1982 50hp Johnson lose cylinder while trolling

[catmotorhead]

starts up fine but even cold while driving through no wake areas I lose a cylinder. Throttle up and it runs great wide open. it doesnt always lose the cylinder while trolling but eventually does. I pulled the plugs out in the garage both have beautiful blue spark. I run klots oil and rec90 fuel.2 separate 5 gallon cans so typically fresh gas. new wire harness last year. thanks for your time
Charley

 

[boobie]

What's the compression on it ?? Will the spark jump a 7/16" open air gap on a spark tester ?? Do you always lose the same cylinder ??

 

[catmotorhead]

118# on both cylinders. I haven't done the spark gap test yet

 

[oldboat1]

Good it happens at trolling speed, at least (easier to check). If in the driveway, use a barrel of some kind (plastic trash can works), and sink the l.u. about half way in water. Test at temp and in gear (monitor temp -- good to keep the fill hose running to keep up water level). You need back pressure.

It might not be a spark issue, but have to test for that first with the adjustable tester (cheap), as suggested -- open air kind. If you have spark when testing, but it still acts up when idling, pull off the plug boots individually and check for changes -- if no change, that cylinder isn't firing. (Heavy work gloves are a good idea....)

It's possible an idle jet is partially clogged, or idle passages, so might turn to carb work if spark tests check out OK.

 

[catmotorhead]

A couple years ago I had it in the shop and they said it needed a thermostat I never got around to changing it. It didn't seem to be causing much a problem so it never got replaced. I did the open air spark check. It's like a little lighting bolt. It's blue and looks thick! What is the recommended plugs for it?im running an NGKB7HS-10 2129

 

[catmotorhead]

UPDATE. Took her out today for 4 hours it ran great never missed a beat. Didn't get any walleyes but we had fun reeling in a dozen catfish. It must be in the carbs. I wont worry about it until it gets worse. Thanks for the advise fellas