Mercury 4hp 2 stroke advice

[djtrauma]

hi everyone need some advice please.
i bought a Merc 4hp 2stroke last week. not sure of exact year as i cant seem to find a serial number plate anywhere. however there is a circular metal plate about an inch across on the engine with 9580531 on it. i'd say its mid 80s-mid90s at a guess.
so i saw it running in a test tank when i bought it. started and ran fine.
took it out into my local bay a few evenings later, started and ran fine then too in fact i loved how quick and smooth it was. but then i had a seagull silver century previously! it starts on 2nd or 3rd pull from cold. pumps water fine.
took it out to sea again yesterday morning and after a mile at full throttle it lost power slowly over about 30seconds and eventually died. it would start again ok but as soon as i put it in gear it died again. undid the fuel cap and checked the breather then re attached. started and went into gear fine, took it back to shore. after leaving it for 20mins and firing up it went about 300 metres before dying out again, and would start then cut out almost immediately on the 7-8 times i tried on the row back to shore.
when i got it home and put it in my test tank it started and ran fine in and out of gear fine. can anyone give advice on what happened? this is my first year of boat owning so not all that clued up, just the basics. i wonder if the carb needs cleaning? or if some gunk was making it cut out that was freed up from laying it down in the car journey home?
when i bought it it had a little fuel left in, maybe it was old fuel that had been left in. the seller said it had been serviced a few weeks before but forgot to ask to see the receipt.

[lightning]

I assume you are using the correct premix of petrol/oil, about 50-1 but if you had a Seagull you will know all about that.
It could have overheated after a mile on full throttle and begun to sieze. Then as it cooled it freed off but you've damaged the bore and it's losing compression so not running properly now.

[Locozodiac]

Is that a internal & external tank model ? other things to check :

1-Clean well carb, fuel lines, check fuel connectors for o'ring wear if using ext tank, might be sucking air.

2-If using internal tank, clean well tank interior, fuel line to carb, small mesh filter located inside of fuel cock, look for debris, obstructions.

3-Check fuel pump condition for cracked, dried, balloned, hardened diaphragms. Change

4-Spark plug for correct gapping, fouled, try with new if possible. Make compression test.

5-Worn, out of torque specs, dried out and hardened head gasktet, replace, clean all inside water passages.

6-Use fresh fuel and correct 50:1 fuel/oil ratio.

7-Could have an electrical related issue, when component heats up while on load, shuts engine down ?

Happy Boating