1975 Johnson 15 HP low speed idle problems.

[Boatist]

Motor ran perfect for 20 years but when the TCW 3 oil came out started having trouble fouling plugs. Trolling all day may have to change twice a day.

Ran OMC Engine Tuner to decarb the engine and also cleaned the carb and replaced the float and float needle. Blew out all the passages with carb cleaned and compressed air.
Did a compression check and got 120 and 125 psi.
Replace points and condensers and also one weak ingition coil.

Motor now starts and runs great but has one small problem Left.

Each Cylinder runs best at a different low speed carb adjustment. One runs best a little richer and one a little leaner. Range where both cylinder fire is very narrow.

Anyone have any ideas what I can do to get them closer to each other??

 

[tashasdaddy]

Re: 1975 Johnson 15 HP low speed idle problems.

this is one of the best threads on the forum

http://www.iboats.com/bbboard/bbBoard.cgi?a=viewthread;fid=28;gtid=818798;gpid=818802

 

[mattjeanes]

Re: 1975 Johnson 15 HP low speed idle problems.

My 1974 model has the same issue. The instructions given in the above link are good but I was still bothered by one cylinder running richer than the other. I did a complete stripdown and rebuild with rebore a couple of years ago but the problem persists. I spent ages checking for air leaks and even replaced the reeds although they looked just about perfect. Eventually I gave up and now I just live with it as it is. I'd love to hear if you find a solution.

 

[mattjeanes]

Re: 1975 Johnson 15 HP low speed idle problems.

This is my original post re this problem: http://forums.iboats.com/bbBoard.cgi?a=viewthread;fid=28;gtid=731336;gpid=731343#gpid731343

 

[Boatist]

Re: 1975 Johnson 15 HP low speed idle problems.

Thanks for the links both very good. I had not thought about the Fuel pump. As I recall the fuel pump is actually run by presure from the top cylinder to push the diaphram. I can see where a leak there could make a difference.

I also wonder if 5 pounds of compression could make the difference?

Mattjeanes
Did you have your problem before the bore and rebuild or only after?

Another thing I am thinking could cause it is if one cylinder is running hotter than the other.

I had to change the thermostat 3 times on my motor but each time because it was stuck open and running too cold.

Thanks and any ideas please respond.

 

[OBJ]

Re: 1975 Johnson 15 HP low speed idle problems.

Go back and recheck the point gap. Make sure each spark at each cylinder will jump at least a 7/16" gap with a hot lighting like spark.

You should be using a Champion UL81C spark plug set at .030. Setting the gap at .040 will help the idle.

At idle, does running the low speed adjustment in or out make a difference in the engines idle? The needle should be gently seated and then backed out 1 1/2 turns for the initial adjustment.

Engine temp.....at idle, you should be able to hold your fingers on the flat area on top of the engine block for several seconds before having to remove them. Can you?

 

[mattjeanes]

Re: 1975 Johnson 15 HP low speed idle problems.

Just before the rebore it was only firing on one cylinder - very low compression

Before that it was very poor idling at any setting of the mixture

 

[Boatist]

Re: 1975 Johnson 15 HP low speed idle problems.

OBJ
I have very good spark to both cylinders, at slow idle will jump from the center to the base and is a good strong spark. I pulled the flywheel and adjusted both points making one closer and one wider then the other way and has no effect. I also changed both points and condenser then swapped from side to side but no change.

Engine came with and the factory manual calls for L7J. Champion changed that to a RL82C or without the resistor a L82C. Motor ran perfect for more than 20 years with that plug. When TCW3 Oil came out and I started having a lot of plug fouling I talk to factory reps and he said if I only using for trolling a hotter plug would work better. So I moved to a L86 C and then a L90 C which helped some but stilled fouled.

Plug fouling solved by useing OMC engine tunner. I think over the years motor carboned up and when the TCW 3 oil came out it started to cut some of the carbon and that fouled the plugs. Now no longer fouls plugs and all the plugs work the same.

Temp runs around 140 as best as I can tell. After warmed up can hold a finger on the top of the block for about 4 seconds.

