best it has ever did 40 mph but then all of a sudden I lost power and could only do 5-10 mph max. I left it over the winter and replaced the plugs from champion to NGK and the engine ran fine. This is the first plug change since I first got it in 2007. I tested it out on the lake and had stalling issues when I tried to accelerate, so I adjusted the mixture screw out 1/8 turn and it got better, and did it one more time and the stalling stopped. So I thought great and took it out on the lake over the 4th and got 2 miles up the lake and lost power again. I limped it back to the dock and got new plugs after I tried cleaning the old ones and it ran good again. On one of the old plugs it had a chunk of carbon between the electrode and the side of the plug. The plugs didn't look abnormally oil fouled to me, they were damp but not dripping wet. I went 1 more mile and again lost power so I took it out of the lake and went home. I have not touched it since. I mix the oil in the gas to 50:1 with mercury brand oil so "I" don't think the mixture or the oil is the problem. Any other ideas??????

 

[Bifflefan]

Re: 78 Merc 140 hp inline 6 keeps fouling plugs

I mix the oil in the gas to 50:1 with mercury brand oil so "I" don't think the mixture or the oil is the problem. Any other ideas??????

If its a inboard, then its a 4 stroke and you dont mix oil with the gas, its just straight gas. That sounds like fouling problem to me.

 

[Fuzzytbay]

Re: 78 Merc 140 hp inline 6 keeps fouling plugs

Sounds like you need to clean the carbs out. Did you leave gas in them over the winter? I would check the fuel system out, plugs should not be "wet" dripping or not. Sounds like its running too rich. I would also try the "seafoam" treatment to the motor as well. If you got carbon on the plug, sounds like the engines got a build up.

 

[AaronIROCZ]

Re: 78 Merc 140 hp inline 6 keeps fouling plugs

No this is an outboard, and no I didn't clean out the carbs this year, I always drain the fuel from the carbs before I store it and this last year I stored it inside my garage at 50* all winter. I did not use sea foam this year either. Just to make sure I am on the same page, just pour the sea foam in the gas or run a more concentrated mix??

 

[Fuzzytbay]

Re: 78 Merc 140 hp inline 6 keeps fouling plugs

Use a concentrated mix, I use a can of seafoam, to one gallon of mixed fuel.
You can try the seafoam first, its an easy start, but if you still have issues after, then I think you should check out the fuel system, ie carbs, fuel pumps, even the fuel line, and tank.

 

[AaronIROCZ]

Re: 78 Merc 140 hp inline 6 keeps fouling plugs

Ok will do and report back when I ran the sea foam through it.

 

[AaronIROCZ]

Re: 78 Merc 140 hp inline 6 keeps fouling plugs

Ok to finally update this thread after giving up for 4 years, yes 4 YEARS later I finally figured out the engine wiring harness was so corroded the main power wire to the switch box actually had only 2 strands of copper left. SO after a while of running it in the lake the wire got too hot and wouldn't allow full voltage to reach the box. Put a new harness on it and all is well again.

 

[reginald34]

Re: 78 Merc 140 hp inline 6 keeps fouling plugs

did fixing the wiring harness fix your fouling plugs problem, if so witch where are the wires located

 

[Chris1956]

Re: 78 Merc 140 hp inline 6 keeps fouling plugs

Reginald, That motor has a battery-powered CDI ignition. The white and red wires must have +12VDC on them to have spark. Likely one or both of those rotted out.

 

[reginald34]

Re: 78 Merc 140 hp inline 6 keeps fouling plugs

thanks Chris i will take a look and see if thats my problem

 

[Chris1956]

Re: 78 Merc 140 hp inline 6 keeps fouling plugs

Reg, Both of the wires I referenced will be on the starboard side of the switchbox, side by side. I did not mean the white distributor wire.

 

[reginald34]

Re: 78 Merc 140 hp inline 6 keeps fouling plugs


hi Chris I took a look at the wireing,the guy I purchase the boat had done the rewiring on this motor.The wires you told me to look at are ran togather and attached to the +12volt that run to the distributor.Will that cause any problem.

 

[reginald34]

Re: 78 Merc 140 hp inline 6 keeps fouling plugs


I found this wire with the other wires it was corroded looks like its red and after testing it its positive lights up on both connections when tested

 

[quicktach]

Re: 78 Merc 140 hp inline 6 keeps fouling plugs

What is the engine S/N ? Have you tested individual cylinder compression ? Have you checked the spark of each cylinder with an adjustable spark tester ?

 

[Chris1956]

Re: 78 Merc 140 hp inline 6 keeps fouling plugs

Reg, I cannot hope to see how that motor is wired, from those pictures. Go on the max rules website and download a wiring diaghram and trace 'em out. Maye sure you get an inline 4 or 6 with a distributor ign, diaghram.

 

[reginald34]

Re: 78 Merc 140 hp inline 6 keeps fouling plugs

ok I took the boat out yesterday,what I did different I let the motor warm up for a while before taken off. it did very well,so I stopped and fished for awhile ,and before I came back to the dock,i did the same.Motor ran great again.i used to start her up and go but letting her warm up did some good....will see how she do next time I take her out again.

 

[wired247]

Re: 78 Merc 140 hp inline 6 keeps fouling plugs

Carbon between the electrodes? What plugs are you running?

 

[reginald34]

Re: 78 Merc 140 hp inline 6 keeps fouling plugs

Surfaces gap Ngk