'82 Johnson 70hp - One cylinder down when cold

[kofkorn]

Hi All,

I have a 1982 Johnson 70hp that I picked up last year for cheap. I didn't get a chance to get my boat running until later in the summer this year. The first few times I took it out, mainly after noon time, it ran flawlessly. It idled well and pulled well all the way from idle to WOT. No problems starting.

More recently, I've had the chance to take it out earlier in the morning. When starting cold, it starts and idles, but the idle speed will slowly drop down to a near stall. I can get it out on the water, where it becomes instantly obvious that it's only running on two cylinders. I pulled the airbox cover off, and I determined that it's the top cylinder that is missing by placing my hand in front of the carbs to richen the mixture one by one. When I do this to the top carb it makes no difference to the engine speed.

I have pulled the top plug and found that it is wet when it isn't firing. Also, spraying a fuel/oil mix into the top carb doesn't make any difference. The plug on the top carb, other than wet, looks near perfect, with a very light brown color to it.

Now when I'm on the water, I can move around at idle for about 4 or 5 min, and occasionally, you can hear/feel the last cylinder kick now and again. The longer it goes, the more often it kicks. Eventually, the top cylinder kicks in and the motor runs perfectly for the rest of the day. No issues shutting down and restarting. I can let it set for 2 or 3 hours and come back and it will run fine.

Next morning though, and it starts all over again.

I checked the compression when I bought the motor last fall and had 125psi on all three cylinders, so compression shouldn't be an issue.

I've read numerous issues online about coils that drop out when they warm up, but i haven't found any other situations where the cylinder runs after warming up. I'm currently on a vacation away from my tools, so I can't measure the coils/power pack/trigger to find out where the problem is. I was hoping that someone on here may have run across this in the past.

Any help would be appreciated.

Thanks!

 

[flyingscott]

That sounds like a powerpak issue.

 

[oldboat1]

Interesting post (and hints)! I have run into something similar, but it was with an Oldsmobile -- had to pull off the distributor cap on rainy mornings and use a hair dryer on it. As you are on vacation without tools, I wonder if the old hair dryer trick might work (aim it up under the flywheel, along with hitting coils and the power pack.) Wouldn't that be the darndest thing! As I recall, the Oldsmobile wasn't floating in water so be careful.

 

[kofkorn]

That sounds like a powerpak issue.

That would be convenient. Its one of the easier things to change. I'll try swapping the coils around first to make sure that it isn't a coil issue, but I tend to agree.

As to the hair dryer, I tried something similar by taking the cover off the motor and turning it so the coils were in the sunlight. Unfortunately, the way it's docked, i couldn't get the powerpack in the sunlight, and only the back sides of the coils.

This also points more to the powerpack.

Thanks for the comments!

 

[James R]

If you got it for cheap then you probably have enough left to invest in a manual. The manual will provide enough information to check the ignition system. As a CHEAP alternative try CDI electronics on the net for the info. Sounds like a good enough motor to qualify for a manual..

 

[kofkorn]

So I checked the resistance of the timing coils and found that they were all consistent. I also used my DVA to check the timer base output and found that the voltages on all three timers were consistent as well. I checked the output voltage on the powerpack and had more variation than I expected. I replaced the powerpack and the motor is up and running well.

Thanks for the help!