Lost a cylinder at high speed '72 Johnson 50

[pbookey]

Hey all,<br />I was running my 1972 Johnson 50 HP motor at WOT for about 90 seconds and she abruptly lost power and ran extremely rough. Powered down and she died and would not start. I could get her to fire very briefly but she would only turn over a few revolutions and die. Pulled the boat out of the water and pulled the spark plugs. I've never seen this before. The gap on one of the two plugs had completly closed and the surfaces of the plug exposed to the combustion chamber were dinged up pretty bad. I looked inside the cylinder through the spark plug hole and with the limited vision could see some dings on the face of the piston. Tested the compression, got 120/60. The 60 came from the bad plug cylinder. Something is obviously wrong and I'm in the process of pulling the accessories in preparation for pulling the powerhead to take a look at the cylinders. I was wondering if anyone had experienced anything similar or had any thoughts on what the problem might be. Thanks.<br /><br />Pat

 

[rodbolt]

Re: Lost a cylinder at high speed '72 Johnson 50

yep<br /> it either scuffed a piston or suffered a ring siezure. its possible a wristpin bearing went through it. the cause is normally a bad pump or other cooling system failure or a lean carb due to gum/varnish. very solid old motors but I am not sure pistons are still availible. I have a 71 that is down with the same malady. took it on trade and never messed with it. its now in the pile outback

 

[pbookey]

Re: Lost a cylinder at high speed '72 Johnson 50

Pulled the head and looks like I blew a ring. Pieces of the ring were imbedded into the metal in the cylinder head. and the piston is pretty messed up and the cylinder walls scored. Haven't pulled the lower unit to take a look at the water pump yet, so don't know the cause yet. Carb was very clean with no deposit. I'll post what i find out.

 

[joblo33]

Re: Lost a cylinder at high speed '72 Johnson 50

You sure those are ring pieces, not needles from the needle bearing on the crankshaft? I've seen that bearing go and cause the piston to travel too far and smack the spark plug. The needles like to embed themselves into the walls too.<br />Looks like this:<br />

<br />Eric