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I am having a bit of trouble with a 1973 65 hp Johnson and could use some advice as to where to start.
Background:
Low hours / father had from new.
Has always ran/started good.
50:1 oil / E10 gas but try not to when I can.
Usually have some seafoam in there too.
A realy good running motor that I have never had an issue with that was not my fault.

Problem:
Starts / Runs normal ...good even (it will sometimes stall right after engaging propeller... low, low RPMs... has done this for years) Can start stop motor often and all is good.
...Then... just wont start.
It does not die by itself or run funny.
I turn it off... change / do nothing and when I try to start again; the starter will spin good but it will not run.

This has happened 3 times in the last 10 times I had the boat out.

1st
1st time the boat was used this year.
Started/Ran on muffs then dropped in lake... putt putted to my lift (5min) turned off motor
15 min later came back and would not start. Would spin but not start.
Next day: did not try again until I changed: new gas (flushed out line) and new plugs.
Jumped to life. - ran without issue for like 5 outings (usually only 1 or 2 hours per)

2nd time:
longer day of boating mostly slow with starts and stops throughout.
Then one more stop for a swim (30min?) Then no start.
Got towed back - 2 days latter - did not touch a thing and it started right up.
On / off about 10 times no problem.

3rd (latter the same day as above)
Ran good, just a slow evening voyage - started and stopped at will.
Then about 1 hr in stopped motor then would not start.
Switched tank to Non-E gas (other was not empty or old but was E10 gas)
Still no start but good spin. - A bit of smoke but no “Brooom”
Pulled of housing (to let cool??? no.. the temp light was off and it was cooling... just thought it might help)
Pulled plugs (new from earlier) A bit dark and oily but that might have been from the starting attempts.
Checked spark on top cylinder- good spark.
Buttoned everything back up and tried again.
A bit of smoke in the beginning but no real engine noise.
Got frustrated and attempted to burn out my starter (just held on the key)
The RPM’s of the started sounded like it would speed up a bit after a few seconds of continuous spinning and some smoke came back. I held it on a bit longer and when I let go of the key it was running (like I just pushed it into a start if that makes any sense) no big Brooom. I could not even hear it over the starter. I throttled up and it came to life... sounding normal.
I left it idle and put away my tools and drove home. Sounded good went full speed (same top end as normal per GPS). Parked it on lift - turned it off and on like 5 times and it started every time.

So... what the heck?

I am stumped almost wishing it would be more broken so I could find something.

Any ideas what I should look for?
... a method for how I can isolate the issue?

Thanks,
Jake

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Could be many things related to the starter. When starter spins the gear on top spins up and engages the flywheel, cranking the motor. The battery may be weak, the wire terminals could be corroded or loose, either the positive or ground wires or both, either at the battery or engine. The starter may be going bad. There may be corrosion on the shaft of that little gear that moves up to engage the flywheel. Then again it could be one of those little invisible demons who follow boaters around laughing at us when they pull a trick on us.

I just re read your post. When you say starter spins do you mean engine cranks but des not start or does just the starter spin?

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No the starter engages the fly and it cranks fine.
It just every once in like 15 starts, won't run... but never the 1st cold start. Only after its been running then is turned off. Then is when it wont start smetimes...Not even rumble a little. A bit of regular old 2stroke smoke but it won't start. Then given some time (like overnight) then with one tuch of the key it Willi jump to life. ... Except the last time when I pushed it when it wouldn't start... letting it crank until it just kind of rolled into running... But then ran good.

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Don't keep cranking if the engine doesn't start. You'll over heat the starter, stress the battery and could even cause a fire if the starter gets hot enough and the fuse doesn't blow as it normally draws a lot of current. Thats why every engine manual will tell you to stop cranking if the engine doesn't start in a few seconds. I'm basically a very lazy person and in your position I would just bite the bullet and seek the advice of a mechanic. A good outboard mechanic will have seen just about everything and can probably pinpoint the problem quickly.