Yamaha 50hp losing power?

I have a 50hp Yamaha outboard autolube (about 1988) that starts up no problem but then after running for a few minutes loses power. If I try to increase throttle it just dies altogether. I've changed the plugs, and cleaned the carbs which has made a slight improvement. When the engine is ticking over and you remove the lead to the plugs the engine dies on cylinder 1 which would imply that is working fine, it struggles when you remove the lead to cylinder 2, but when you take the lead off cylinder 3 it makes no difference at all so it looks to me as though it is only really running off 1 cylinder properly but I'm stuck as to what to do next.