1984 50 hp johnson won't get to full power

plywoody · Aug 12, 2002

My 84 johnson 50 hp won't get to full power. It will plane my 17' aluminum boat, but not without a struggle, and I don't have a tach, by my guess is it is getting to about 3k to 3500 RPM max. This just happened suddenly, as I was cruising along just fine when the power dropped (?) I've tried the usual, new plugs, using a different tank with new (and higher octane) gas, checked the carb linkage, and butterfly position of the carbs (fully open)... Anyone have any suggestions? The local repair shop can't get to it for 2 weeks, and I am sure it is something simple, as in all other ways it sounds fine.

mrc · Aug 12, 2002

Re: 1984 50 hp johnson won't get to full power

Pull the spark plugs. 1) Check condition checking for one that might be steam cleaned indicating water entering the cyclinder. 2) Check for spark while all the plugs are out by grounding all the plugs outer shell to the block while cranking. 3) Check compression on each cylinder. Fix for 1) If you have indications of water entering piston, most likely cause is the head gasket, and easy and cheap fix. Get a chart indicating head bolt torque sequence and remove bolts from head in the reverse order. Such a chart would be in the clymers or seloc service manuals for your motor. Check condition of piston, cylinder and head. Place new head gasket on block, replace head. Coat head bolts with anti-sieze compound and torque them down in the charted sequence. Make sure to use torque wrench and once bolt is finger snug, go 1/4 turn per bolt in sequence until final spec torque is reached. Fix for 2 (no spark) Swap plugs to see if spark loss follows the plug or the wire. If the wire it is something back along through the ignition system. Could obviously be the wire. Situation with 3) What you are looking for with the compression check is that the numbers are roughly equal across all cylinders, if one is much different from the rest you might be in trouble. Of course it could be none of the above, plugged carb jet comes to mind. Hope this helps, Mike

plywoody · Aug 12, 2002

Re: 1984 50 hp johnson won't get to full power

How obvious is this "steam cleaning" effect? The insulator on one plug is definitely a different color from the other--one is virtually black (but not wet), and the other is kind of light brown. Also, I didn't mention in my original post, but the engine seems to want to accelerate on initial power up--you can hear the high speed kick in for maybe a second or two at WOT, then drops back down immediately. Thanks in advance

mrc · Aug 12, 2002

Re: 1984 50 hp johnson won't get to full power

The "steam cleaned" effect is on the part of the plug inside the head (ie around the gap) Normally this area would be a bit oily and dark. If it is dry and clean... water in head (or no fuel) Mike

BKool68 · Aug 12, 2002

Re: 1984 50 hp johnson won't get to full power

The black plug sounds like your problem cylinder. Try switching the coils and plug wires and install new plugs again. See if the dirty plug moves. If it moves you will the coil or wire is your problem. If the problem stays I would check to see if a float needle is not seating in a carb causing fuel to flood that cylinder and foul the plug.

plywoody · Aug 12, 2002

Re: 1984 50 hp johnson won't get to full power

The top of the insulator around the electrode on one of the plugs is definitely "cleaner" than the other plug, in that it is almost, but not quite, the original white. That could well be the problem. I don't suppose that the torque spec, and the torque bolt sequence, is on line anywhere? I did order a manual online, but not sure when I will get it. I borrowed a neighbor's compression tester, and both cylinders registered 150 (I'm assuming that's psi, although their was no label on the dial, and that sounded a bit high to me) They were both the same, though. Wouldn't a water leak into a cylinder reduce the compression in that cylinder? I also checked the spark as you suggested, and the spark was easily visible and seemed strong on both cylinders. Thanks much, John W

mrc · Aug 12, 2002

Re: 1984 50 hp johnson won't get to full power

Swap plugs and run it for a bit and see if the other one gets "cleaned" If so, you have water leaking into the cylinder. Mike

plywoody · Aug 14, 2002

Re: 1984 50 hp johnson won't get to full power

Ok, back to the old thread. Swapped plugs, took it out, and tried hitting the primer button at the ignition while in full throttle. Engine immediately responded going to full RPM. Came home, Got a carb spray cleaner, took the carb apart, replaced the floats and the valve and seats for the floats, and sprayed the heck out of the jets, and rodded them out with fishing line. BTW, the dirty plug was indeed cleaned when it was swapped. Could that be a lack of fuel problem, or do you still think there is water getting into the cylinder? Thanks in advance. John

