Saving Old Seagulls

Basically it runs fine with no prop, in gear and out. I put the prop on and it just stalls.
The prop is the kind with the internal spring.
I would normally say it is fuel starvation, but the carb has just been cleaned and is fine.
Plenty of fuel getting there, and the breather on the tank is open!
I can put it in gear and turn it over freely so something else might be at play.

Is the prop on the correct way?

Thanks

Try increasing the tickover revs.

Put it on a boat, that may solve it.
I have a 2hp Honda that runs like a pig in the tank, sweet as a nut on a boat.

Test tanks do sometimes make engines run poorly.

Yes, as Charles says your tickover could be a tad to low.

Well, before I took the plunge and stripped the gearbox again, I did a quick run through.
I have modified this engine to be 9” longer than standard.
It is now 38” (965mm) from the transom to the skeg.

This gives me clear water to the prop and keeps the intake under water if I move forward in the boat!

The result was too much back pressure.

Holes drilled in exhaust and now it runs sweet!