tphoyt, I get it and do appreciate the effort and thought sharing info means. I sought out this forum and look forward to sharing as time goes on. Hopefully whatever I have to do to fix this 40hp Johnson will help some other person deal with a similar issue. Thanks for the post, mebehoofA valuable lesson I have learned on here over the years is Racers knowledge is worth its weight in gold. If you want his help you need to answer the questions with findings and or results and not more questions. I could go into a rabbit hole trying to explain what I mean but in a nut shell if you do the leg work so you can answer the questions you will get the results your looking for.
Best
I'll be tearing into it later this afternoon. Also curious why pulling the head is what seems to be the first option in this diagnosis? Regardless, based on the input from the good people that took the time to post (such as yourself) I'm going to follow their lead and see where it takes me. FYI my biggest question is whether it's electrical or mechanical??? I'm like the new student when it comes to things 2 stroke. So, as I progress I'll be posting any and all results. mebehoofI'm watching this thread, will be interested to know what the problem was. Have rebuilt a couple of 2 stroke outboard motors but I'm no 2 stroke expert. One of the 2 strokes I rebuilt was a 1982 70hp 3 cylinder Mercury (cylinder head is part of block casting and can't be removed), put new pistons in that (pita putting them in from the crank side), then it ran OK for a while but suddenly had the same symptoms as your engine that I never got to the bottom of.
Now after reading others posts on this thread I think maybe the problem with my Mercury 70 was a reed valve, crankcase leak or top seal. I don't know how pulling the head might help reveal the problem but if 2 stroke experts said that was a good idea that's what I'd be doing... as long as they were willing to re-engage and talk me through diagnosis after pulling the head, otherwise after checking the inlet and exhaust ports seemed to look the same on both cylinders I wouldn't know what other checks I could do that could tell me something the compression test hadn't already provided critical information about. It would be nice to know the reason for pulling the head before doing it but if experts didn't want to explain each step in their thought process I'd be happy to go along with what they said anyway, doesn't seem a lot of work to pull the head off the 2stroke.
reeds do not contribute to loss or increase of compressionIs it possible that the reed valve assembly on that cylinder is installed BACKWARD ? It would block faster airflow. But allow enough crankcase air for a good compression reading ? Possible?
jimmbo,"Very little Suction"... Was Fuel getting sprayed back on your Hand when you covered the Carb?
Are the Oil Streaks on the Block, along the Crankcase Seam, or coming down from under the Flywheel?
cyclops222, I was wrong regarding no suction on top cylinder at carb. I started the engine and allowed it to run on the lower cylinder and rechecked all of the items I had posted. As stated, I was WRONG regarding low or no suction on top cylinder it is the same as the bottom cylinder.Can we all read post # 11 again ? His first sentence is critical that there is no suction available. No suction in is no fuel drawn into the combustion chamber. As racerone stated. Compression should be about 140 # Both cylinders are way down. The top cylinder being the lowest compression.
racerone,I read post #1 and read many posts.-----It seems the simplicity of a 2 stroke and trouble shooting of them is not well understood !
Crosbyman,reeds are just one way mechanical valves that let fuel get sucked in the crankcase but not pushed out on the piston down stroke . This effectively guide the new fuel vapours in the cyl IN ports to refill the cyl and.... push out the exhaust.. Reeds work like check valves on a sewer pipe or a diode in electrical cct.
.
