1966 Johnson 20hp FD-20 issue (starts but sputters out after first time out on river)

So, I had this motor running well at home after rebuilding the lower unit, new water pump, all new electrical (condensers, points gapped to .020, coils, plug wires and plugs) and rebuilt the carb. Seems to still have good compression.So i took it out to do some steelhead fishing and it ran fine til the end of the day when it encountered it's heaviest load of the day working our way back up the river. I opened it up to full throttle to get up on plane. The motor made a loud pop sound and backfired a few times... It died to about half power but ran for a few minutes at full throttle. Since then it will not stay running. I can turn it over, it sounds like it wants to run but just sputters out or backfired now.... I don't see a blown head gasket or crack in the block so am not entirely sure where to start from here.Any suggestions other than driving it off a cliff or scrapping for parts are gladly appreciated!

kimcrwbr1 said:

40-45 ft lbs for the flywheel nut.

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I thought I did but bought a cheap clicker torque wrench and it wouldn't click so just wrenched it in till the wrench head flexed and touched the side. I need a new strap wrench to pull the flywheel and will check that key for sure! (And get a new torque wrench).

Also it did pull the plugs and the bottom one looked a little burnt so cleaned it, checked gap and reinstalled. Seems to get great spark still neither plug looked smashed.

kimcrwbr1 said:

Did you tighten the flywheel nut to spec it could be the flywheel key sheared? All it takes is a small crease to throw it way out of time!

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So, got the flywheel off tonight, and the ring that fits over the key and that opens the points was in 4 pieces. Luckily I have a parts motor and it was an easy swap out! Cleaned everything very well after removing the stator plate and once I get a new torque wrench and torque this back in properly am sure I'll be back out on the river! Thnx kimcrwbr1!!! You the man!

kimcrwbr1 said:

That will keep it from running, did you determine why that happened? My guess would be the woodruff key wasnt fully seated in the crankshaft and when you tightened the flywheel the key put pressure out on the points cam? Did you use a good high quality grease for the armature plate? It should move nice and smooth with zero wobble up and down and also check/adjust engine sync is easier with the flywheel off.

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I would guess I didn't have it seated all the way in but not really sure what to look for here. This is my first time working on an outboard. I do have the service manual and will read up on the magneto rebuild to see if I missed something.I do know the armature plate is greased well all ready and moves nicely. Is there a way to balance it so it sits level? There are 4 scewes holding it in. I have removed the plate when I replaced the plug wires. Maybe I i need to balance the armature plate?I did not adjust the engine sync, will need to read up on that too.

I also noticed that I have to run the motor in the start to fast position on the throttle control arm just to idle. Is there a way to adjust that? Would that have anything to do with this?

Make sure that the flywheel key is a factory replacement one.

Anyone have a good diagram of the synchronization positions for this motor? The service manual I am referencing is online and looks like it's been copied 10x too many for the pictures to show up...