Re: High speed jet or something else?
Do a compression check as suggested. It'll run on 60, it'll run better at 80, 100 would be good, 120 probably the gauge reading wrongly. Should get the same on both cyls within 10%.
Check for good sparks on both cyls. Use a spark tester if possible.
Bad sparks: need to investigate the ignition system. Points , coils, condensers, plug leads.
Good sparks check clean reset but better replace plugs with correct type. current recommendation J4C, gapped at 0.030". (J6C if tending to foul perhaps.)
Dont understand what you say about "leaned right out" and "running rich as hell."
The carb has only one adjustment, the idle mixture screw.
Joe Reeves' instructions for adjusting are
(Carburetor Adjustment - Single S/S Adjustable Needle Valve)
(J. Reeves)
Initial setting is: Slow speed = seat gently, then open 1-1/2 turns.
Start engine and set the rpms to where it just stays running. In segments of 1/8 turns, start to turn the S/S needle valve in. Wait a few seconds for the engine to respond. As you turn the valve in, the rpms will increase. Lower the rpms again to where the engine will just stay running.
Eventually you'll hit the point where the engine wants to die out or it will spit back (sounds like a mild backfire). At that point, back out the valve 1/4 turn. Within that 1/4 turn, you'll find the smoothest slow speed setting.
Note: As a final double check setting of the slow speed valve(s), if the engine has more than one carburetor, do not attempt to gradually adjust all of the valves/carburetors at the same time. Do one at a time until you hit the above response (die out or spit back), then go on to the next valve/carburetor. It may be necessary to back out "all" of the slow speed adjustable needle valves 1/8 turn before doing this final adjustment due to the fact that one of the valves might be initially set ever so slightly lean.
When you have finished the above adjustment, you will have no reason to move them again unless the carburetor fouls/gums up from sitting, in which case you would be required to remove, clean, and rebuild the carburetor anyway.
If you cannot get it adjusted following this then the carb probably needs an overhaul. If it really is running rich then focus your attention on the float and the needle valve because it might be flooding.
Dont know what you mean by lower unit fouled.
Check that the prop rotates freely when not in gear.
Check your gear oil. It should be "clear and bright" and up to the level of the level screw. The drain and level screw are the two broad flat slotted head screws on the side of the gearcase assembly. DO NOT undo the smaller screw near the lower one.
If the oil is milky it indicates water ingress which must be investigated and the cause rectified.
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