1974 Johnson 50Hp outboard, fouls plugs when trolling

[TkdSilat]

Hello. I am finally back to the forum for some help!. I did a restoration on a 1970 starcraft supersport. here is the thread

http://forums.iboats.com/showthread.php?t=362923&highlight=starcraft+supersport
(at the end, there is a video of the motor running)

anyway, I have had the boat out several times, no problems except until today. I was trolling, and I noticed that I fouled out both plugs. I trolled for probably 1 hour. Prior to this, I never trolled and I never fouled out any plugs. After I had the engine professionally serviced, I put in the factory recommended spark plugs the champion plugs, and never had a problem. I read in lots of threads that people are now using the NGK equivalent to the factory champion plugs.
My question is , will that even help? or is this motor not built to troll at low rpms for a long time?
Everytime I see one of these boats, they always have a kicker motor, anywhere from 5-10 hp in addition to the more powerful motor.

I ran the boat all last year, and it fan fine, no indication of fouling plugs. I didn't change my fuel mix, actually didnt change anything except that I trolled for a long time.

Thanks!

 

[boobie]

Re: 1974 Johnson 50Hp outboard, fouls plugs when trolling

Could be the mtr is running to cold at idle. Have you checked the mtr temp at idle ??

 

[TkdSilat]

Re: 1974 Johnson 50Hp outboard, fouls plugs when trolling

No I haven't. I had the motor professionally serviced. I remember the mechanic told me all the cylinders were at factory recommended compression and the voltage was right on. what's the recommended temp? How do I check it?

 

[boobie]

Re: 1974 Johnson 50Hp outboard, fouls plugs when trolling

The idle temp should be 140* +/- 15* at idle. You can buy a cheap infrared gun at Harbor Freight to check it with. Or, poor mans way. If you can hold your knuckles on top of the block just before the cyl head for over 5 seconds after the mtr is warmed up and idling, it's running to cold.

 

[FishinDave]

Re: 1974 Johnson 50Hp outboard, fouls plugs when trolling

The idle temp should be 140* +/- 15* at idle. You can buy a cheap infrared gun at Harbor Freight to check it with. Or, poor mans way. If you can hold your knuckles on top of the block just before the cyl head for over 5 seconds after the mtr is warmed up and idling, it's running to cold.[/QUOTE


I had the same issue with my 30 hp mariner. The guy who had the motor before me took the thermostat out and it would foul plugs at idle.

 

[FishinDave]

Re: 1974 Johnson 50Hp outboard, fouls plugs when trolling

Hello. I am finally back to the forum for some help!. I did a restoration on a 1970 starcraft supersport. here is the thread

http://forums.iboats.com/showthread.php?t=362923&highlight=starcraft+supersport
(at the end, there is a video of the motor running)

anyway, I have had the boat out several times, no problems except until today. I was trolling, and I noticed that I fouled out both plugs. I trolled for probably 1 hour. Prior to this, I never trolled and I never fouled out any plugs. After I had the engine professionally serviced, I put in the factory recommended spark plugs the champion plugs, and never had a problem. I read in lots of threads that people are now using the NGK equivalent to the factory champion plugs.
My question is , will that even help? or is this motor not built to troll at low rpms for a long time?
Everytime I see one of these boats, they always have a kicker motor, anywhere from 5-10 hp in addition to the more powerful motor.

I ran the boat all last year, and it fan fine, no indication of fouling plugs. I didn't change my fuel mix, actually didnt change anything except that I trolled for a long time.

Thanks!

I had the same issue with my 30 hp mariner engine. The guy i bought the motor from took the thermostat out and ran the motor like that. Cant really hurt the engine but will make the engine run too cool and foul plugs at idle.

 

[TkdSilat]

Re: 1974 Johnson 50Hp outboard, fouls plugs when trolling

Where would the thermostat be on this motor? I was unaware that these old 2 strokes had a thermostat...

 

[Joe Reeves]

Re: 1974 Johnson 50Hp outboard, fouls plugs when trolling

The thermostat and water bypass valve assembly is between the cylinder head and the cylinder head cover plate... at the bottom portion of the head/cover. It's a real pain to get at and usually requires the cylinder head to be removed.

You don't say exactly what Champion plugs you're using. They should be either L77JC4 or QL77JC4 plugs, gapped at .040 (original recommended gap) or .030 (revised gap of late 1990's). Whatever gap you're using, try the other.

 

[TkdSilat]

Re: 1974 Johnson 50Hp outboard, fouls plugs when trolling

The plugs that I used were Champion ul77v. no gap setting on the plug. Thats what the manual called for, and that is what was in the engine when it was stored a long time ago. Should I try the QL77JC4 at .30 gap? I do not want to rip the whole cylinder head off to see if the thermostat was removed

 

[Joe Reeves]

Re: 1974 Johnson 50Hp outboard, fouls plugs when trolling

The plugs that I used were Champion ul77v. no gap setting on the plug. Thats what the manual called for, and that is what was in the engine when it was stored a long time ago. Should I try the QL77JC4 at .30 gap? I do not want to rip the whole cylinder head off to see if the thermostat was removed

The UL77V plug is what is called a surface gap plug, and although the heat range is listed on the plug as being "77", it's really a cold running plug and quite prone to fouling. Switch over to either the L77JC4 or the QL77JC4 plugs and try the two mentioned gap settings.

The "Q" simply indicates that the plug is a suppressor type plug designed to help eliminate electrical/ignition noise from ones installed electronic and electrical accessories.

If the enqine runs unusually cold (or hot) at the higher rpms, regardless of the work involved to install a new thermostat, it is strongly suggested you do so.

(Thermostat Purpose)
(J. Reeves)

Many engines are considered high performance engines and demand a thermostat(s), and many smaller hp engines also require a thermostat for the following reason. The pistons actually distort/twist off round when running. The thermostat is required to keep the outer wall surrounding the steel cylinder, and the cylinder wall itself at a temperture to compensate for the distortion.

With the thermostat removed, the difference in temperture between the inside the cylinder and the water jacket quickly reaches a point whereas (in effect) you have a oblong piston running up and down a round cylinder, or vice versa. Bottom line is without the thermostat, the piston and cylinder wall could be damaged in a short period of time.

The thermostat in your car isn't there simply to have the heater function properly..... think about it.

 

[boobie]

Re: 1974 Johnson 50Hp outboard, fouls plugs when trolling

Check your mtr temp before you do anything else and then put in the plugs that Joe recommended..