15 HP Johson - 1988 Coil Check?

[gamarines2]

I have an old very reliable Johnson 15 that seems indestructable....been enjoying it for many years in very harsh duck hunting conditions.

Last time out, I had an overheat....thought my impeller had failed. Even after engine cooled, could not start it up again.....lotsa almost starts, but exhausted after many pulls, I broke my pride and flagged down a tow to the boat landing.

First thing was to replace water pump. I was surprised to learn that the pump was in excellent shape, not one piece of rubber shedded from impeller. Replaced it anyway. Ready, set, pull, pull, pull......and to use Yukon Cornelius' famous line......"nuttin".

Well, it did start after I pulled the plugs and cleaned 'em real good (a little wet, but didn't look terrible). Water came shooting out of like a brand new engine. so impeller problem fixed.

Shut her down, and could not restart warm, or several hours later cold. Every once and awhile it will fire but won't get and stay running. Still think it's fire. But not yet ruling out fuel.

Is there a way to test coil resistance/output?

Any of you pros eliminating fire based on above and think I should go with fuel troubleshooting? Before you go down that road, I will admit I added some Seafoam to fresh fuel for my first fall run (the day this failure first occured). I have read on this forum that Seafoam actually reduces flash point, so my first trick will be to try fresh fuel/oil mixture.

Thoughts?

 

[boobie]

Re: 15 HP Johson - 1988 Coil Check?

First thing to do is check your compression. Second, is make sure your spark will jump an open air gap of 7/16'' on a spark tester.

 

[OptsyEagle]

Re: 15 HP Johson - 1988 Coil Check?

I agree with boobie. Most likely it is not your coils since you have two of them and it is very unlikely that both would fail you at the same time. These motors will start and run fine on one cylinder, albiet at a very reduced power. So check the compression and spark. If those are OK, I suspect your seafoam probably dislodged something that is now in your carburetor, clogging things up.

 

[gamarines2]

Re: 15 HP Johson - 1988 Coil Check?

OK. Did a compression test. 70 PSI in top cyl; 85 in bottom.

Nice bright spark at 7/16".

Fuel now?

 

[1Crazymechanic]

Re: 15 HP Johson - 1988 Coil Check?

Sounds like fuel would be a good place to go. Has the engine sat for a while?

 

[OptsyEagle]

Re: 15 HP Johson - 1988 Coil Check?

Well from what you have said it is probably a dirty carburetor or perhaps those compression numbers. One usually doesn't like to see a larger then 10% difference in compression between cylinders. To narrow the choice down a little you could mix up some gas and two stroke oil into a spray can and try and squirt some into the barrel of the carburetor and give it a pull. If it runs for a short time on the spray then I would say it is a dirty carburetor and if it doesn't it is possibly a compression problem.

For compression, you could pull the cylinder head and look for a blown gasket or anything not right with the cylinders. I would still think that you could get it to at least run badly with those compression numbers but I am just putting some more ideas out there. If you do look at the head gasket be sure to re-surface the head well and of course use a new gasket whether the old one blew or not.

 

[gamarines2]

Re: 15 HP Johson - 1988 Coil Check?

Sat all summer; early spring fishing, then sat for 8 months. But when taking out of storage, started up fine (on fresh fuel +seafoam), and ran like it always did until it overheated on the big lake (on a windy day, to boot). It has started and ran poorly since I replaced the impeller, but can't get her running again. Good fire.

Gonna try fresh fuel without seafoam. Probably will rebuild carb if this doesn't work.....is that the right next step?

 

[gamarines2]

Re: 15 HP Johson - 1988 Coil Check?

How does the fuel pump work. I have a bulb primer on the external tank, but now, it almost immediately loses pressure in the line when the motor starts (very briefly).

 

[OptsyEagle]

Re: 15 HP Johson - 1988 Coil Check?

Sure try the new fuel but if it doesn't run then I would try the fuel spray right into the carb barrel first. The fact that you had an overheat and with your compression numbers, they make me weary of the integrety of your head gasket. However, it never hurts to clean and rebuild the carb, at any time.

