Unhappy Yamaha 25.
So Saturday was the first outing of the new inflatable (Bombard Aerotec 420) with Yamaha 25hp 2-stroke (built around 2000). The engine's been running fine in a bin of water and started out fine on Saturday but a mile or so up the Deben, power seriously dropped so that WOT was making maybe a couple of knots and the engine sounded like it was really labouring. It kept running long enough to get us back without paddling but as soon as I got off WOT the engine would cut out.
Had a play with the engine today in the bin of water and it's still messing around - very hard to start (was fine before yesterday) and cuts out shortly after starting. Started easily enough with a sniff of ether but then died as soon as the ether was burnt. Checked the fuel filter in the engine and it was clean, gave it a clean anyhow and the filter on the dip tube in the tank was clear as well. Pulled the plugs and they seem fine - dry, light tan on the insulator, slightly oily on the body and no smell of unburnt fuel. No obvious signs of distress on the CDI coils or HT leads.
So my guess is a fuel problem, maybe blocked jets due to dirt, although I'm not sure where the dirt would have come from, everything looks clean. One thing I noticed is that I couldn't get the bulb on the priming pump on the fuel line to go hard when connected to the engine. Playing around with it off-engine it would go fairly solid if held upright (outlet up) but not if horizontal - fairly sure that's not right and probably a stuck non-return valve, but would this cause engine problems (bearing in mind the fuel filter in the engine was full)?
Failing that I'm going to have to take the carbs off and clean them, but before I do that I'd like a service manual for the engine so I can get the settings right after I put it back together again. Is this available online somewhere or can it be bought?