revs drop after 5 minutes

[Would-i]

Was loaned a mid seventies 20 hp 2-stroke. (Merc model 200). Under the cowling looks as new - REALLY! Has new gas tank and lines. Plugs are new. Wiring looks great.
It starts easily, accelerates quickly and runs smoothly at all revs. From cold start, after 5 or so minutes the revs drop to trolling speed. Only feathering the choke gains back a few rpms, but not enough to get boat back on plane. Retried the second day with same results. Doesn't stall, just won't rev up.
Discussions with others point to carb problems based on the reaction to choke. I'm not convinced as the timing of the problem suggests to me it could be heat related.
(Engine cooling system seems fine. Water output seems normal)
Any suggestions where to start?

 

[Mi duckdown]

Re: revs drop after 5 minutes

Serial number would help. ALOT

 

[Would-i]

Re: revs drop after 5 minutes

Serial number would help. ALOT

The motor is across town so I don't have the full serial #. I do recall is started with 70, which should make it a '73 to '78 model.

 

[Would-i]

Re: revs drop after 5 minutes

The serial number is 7061036.
After running some seafoam in the mix and a few shots directly into the carb air intake on Thursday, when I took it for a spin today things were a bit better. I found a sweat spot on the choke. With the choke pulled out less than a quarter inch I can get the maximum rpms and the boat planed steadily for 15 minutes without dropping back to a troll.
Have I missed something or is this a simple case of gummed jets that are clearing?
I have 5 days left before taking the boat on a trip. Should I take it into a shop for carb or float work?