'75 15 HP Johnson Seahorse

Tim Wagner · Jan 6, 2011

This thing keeps fouling a plug for some unknown reason and forces me to use my big motor to troll with.
The compression is balanced, the carb is cleaned, decarbed, replaced both coils, gas is new and mixture is 50:1.
I havent pulled the flywheel yet and checked the points and stuff but it will run for a period of time normal.
Then all the sudden Ill kick it back to idle, then when I need to get going again it wont run any more.
I check it out when I get home and its got a fouled plug.
Serial number 15RL75C
I use this to troll (ludicrous speed) 5 MPH on a 19' Starcraft (the one in my sig) so its running at 2500-3000 rpms.
I ran this motor on my other boat and it was fine.
Now I put it on my new boat and it fouls plugs.
Im using NGK plugs gapped to 0.30 thousanths. (I forget what the part number was but it was the equivalent to the Champions)
The difference now is I had to get a seperate tank for it. I used to draw from the big tank cause the motor took mixed gas. My new motor is oil injected and ended that.
I used alcohol resistant hoses for it.
I had notices by pulling the cover off the head that there wasnt a thermostat in it.
It actually looks like there never was one either.
Could this be an issue??
Any other suggestions????

g-man22 · Jan 6, 2011

Re: '75 15 HP Johnson Seahorse

first im wondering how you replaced the coild without pulling the flywheel....you should pull the flywheel and check the timeing or when the points come in contact.... also switch to champion plugs they run much better..... you might want to take the powerhead apart after all of that if that doesnt help and check your cylenders and pistons and rings...

anyother questions just ask

lindy46 · Jan 6, 2011

Re: '75 15 HP Johnson Seahorse

Try a little hotter plug and open the gap to about .035"

kbait · Jan 6, 2011

Re: '75 15 HP Johnson Seahorse

coils are external to the flywheel on that model. Pull the flywheel and clean up/reset the points, and check for lateral play in the armature plate. If you find some, post back for remedies.. Any lateral play will change the point gap and screw things up. Good luck!

Tim Wagner · Jan 6, 2011

Re: '75 15 HP Johnson Seahorse

Ill go with the hotter plug, Champion and Ill gap them to 0.035.
Id love to be doing it now but the boat is all tucked away in the backyard.
Ill pull the flywheel and check things out. I bought this guy at a garage sale for 75 bucks.
I just figured since it runs ......... till it fouls a plug, the timing was cool.
then since it runs ................ till it fouls a plug, the points were cool too.
Ill check it out kbait. Thanks.
Nice 'eye in the Avatar.
They are starting to get nice ones (and a ton of "in betweens") here in the Allegheny River. Them things stack up in the winter like crazy.
Still not a muskie though.

Tim Frank · Jan 6, 2011

Re: '75 15 HP Johnson Seahorse

I'd change the plugs....then try it and see what happens.
I've used the NGK "equivalent" a couple of times in my E/rudes....never again!
In fairness, the Champion "equivalent" ran like crap in my Nissan, so I am back to NGK for that.

If that is not the solution, try one thing at a time.

OptsyEagle · Jan 6, 2011

Re: '75 15 HP Johnson Seahorse

I have the identical motor and the magneto ignition system does not put out a lot of voltage and they are prone to foul plugs quite often, especially if trolling a lot. Mine hated the Champion plugs. I used the NGK B7HS with no problems.

The motor also needs a thermostat. Most likely the water is pretty cold wherever you are and the T-stat is to ensure the motor warms up to about 140F. Without one and running at low RPMs, you will definitely keep fouling plugs. If I were trolling a lot, I would go with the NGK B6HS, a grade hotter plug, and I would install a T-stat no matter how fast I was going.

Your cylinders will also be carboning up. If carbon is getting on your plugs (that's what plug fouling is), you can be sure it is getting on your piston rings as well, so a decarb is in order once you get it fixed.

iwombat · Jan 6, 2011

Re: '75 15 HP Johnson Seahorse

By any chance, is it the same cylinder that operates the vacuum pulse on your fuel pump that's fouling?