The motor actually runs pretty good, starts by the second pull when cold and first pull after warmed up.
Only problem is neither cylinder can be adjust to it best spot without the other cylinder dropping out.
When adjusting low speed idle turning one way the top cylinder drop out the other way the bottom cylinder dropps out.
By pulling one plug wire at a time can adjust the other cylinder for best power and note position then do the other plug and note position.
To get both to run at the same time each is running at a position down on power. The range where both run is pretty narrow not like it was in the old days.

Since I use this motor as a Kicker on a 21 foot Ocean going boat I need to know it in top condition. I do not want to be 26 miles out at Cordell banks and have something happen to main motor and the 15 not start an run.
Main motor has never failed but with murfey around do not want to take any chances with the family on board.

Going back thur my records never noticed this problem before I ran the OMC engine tuner. Maybe I should try doing one cylinder at a time and see if that will bring them closer together. Have never tried Napa Seafoam either.

Last resort would be a 20 HP Honda on the other side or replacing this motor.

Anyway thanks for the ideas.

 

[F_R]

Re: 1975 Johnson 15 HP low speed idle problems.

For whatever it's worth, we had a LOT of trouble with those motors when they were new. They were heavilly promoted as sailboat auxillaries. Trouble is you couldn't keep one running on a sailboat. OMC came out with a number of kits that were supposed to cure the problem. We would install the kit and send the motor out and it would be back the next day. Plug fouling was the main problem. The kits included different carburetors, different magneto stuff, different cylinder head, etc. None of them worked. After awhile, word got out to the sailboat crowd and you couldn't give one of them away to a sailboater. Problem solved...in a way.

It is my humble opinion that the angle of the plugs points them directly at the incoming fuel/air charge which blows any liquid fuel or carbon particles right into the plug. In fact the head "fix" had a little dam around the plug hole to stop the flow of raw fuel right into the plug. Didn't work. Poor fuel control seems to be another part of the problem, which you are experiencing right now. The carb replacement fix didn't work either.

 

[cobra 3.0]

Re: 1975 Johnson 15 HP low speed idle problems.

I have a 74 15 hp Evinrude that also runs a little crappy at idle, but goes well at all other speeds. I have it on a 12 foot toon and use it to put-put around on our lake at the cottage, so it's really not that critical for me.

For your situation where it must work and not leave you far from shore. I'd say it's time for a new(er) engine as it's had it's day in the sun!

 

[BF]

Re: 1975 Johnson 15 HP low speed idle problems.

Interesting info F_R... I have an '82 15hp that idles the best of any of my outboards. (even better than a smooth 6 hp).

Maybe the idle prob was cured by the better ignition in later years (?). Or a different plug angle (?). Or maybe mine is the exception? I thought '75 was the first year of the more modern 15 hp model?

 

[Boatist]

Re: 1975 Johnson 15 HP low speed idle problems.

F_R, Cobra 3.0, BF, Trashasdaddy, Mattjeanes, OBJ.

Thanks to all for your time and ideas. I have some more things to try thanks to you guys. Do you think OMC engine turner or Seafoam works better to de carb.

First thing I am going to try in spring is to decarb the cylinder with the lower compression and see if can get it up to 125 also. I do not think I had this problem before I decarbed with OMC engine turner. Running the engine tuner did bring up the compression and fix the plug fouling problem I had.

Also did not think about a fuel pump diaphram problem so will check that.

Last want to see if I can tell is one cylinder running hotter than the other. I know I have some water passage corosion near the thermostat that I did clean some when replaced the thermostat. A difference in temperature in one cylinder would require more or less fuel. Probably need to buy a IR heat gun. Always wanted one anyway.

Last hope is the BRP Factory Reps, at the boat shows since they are the same old OMC reps that came to the shows for years.

Been a great motor for over 20 years and the last 10 have had some minor Problems but still starts by the second pull and runs all day. Use mostly for slowing our drift offshore when the winds kick up over 25, back into the wind and swells. Also use for some Salmon trolling in the rivers. Still works well but idle adjustment only has a small range where both cylinder will fire at idle.

Thanks again.

 

[tashasdaddy]

Re: 1975 Johnson 15 HP low speed idle problems.

take a look at this.

http://outdoorsbest.zeroforum.com/zerothread?id=2589