Johnson Engine Issues ****HELP HELP

[waterdogMP]

Hey guys,

So I've got an issue with an old engine and I was hoping someone had some knowledge on here as I'm not very knowledgeable when it comes to boat engines. The easiest way is to just tell the story. First time the boat has been ran since I've had it and I don't know the last time it was ran before today.

The engine is a

1979 Johnson 140TL79R 140hp Outboard.

It started fine and ran fine at idle and throttle while in neutral. I put it in gear, gave it throttle and it took forever to get up to speed to plane out. Once planed it ran fine and was fast for about a minute then it sputtered and died. I was able to start the engine back up but the boat wouldn't go over 5 mph. I took off the cover and noticed a wire supplying power to the upper right coil pack was broken. I spliced a new wire into it, started the engine back up and it still wouldn't go over 5 mph. When I give it full throttle the drive gear fully engaged and the carbs opened up to full throttle but it just wouldn't go. Also, now in neutral if I give it throttle it sputters up before it gets to 4500 rpms and then will run ok. Fuel is new and mixed at the proper ratio, the boat isn't overloaded, lower unit is full with good oil, good batteries, water impeller pump is working fine. My question is what do you guys think is wrong? Should I clean the carbs and replace the needle valves and gaskets? I've heard that fuel could evaporate and varnish the needle valves. Also, it has the factory coil packs from 1979 on it, should I replace those?

Please help guys, I need to get back on the water soon. Any help will be greatly appreciated.

[fishinwrench]

Before you spend a dime on carb kits, coils, ect. You need to do a compression test on all 4 cylinders.

It probably does need a fuel system overhaul and some ignition/charging system work, but there's also a good chance that your 1979 140 has some sticky rings or cylinder damage.

Always check compression first, then ignition system, then fuel system... whenever diagnosing engine running problems

Where are you located ?

[Ketchup]

carb clean, possible rebuild would be where i would start, since it has sat so long.

[waterdogMP]

Thanks guys, and this may be a dumb question, but how do I do a compression test? Oh, and I'm located in Springfield. Thanks for the proper order on diagnosing outboard engine problems fishinwrench. Thanks ketchup.

[Ketchup]

remove spark plug wire. remove spark plug. screw in compression test guage into spark plug hole. turn engine over for a few seconds. look at guage to see what compression is. Repeat for each cyclinder.

Write down compression after each test to compare. Test each cyclinder twice, one after another without removing guage from each.

[waterdogMP]

Ok thanks. I read that compression should be within 10 percent of eachother, but what is a good compression for that engine? Sorry to ask so many questions but I'm looking online and can't find answers

[fishinwrench]

You'll probably have 130ish for highs but compression PSI above 90 is OK as long as all cylinders are within 15% of each other.

[waterdogMP]

Thanks, you guys are awesome. When I get it done I will update you and let you know what I discovered. Thanks again!

[waterdogMP]

Alright guys, update time.

So I did a compression test and I will explain my findings the best I can. When standing behind the boat looking at the engine, the right top and bottom right cylinders both had 125 psi, both plugs need replaced as the electrodes were almost nonexistent. Top left cylinder 125 psi, plug in same condition. bottom left cylinder 75 psi (I checked multiple times for gauge fit and it was good, still 75 psi), the spark plug was junk. The electrode was burnt so bad it was nothing but a large ball of carbon, it melted the side and center electrode into 1. I also had no fire going to the plugs on the left side. I checked the power pack and it is bad on the left side.

I am ordering a new power pack, and already have a new set of plugs. I used a mechanics camera to look inside the cylinder on the bottom left, there is no scarring on the piston or cylinder wall, there does seem to be carbon build up on that cylinder more than the others (some buildup on all cylinders).

I am wondering what to do next, also wondering if putting seafoam in the cylinders and letting them sit for awhile will clear up some of that carbon and maybe bring the rings back up on the low psi one.

[fishinwrench]

This is why I said not to go buying stuff until you did a compression test.

You need to pull that port side cylinder head and inspect the cylinder and piston. You've probably broken a piston ring and the power head needs to be rebuilt. No sense in buying a power pack or spark plugs at this point !

Do not attempt to run the motor, not even for a few seconds, until the power head has been repaired, or you risk throwing a rod out the side of it and turning the motor into scrap metal.

There is a slight chance that a head gasket has failed...but it is a "slight" chance. Pull the head.