Tim Wagner · Jan 7, 2011

Re: '75 15 HP Johnson Seahorse

iwombat said:
By any chance, is it the same cylinder that operates the vacuum pulse on your fuel pump that's fouling?

Well, Id really love to answer that but Im a bit of dumb azz and I havent paid attention to which plug is the one the keeps fouling. (even if its the same one)
Im on my 4th set of plugs and at least have the problem narrowed down.
I could kick myself for not paying attention to this detail, since I work in technical support and it is my job to pay attention to detail.
Anyway, another thing I didnt mention is that I did rebuild the fuel pump already.
For some reason, I wanna say its the bottom one but honestly, Im not sure.

jmendoza · Jan 10, 2011

Re: '75 15 HP Johnson Seahorse

Your problem is the engine is not warming up, and the cooler the body of water, the worse the problem is. If it's been in salt water for any length of time, most likely the area where the thermostat seal goes will be so corroded that a new thermostat and seal will leak and still over-cool...I know, it happened to me. You can replace the head, or use a piece of aluminum tube to cut a very thin washer from to press in where the seal went to create new flat sealing surface, coat the bottom of it with high temp sealant and press it into place, then fit the new thermostat and seal. Took me a couple months back in 1982 to diagnose this problem, but you can learn from my misstakes!

bktheking · Jan 10, 2011

Re: '75 15 HP Johnson Seahorse

Do that stat, if that doesn't fix it then check your reeds, a constantly fouling SINGLE plug can be caused by a bent reed or something stuck in the reeds, don't ask how I know.

samo_ott · Jan 10, 2011

Re: '75 15 HP Johnson Seahorse

bktheking said:
Do that stat, if that doesn't fix it then check your reeds, a constantly fouling SINGLE plug can be caused by a bent reed or something stuck in the reeds, don't ask how I know.

$75? What a steal! I'd buy em all day at that price!

bktheking · Jan 11, 2011

Re: '75 15 HP Johnson Seahorse

That's why I scooped it, it's been rebuilt of course.

Tim Wagner · Jan 12, 2011

Re: '75 15 HP Johnson Seahorse

bktheking said:
That's why I scooped it, it's been rebuilt of course.

Hey, whos thread is this???
Anyway, yea it might have been 75 bucks but since then, I got 20 some bucks in an impeller, 15 bucks in the carb rebuild, 15 bucks in a fuel pump rebuild, 30 bucks in coils, 16 bucks in plugs (an more to come), 75 bucks in a control box from ebay and maybe 40 bucks in cables. Now its another 25 bucks for a thermostat, gasket and seal.
Not to mention the hours screwing around with it, emptying beer cans, dirty fingers and a teeny bit in heartache.
It does have a long shaft though which my boat requires so its all good.
But the long shaft does me no good with a fouled plug.
I am determined however and as soon as all the snow is gone and I can take the giant plastic cover off my boat Im gonna jump right on it!!

bktheking · Jan 12, 2011

Re: '75 15 HP Johnson Seahorse

He and I were referring to the $75 motor I bought that had the same problem, 1 plug fowling, ended up being a reed issue.

It's still your thread, it was a sidenote.

dazk14 · Jan 12, 2011

Re: '75 15 HP Johnson Seahorse

Let's start from the top....

"The compression is balanced"...
What is it? 125psi and it'll run on vegetable oil, 85psi and it'll need to be set up perfectly.

"I havent pulled the flywheel yet and checked the points and stuff "
This is really a must. PO may not have had that ability and a weak spark is very common when the magneto isn't right. This would be my 2nd guess as to the problem.

1st start with a simple link and sync and adjust the idle mixture leaner (clock-wise). As I recall, 3/4 turns out from lightly seated is the factory spec on that 1975 15.

"Im using NGK plugs gapped to 0.30 thousanths. (I forget what the part number was but it was the equivalent to the Champions)"
What specific NGK plug are you using. FYI- there is no exact cross for these motors. b7hs is close enough that it would not be an issue(in your application) if all else it good, but get the Champions.

