'72 50 hp Johnson intermittent power loss at WOT

I have a 50 hp Johnson that has been helped a lot by advice here, but now we have a problem that has stumped us - the motor intermittently loses top end speed. For example, on a recent fishing trip three of us went upstream a little over a mile and the motor ran fine - planed out and ran along near 30mph which is its peak so far. We fished for an hour or so, then started it and went down stream and it would only go about half speed. It is like half of the throttle has been removed. This has happened several times where we went out ok and came back at only half speed. There is no roughness about it - no backfire - no hint of dying. It seems like it may be starved for fuel, but pumping the primer bulb has no effect. Also, spraying fuel in the carbs has no effect.<br /><br />Things we have recently done to the motor include the following: <br />1. Ignition - new coils, new powerpack, new plugs, all the electrical checks out according to the Johnson service manual specs. It shoots spark like blue lightning. Timing checks out OK at 4 degrees and 19 degrees. Looked under flywheel and timing disk seems to be moving as it should when the throttle is advanced.<br />2. Compression reads 160 on both cylinders<br />3. Fuel - new tank, new hose, new fuel pump, all new lines, fresh fuel, two cans of seafoam recently run through, carbs throughly and completely cleaned twice, carb kit with new floats installed, new reed valves. <br />4. Prop new.<br />5. No sign of overheating.<br />6. Throttle linkage, carb linkage, and remote control box seems all ok.<br /><br />We have racked our brains - what have we missed?

Re: '72 50 hp Johnson intermittent power loss at WOT

Do you have a fuel filter installed?

Re: '72 50 hp Johnson intermittent power loss at WOT

you could be loosing a cylinder due to clogged carb or to much oil in gass .use a L77JC4 . plug gapped at .040 . the factory plug is a non gaping plug and will fowl easy with a rich running motor or a lot of idling . papasage

Re: '72 50 hp Johnson intermittent power loss at WOT

Thanks papasage and Reel Appeal for responding. We just put a new fuel filter in the line going into the fuel pump to reduce the possibility of anything getting into the carbs after removing and cleaning them twice. Have not run the motor yet since putting the filter in. The oil in the gas is Pennzoil and carefully measured. The plugs are L77JC4 gapped at .040. Losing a cylinder due to a clogged carb we have considered and I guess remains a possibility but we have been over them very thoroughly including all jets, orifices, etc. They really shine. If there is a clog or malfunction in there we cannot locate it. Is there anything we can do next time it acts up to diagnose a clogged carb?

Re: '72 50 hp Johnson intermittent power loss at WOT

papasage could be on something there. those big twins run real good on just one cylinder. i had one(still do, technically) that would do as you state, till it puked a rod.<br /><br />check those carbs, THOROUGHLY

Re: '72 50 hp Johnson intermittent power loss at WOT

sounds like you may be loosing a rod berring. that is what happened to my 40hp. minor performance problems untill it just quit and siezed up after knocking a hole in my crankcase.

Re: '72 50 hp Johnson intermittent power loss at WOT

yau say you've tried squirting mix in the carbs. Are you saying you've done this to each carb in turn and it makes no difference?? Have you noticed any strange noises coming from the engine? Have you checked the gear oil for contamination?

Re: '72 50 hp Johnson intermittent power loss at WOT

just spray gas oil mixture in the throte of the card and see if it picks up or dies out . some times bumping the chock will tell a difference but that will add gas to the starving cylinder and and to much to the good cylinder . so sprey into the carbs and that will definatley tell the difference . papasage<br /> P.S.if can git it to run right bring it to so. ga. and i will help you with it. i have a 50 hp and have ben thrugh one like it .

Re: '72 50 hp Johnson intermittent power loss at WOT

Thanks everyone very much for the ideas. Will try the spraymix in the carbs again this evening. When we tried before it seemed to make no difference. That was before taking the carbs off and cleaning the second time. <br /><br />Phatmanmike and Have tools the mention of flying rods gets my attention. I do not know small motors well enough though to understand the connection to these symptoms? Not enough lubrication getting into one cylinder?<br /><br />Stairlft - no strange noises. Motor sounds good to me - smooth as can be. Gear oil is in great shape. Getting the hydroelectric shift working was a whole other painful, but ultimately succssful, saga that brought me here several times before seeeking insight.

Re: '72 50 hp Johnson intermittent power loss at WOT

Mystery apparently solved. We took it out tonight to fish and try spraying premix in the carbs. Same thing happened - went out about two miles smoothly but not quite full power. Then, on the way back it went down and went down decisively. Turned out that the piston in the top cylinder had disintegrated, and apparently was faulty since I got the motor because this has been a problem from the first. It must have been deforming when it got warm and then resuming its normal shape when it cooled down. Does not seem to have caused much damage to the cylinder wall. Any ideas on possible causes for a piston to come apart at the top? It was all beat up on the top.<br /><br />The people I got it from seem to have lied completely about having rebuilt the powerhead. Does not look like it has been seen by human eyes in a very long time. It is the one of the few areas of the motor that I had not seen yet. I am surprised it showed such good compression when I got it, but not anymore. Thanks to everyone for your help. It looks like we have our work cut out for us for a while.

Re: '72 50 hp Johnson intermittent power loss at WOT

lack of fuel=lack of oil, badly functioning carb could cause a cylinder to go down very quickly. You should rebuild them both after the powerhead is sorted.<br />good luck

Re: '72 50 hp Johnson intermittent power loss at WOT

I guess we need to figure out why the piston came apart to avoid it happening again. It is really hammered on the top, especially around the edges. The top ring is visible looking down from the top. The damage is slightly worse next to the exhaust port. There are also lots of indentations in the head due to flying chunks of piston. Do these have to be repaired (and if so, how?) or the head replaced?<br /><br />The lower piston head shows no damage but there are indentations in the lower head which I guess mean that the lower piston has come apart sometime in this motor's past life? <br /><br />From what has been said and what I can read of other posts it looks like the best guess for cause is lack of lubrication due to clogged high speed jet in the top carb? Since we have already cleaned the carbs thoroughly twice, is there any other possible cause of fuel obstruction? Or, since this motor has done this since I got it this Spring, was the piston perhaps already damaged and continued to decline despite getting the carbs cleaned up?<br /><br />I see that another frequently mentioned cause of piston damage is predetonation. Wouldn't that affect both pistons equally? Also, wouldn't that affect the middle of the piston rather than the edges? Can I rule that out since only one piston shows damage?<br /><br />Any other possible causes that need to be investigated?