Thank you for your question XXXXXXX,
Probable cause would be the carburetor on this type of issue. And a thorough inspection, cleaning , and adjustment will likely be the repair.
We do need to take a look at a few other possible causes.
Stuck piston rings will cause this type of problem, when cold the rings are stuck, and when the engine warms up they are released to give more compression and thus a better idle power output.
The distributor on your unit is mechanical in nature and if the timing is not returning to the same setting each and every time the engine is idled down, this will cause the same poor idle.
The overall health of the engine needs to be investigated and any issues found will need to be addressed. We should start with a compression test cold and hot. and timing light test on the ignition system. Also spark strength should be checked. This engine originally came with points and condenser.
Yes they would be possible, but less likely, unless there is some history that I am unaware of. Poor storage, lack of use, sinking, water damage from rain and sun.
I would still point my finger at the fuel system, this is where we have most of our issues. The fuels of today are manufactured to be used when produced, they store poorly and will foul a fuel system in just a few months of storage.
Marine Mechanic
21 Years Experience as Mechanic, 18 Yrs Boat Yard Owner/Mechanic, Factory Certified