mrc · Aug 14, 2002

Re: 1984 50 hp johnson won't get to full power

Have you had a chance to try it in the water since the carb cleaning? I think you probably still might have a head gasket leak, but the plug might also have been cleaned during the carb cleaning process. Mike

plywoody · Aug 14, 2002

Re: 1984 50 hp johnson won't get to full power

Just got back from the water, and even after carb cleaning, symptoms are exactly the same. Not precisely sure, but fuel consumption seems excessively high as well. There was no noticable problem with the jets, and I cleaned them as good as I could, but not sure. I'll see if I can't get a head gasket tomorrow, and replace it. It can't cost too much, and is not too big a job, so what the heck. Thanks again, John

BKool68 · Aug 14, 2002

Re: 1984 50 hp johnson won't get to full power

If the extra squirt of fuel from the primer got her going it sounds like a fuel restriction still. You need to find your high speed jet and run a piece of wire thru it along wih the cleaner. Replacing th head gasket won't hut as you can check the carbon build up and cylinder scoring. Good luck.

plywoody · Aug 15, 2002

Re: 1984 50 hp johnson won't get to full power

OK, I pulled the head, and both cylinder walls seem to be in perfect shape (no obvious scores, clean and polished, etc) On the top cylinder, the piston top is absolutely clean--no deposits of any kind anywhere. On the opposite head, in the recess just below the spark plug opening, is a sticky, gummy deposit. On the bottom cylinder, there is some minor carbon deposits on the top of piston--clearly visible, but does not appear to be very severe. On the opposite head, the head is absolutely clean, with no deposits of any kind whatsoever. The head gasket itself does not show any obvious signs that there is a water leak. If there is any leak at all, it is very minor. Any comments?

mrc · Aug 15, 2002

Re: 1984 50 hp johnson won't get to full power

Was the top piston the one that was cleaning spark plugs? If so, I think you had/have a minor water leak into that piston. Mike

plywoody · Aug 15, 2002

Re: 1984 50 hp johnson won't get to full power

yes it was the top piston that was cleaning the plugs. I cleaned it up, put in a new gasket and have it all back together, and am going to the water to check it out. thanks Thanks

mrc · Aug 15, 2002

Re: 1984 50 hp johnson won't get to full power

Remember to ReTorque head after test run.

Mike

plywoody · Aug 16, 2002

Re: 1984 50 hp johnson won't get to full power

Thanks, I will. But it sadly did not fix my problem, as the symptoms are precisely the same. I've checked all fuel connections, and pump output. The pump puts out enough gas to put me in the poorhouse in a hurry if the engine consumed that much. I don't know what it could be besides the carb. Is there any secret to cleaning the jets that I need to know? The end of the jet that goes into the bottom of the float seems to be clean. I'll try wire, but all I had at the time was 20 lb fishing line, and I rodded it with that. I can't see an easy way, without mucking up the soft brass, to remove them. Is there a secret to this? The opening to the bottom of the jet is slighly larger in diameter than 20 lb fishing line, is there a similar restriction at the top of the jet, and is there a way to be *sure* it is clean?

mrc · Aug 16, 2002

Re: 1984 50 hp johnson won't get to full power

I'm sorry it did not fix your problem

The next guess would have to be Carb or the reed valves. At this point I'm out of my depth, so I'll let one of the real mechanics on the board take over. Mike

plywoody · Aug 17, 2002

Re: 1984 50 hp johnson won't get to full power

Took the carbs all part again and tried once more. As a sidenote, I was pretty curious that the two carbs in this engine are quite different. One has much narrower throat than the other, and my stainless steel wire that I got to clean the jets would fit in one of them, but was too wide to fit in the other. At any rate, after playing around poking and spraying and such, a little (1/8" dia or so) glob of white crystalized junk came falling out of one of the carbs--not sure exactly where it came from, but it came out. Thought that might be a good sign, so I put the motor back together and took it out...And low and behold it runs beautiful! Thanks to everyone for their help!!! John W

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