The primer bulb will not stay hard once the motor starts running, so I wouldn't worry about that. The fuel pump works on pulses from the engine to suck fuel into the carb. It actually sucks and blows depending on how the cylinder is moving but the check valves in it ensures that fuel only flows one way. If your fuel pump was malfunctioning you would either get a fuel leak, which you could see, or a flooding condition in one cylinder only. The motor would still run, just not well. By priming the bulb, you are manually pumping the fuel into the carb. so even if it didn't pump anymore, the motor would run for a few minutes until the carburetor bowl emptied of fuel. So your problem is unlikely to be the fuel pump, in my opinion.

As for the seafoam, many on this board have done a solid decarb by adding large amounts of it to the gas. I prefer to decarb by using the sprayable seafoam or OMC Engine Tuner spray and spray it liberally right into the carb. I then really spray it until the motor stalls, take out the plugs and spray it right into the cylinders, turning the flywheel by hand to get really mix it in there. Leave it for 30 minutes and then start it up and spray some more into the carb. Then I take her out in the water and run her hard. This is to blow out the carbon and any remaining seafoam and also because I just love running a 15Hp motor, hard.

Anyway, I have done it in the gas before and I do add seafoam (about 1.5% by volume) to every tank of gas with no problems. I worry about dislodging chunks of crud into my carb but I suspect the chunks, if they are there, would get into it eventually anyways. You should make sure you clean and blow out your fuel filter at the end of every season.

 

[kbait]

Re: 15 HP Johson - 1988 Coil Check?

With your compression numbers + overheat issue, I'd change the head gasket and check the mating surfaces for flatness (I use a flat file). If still no go, squirt a bit of premix down the carb barrel and if it runs ok for a few seconds then dies, look toward a fuel delivery issue.. likely gunked carb. Good luck w/your awesome motor!

 

[gamarines2]

Re: 15 HP Johson - 1988 Coil Check?

With your compression numbers + overheat issue, I'd change the head gasket and check the mating surfaces for flatness (I use a flat file). If still no go, squirt a bit of premix down the carb barrel and if it runs ok for a few seconds then dies, look toward a fuel delivery issue.. likely gunked carb. Good luck w/your awesome motor!

Kbait, the Vikings look alot better with Ponder; gave my Packers a pretty good game last week. And oh, us Packer fans know about lean years, even with Brett Favre at the helm......remember the Dan Devine and Lynn Dickie years in the 70s?????

 

[gamarines2]

15 HP Johnson.....more wonderful discoveries

15 HP Johnson.....more wonderful discoveries

Alright. Rebuilt carb and replaced all fuel lines. Carb didn't look all that gunked up, and lines were in surprisingly good shape. Rebuilt and replaced lines anyway. I got to thinking more about my original problem. I had an overheat and assumed it was my impeller (never before replaced it). But when I went to replace it, the impeller and water pump housing were in impeccable shape. So I ask myself....self, why did my boat overheat?

So I went to check the thermostat. Honestly, I never even imagined that a motor this small would even have such a device. Top two bolts came out with no problem, almost TOO easy. Bottom one twisted off easily and remains in the cylinder head. First question. Any suggestions for removing a stainless bolt? Use traditional come out tool? Will that work, or should I even try?

What came out into my hand when the housing cover came out? A severely rusted couple of springs, and what looked like what have been at one time a functional thermostat. Will obviously replace that, but now, I wonder if there aren't some parts of that deteriorated mess clogging up a water passage somewhere. How do I even begin to run that to ground? Any suggestions will be greatly appreciated.

 

[gamarines2]

Re: 15 HP Johnson.....more wonderful discoveries

Re: 15 HP Johnson.....more wonderful discoveries

ALright, some complications overcame on the thermostat housing bolt, removed head, drilled out old bolt, tapped in some new threads.

Compression numbers with new head gasket went up to 91 and 82 PSI. Started right up and ran great without load. Can't wait to get it back on the water.....here fishy, fishy, fishy.....

Thanks for all the advice everyone, this forum ROCKS!!