"I had notices by pulling the cover off the head that there wasnt a thermostat in it."
A poor idle is a classic symptom of not warming up. If all else is good, the motor will still idle fine without a thermostat(seen lots of them), but put one in for better everything.

Now that we know the motor has been loading up, do a de-carb when you get it running well.

My direct notification is not working on this forum - maybe someone has a tip to fix it - but I'll try to keep an eye on this.

There's some real good 9.9/15 experience/knowledge on this forum, so you're in good hands, here.

F_R · Jan 12, 2011

Re: '75 15 HP Johnson Seahorse

I hate to tell you this, but I was an Evinrude dealer mechanic when that motor was new. Brand new ones fouled plugs too. OMC supplied a bunch of "fixes" for them and they still fouled plugs. As far as I am concerned, they never did fix them till they came out with CD ignition.

BTW, the thermostat was one of the fixes. That didn't work either.

boobie · Jan 12, 2011

Re: '75 15 HP Johnson Seahorse

Geez FR I was doing the same thing as you were doing when that mtr was new. I think OMC changed the cyl heads on them to try and fix the prob. But the ignition system on them wasn't worth a crap either. Some of the heads I changed did help tho'.

F_R · Jan 13, 2011

Re: '75 15 HP Johnson Seahorse

Yep, if you examine the design of those powerheads, the fuel/air mix enters one side, is deflected up toward the head by the piston crown, then across the head and back down toward the exhaust ports....Just like any crossflow 2 stroke. However on the 9.9 and 15, the spark plugs are at a sharp angle, faceing the fuel flow across the head. So if the fuel is not completely vaporized, any liguid gas splattering against the head runs right into the spark plug. The redesigned head has little dams around the spark plug to try to prevent that. It helped, but certainly didn't cure the problem. They still fouled.

Been there, done that, got lots of grey hairs.

Tim Wagner · Jan 13, 2011

Re: '75 15 HP Johnson Seahorse

dazk14 said:
Let's start from the top....

"The compression is balanced"...
What is it? 125psi and it'll run on vegetable oil, 85psi and it'll need to be set up perfectly.

97-top 96-bottom

"I havent pulled the flywheel yet and checked the points and stuff "
This is really a must. PO may not have had that ability and a weak spark is very common when the magneto isn't right. This would be my 2nd guess as to the problem.

When the snow thaws, Im on it.

1st start with a simple link and sync and adjust the idle mixture leaner (clock-wise). As I recall, 3/4 turns out from lightly seated is the factory spec on that 1975 15.

Ive actually gone this route. I made an index mark on the lower cowl where the mixture knob is and went an 1/8 turn both ways and wound up with the same results.

"Im using NGK plugs gapped to 0.30 thousanths. (I forget what the part number was but it was the equivalent to the Champions)"
What specific NGK plug are you using. FYI- there is no exact cross for these motors. b7hs is close enough that it would not be an issue(in your application) if all else it good, but get the Champions.

Yes, they are B7HS. The first set that fouled were L86C's.

"I had notices by pulling the cover off the head that there wasnt a thermostat in it."
A poor idle is a classic symptom of not warming up. If all else is good, the motor will still idle fine without a thermostat(seen lots of them), but put one in for better everything.

It does idle perfect with new plugs in it. After some run time and its done its little trick, its dead in the water.

Now that we know the motor has been loading up, do a de-carb when you get it running well.

When I winterized it, I finished off with a huge blast of sea foam in each plug hole.
Its gonna have a 4 month soak!!

My direct notification is not working on this forum - maybe someone has a tip to fix it - but I'll try to keep an eye on this.

There's some real good 9.9/15 experience/knowledge on this forum, so you're in good hands, here.

I know there is. For the winter, Ill tackle a few things and get some help here. Come spring, Ill be on it.
I appreciate the help and all the suggestions.

'75 15 HP Johnson Seahorse

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