Re: 1982 johnson 150 half speed
yup, we've all been there before...when the lower unit is submerged, backpressure on the underwater exhaust puts a load on the engine; on the muffs even a crappy engine can sound great all day long.
Classic symptom of clogged hi-speed jets, carb rebuild is the solution. It's generally a good idea to rule out compression and spark first, as those are quick and easy tests, and also potential suspects in a case like yours.
Compression should be about equal in all cylinders (about 5 or 7% difference between worst & best). Spark test (not a "spark plug" test): get an inline spark tester (couple bucks at autozone), set the gap for 7/16", clip it to the engine block, attach the plug leads (one at a time) and look for a strong blue spark while cranking. If it can't jump a half-inch gap in the open air without a spark plug, it's not firing in the hole under heat & compression.
Good compression & spark, proceed with the carb rebuild. See the "carb cleaning" link in the "Top Secret" thread at the top of this forum.
Keep us posted.
