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Old 04 August 2013, 16:13   #1
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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?

Cheers,

Andrew.
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Old 04 August 2013, 16:22   #2
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I'd humbly suggest that air is getting into the system. What's the condition of the fuel line upstream from the bulb? Maybe a thorough check of the integrity of the hose and the connections before setting about the carbs....
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Old 04 August 2013, 16:28   #3
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Fuel pump diaphram?
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Old 04 August 2013, 16:28   #4
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I'd humbly suggest that air is getting into the system. What's the condition of the fuel line upstream from the bulb? Maybe a thorough check of the integrity of the hose and the connections before setting about the carbs....
Looks pretty decent, although I haven't particularly looked for air leaks (no fuel leaks though!) but you're right - I'll happily replace the fuel hose and bulb pump before I start mucking about with the carbs (said things being frankly hateful devices anyhow).

Cheers,

Andrew.
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Old 04 August 2013, 16:29   #5
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Fuel pump diaphram?
Possibility - is it fairly obvious to find? Between the filter and carbs I imagine...

A.
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Old 04 August 2013, 18:10   #6
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Looks like there is more than 1 c.2000 25HP 2 stroke Yammie... but here's a possibility of what you are looking for:


Part 1 is the pump. 9 is the diaphragm
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Old 04 August 2013, 18:22   #7
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Oh and what happens when its badly running and you try and prime the fuel line? If it improves its more likely a leaky line or the pump than the carb...
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Old 05 August 2013, 02:21   #8
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some primer bulbs only work when upright - i was told its because they have a ball inside which sits in and so seals against a hole. crap idea if you ask me!
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Old 05 August 2013, 04:24   #9
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Looks like there is more than 1 c.2000 25HP 2 stroke Yammie... but here's a possibility of what you are looking for:


Part 1 is the pump. 9 is the diaphragm
Thanks, that's useful. It looks like it's not quite the same engine as the fuel filter is different - mine's a vertical one with a filter bowl that can be unscrewed to get muck out of the bottom. Fuel pump's probably not dissimilar though. Looking at the id plate on the engine I think it's a 25NMHO.

re. trying the pump while the engine's running - I didn't try that, it would be tricky as it literally runs for a second or two before it dies, maybe I'll rope in my young assistant to squeeze the pump bulb (or maybe try to raise the fuel tank above the engine).

Cheers,

Andrew.
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Old 05 August 2013, 04:25   #10
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some primer bulbs only work when upright - i was told its because they have a ball inside which sits in and so seals against a hole. crap idea if you ask me!
Yes, I can hear a little ball inside, so it may be that. You'd have thought they might have heard of springs eh!!?

Thinking about it though, the pump was 'going solid' when I tried the engine before the problems started. I may have been holding the bulb upright by coincidence though...

A.
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