Confused

[The maddhatter]

Ok so first off Thank you for this forum I've used it for numerous repairs over the years. This time I can't seem to find out what is going on.
working on a 1982 Johnson sea horse 75 hp 3 cylinder sn j5683939. Model number j75tlcnb
so I've cleaned and set carbs checked compression 115 cyl 1 117 cyl 2 115 cyl 3 great spark on all 3. Idled great for a while but was slow to plane out (I would have to trim way up and then bring it back down to get it to speed) . Yesterday I was fishing and it acted like I dropped a cylinder ( started idling rough and vibrating. Today I went back through everything checked compression ,spark,fuel, so on and so forth while running it acts like it's not running on the top cylinder. If I cover the carb while running it doesn't affect the way it runs. If I cover the bottom or middle cylinder it dies. So my question is what am I looking for at this point?

 

[jimmbo]

If it still has spark and compression it might be a plugged carb. Not good as no gas means no oil. It might also be a bad upper crankshaft seal.

 

[Silvertip]

Put some pre-mix fuel in a spray bottle and spray it into the top carb. If it picks up, you have a fuel problem. If not, you have a spark problem or it is possible the piston on that cylinder decided to part ways with its "skirt". You might also look at the side of the engine block in the area of the top cylinder to see if the skirt punched a hole or created a flap that is pushed outward. This looses crankcase pressure/vacuum and hence no fuel flow. You checked for spark but how did you check? It is the quality of the spark that matters.

 

[The maddhatter]

Thanks how do I check the upper crankshaft seal? As for the plugged carb I don't think that's it but I'll tear it down and go again

 

[The maddhatter]

I checked the spark with a gauge. Had 1 coil that wouldn't jump more than a quarter inch so I replaced it. Tried the fuel in a spray bottle and couldn't tell a difference. The motor is vibrating pretty badly as well

 

[The maddhatter]

No visible holes or flaps in the block

 

[fireman57]

Sounds like no spark on that cylinder. Look under your stator and see if there is goo oozing from under the flywheel. You can also go to cdi electronics website and they have great testing procedures as long as you have a good meter.

 

[jimmbo]

Thanks how do I check the upper crankshaft seal? As for the plugged carb I don't think that's it but I'll tear it down and go again

You have to pull the flywheel and inspect the seal area for oil. If air is getting in, oil will be getting out. Some shops would replace them as preventative maintenance, whenever they had the flywheel off

 

[Sylva-Ranger]

I saw 2 times on similar ( 3 cyl OMC) engine………..One of the cylinder (sleeve) turning about of a turn in the bloc, and ports now not aligning correcly ………..Good compression, good spark , good carburetor etc but still missing one cylinder….



Just need to remove cylinder head and compare ports position betwin differents cylinder….

But in the 2 cases, thats apens after an overheat condition,

Just an idea, may be not your case…...

 

[racerone]

The sleeves on an 82 model can not rotate if factory original.-------On the newer 56 cubic inch they can rotate !!

 

[Joe Reeves]

Your very first post (#1), you state "I would have to trim way up and then bring it back down to get it to speed"... and in that post you indicated that the engine was running normally... not actually saying so..... BUT..... you did not state any actual engine malfunctions.

You indicated that it was on later trips that you encountered the missing problem.

My reply here pertains to the initial problem... riding bow high, hard to plane, but when finally planing, all was well.

The only times I encountered a problem like that was when using a prop with too much pitch or the boat has a load of water in its false bottom.

 

[Sylva-Ranger]

The sleeves on an 82 model can not rotate if factory original.-------On the newer 56 cubic inch they can rotate !!

Ok tank you , youre right, thats apen on a mid 80's 70 hp if i remember…...

 

[The maddhatter]

Correct Joe, I did a little bit of testing over the last couple of days. The miss was a crack in a coil and the slow to plane is overpitched prop I believe. I took it back out Last night and trimmed it up got on plane (seemed like forever) probably took a minute and a half. But once on plane I was topping out at 4800 rpm. I don't know what the pitch is on this prop but I think it's too high for the setup. Can't find any stamping on the prop either. I may take it to the prop shop and see if they can tell me.

 

[jimmbo]

Your engine has an WOT range of 5300 - 5800 rpm. Due to the torque(or lack of) characteristics of the 49.7 inch engine as a 75 hp, it needs to be propped near the 5800 mark to get reasonable holeshot. Venting the prop was covered in one of the Service Bulletins to help this little engine along

 

[Joe Reeves]

Prop ID is usually on the face of the prop behind the prop nut, spacer, etc.