J50PLSTD miss fire over 3000RPM Idles fine. Compression is good

[oldboat1]

What's the operating temp in the lake, not on muffs? Check surface temp at top of head.

The operating temp of the engine, measured at the top of the head. Too hot to touch is too hot. I wouldn't run it until I changed the impeller.

 

[JerryDH]

The operating temp of the engine, measured at the top of the head. Too hot to touch is too hot. I wouldn't run it until I changed the impeller.

The temp at the upper head bolts using warm water at the muffs was 139 degrees F. I used the bolt head instead of the head to get a reading on a clean unpainted surface. This problem has been ongoing for the last 3 or 4 years, but since 90 plus percent of the time I run it no wake areas, I haven’t worried about it too much. I’m going to sell this boat because I purchased a newer one, and I’m trying to get this fixed before I do. Thanks for all your help. I’ll post what happens after I take it on the water.

 

[JerryDH]

The temp at the upper head bolts using warm water at the muffs was 139 degrees F. I used the bolt head instead of the head to get a reading on a clean unpainted surface. This problem has been ongoing for the last 3 or 4 years, but since 90 plus percent of the time I run it no wake areas, I haven’t worried about it too much. I’m going to sell this boat because I purchased a newer one, and I’m trying to get this fixed before I do. Thanks for all your help. I’ll post what happens after I take it on the water.

Put the boat in the water this afternoon, and surprisingly it ran great! Had a misfire at initial acceleration, but cleared up and ran as good as it ever has. Weather is a little cooler and lake temperatures have dropped about 5 degrees in the last week. I did not have any of the engine cowlings or cover on, so had a little more heat dissipation than with cowlings on. I did tweak a couple of reed leafs because they were leaking a little light when checking with a light, and set float levels back to spec, and rechecked main jets to be sure they were clean. Both were good. Water pump impeller on order, so will leave cowlings off till after I install impeller. Thanks everyone for all your help and ideas. JerryH45

 

[GunnerH]

I recently had similar problems with my 60 HP Johnson. I'm convinced it had something to do with the engine monitoring system (ironically named SLOW). Below is a page from the owners manual describing the system limiting the engine operation to 3K RPM and "noticeable vibration". She idled fine and would throttle up to a point (in neutral or under a load) but fall on her face above that. Link to my post a few weeks ago with details of how I chased the problem from fuel delivery to spark. Link to 18 second video of the motor at idle, part throttle and when the problem occurred. Happy to report all good now!