I live in Australia and have been given a 30hp Yamaha 2 stroke 2 cylinder 1986 model (30M?), my son in law bought it but only took it out twice with the last time him needing to paddle due to the motor stopping. When I first received it I gave it a fresh tank of petrol started it up and warmed it up with no apparent problem, took it out and thats when it happened again to me.
The motor starting loosing power and then stopped and wouldn't start for 30 pulls or so. I was towed part way to the ramp and manage to start it again as I was being towed. After this episode, I investigated a few things and found my spark plugs were fowling. I put new plugs in and she fired straight away, but later stalled again while I had her under test in a drum of water to flush her. I tried to start her again and same thing hard to start and fowled plugs. I did a compression test to find out the bottom cylinder was 120 psi and the top 135 psi which was just in Yamaha tolerence (15psi or 25%) but an obviuos reason why she was playing up as it was the bottom spark plugs doing most of the fowling.
After testing her with compression, I decided to read more regarding the power head and asking some outboard dealers their opinion, the mercury dealer advised the motor was worn, the Yamaha dealer advised that "Ring free may work on an old engine, don't use new plugs till shock treatment is finished." So in saying all this I am in the process of using Yamaha's Ring Free which is like hen's teeth in Australia, After running it for 30 minutes with a mix of 100:1 ratio, ring free at it's spec ratio, I found the motor idling a lot better than it has, I am yet to check compression as I want to finish all Ring Free tank. I will post what my findings are after running a full tank