Yamaha 25hp Presision Blend, why can't I get it running properly?

[Orky]

Ok, bare details first then the more in depth explanation: Yamah 25hp 2 stroke autolube/presision blend, starts up fine, runs on muffs or in a barrell of water fine, will throttle up to high revs in neutral, under load on a boat however it has the same predictable issue - start to throttle up, reach the point just before planing and it loses all power, goes slower and slower the more throttle you give it, the more you open the throttle the worse it runs, vibrating and misfiring as soon as that point is reached. Tried cleaning the carbs, changed the plugs, checked electrical connections, different fuel tank and different fuel line and bulb, adjusted idle screws, timing linkage, changed the oil in the oil injection tank and made sure it was full of oil at all times before running, suspected a leaking reed valve in the bottom cylinder (2 cylinder engine) because of oil blowback in the bottom carb so I fitted a new reed valve assembly, nothing has made any difference, no matter what I do the motor runs the same afterwards every single time. I am at a standstill with this problem and any suggestions or help would be very welcome. One more thing for now, the warning light on the centre of the control panel has had its wires cut off and this is making me wonder if the problem starts here? Thanks for reading.

 

[99yam40]

have you checked compression, spark, timing. and fuel pump?

Getting the alarm system working properly would be the best so you could know when there was a problem.

There could be an alarm and RPM reduction happening because of the alarm condition

 

[Orky]

have you checked compression, spark, timing. and fuel pump?

Getting the alarm system working properly would be the best so you could know when there was a problem.

There could be an alarm and RPM reduction happening because of the alarm condition

Thanks, Compression good, top cylinder slightly lower than bottom cylinder but only under 10 psi. Spark good on both plugs but bottom cylinder has fuel fouling after running, top plug dry and clean and ht leads seem in good condition. Timing? I dont know how to check this exactly but have tried adjusting idle screws and the arm which attaches to the moving disc under the flywheel (sorry but don't know the name, throttle advance?). Fuel pump? I have not had a good look in here but need to soon, I was going to check for blockages and maybe holes in the diaphragm, also want to check the fuel feed between the pump and the upper carb, I will check all fuel lines to and from the fuel pump. It is the bottom cylinder and carb where the blowback is happening if that means anything, I misdiagnosed this as reed valve leakage because of the hiss coming from the bottom set of reeds I only noticed the hole for the fuel pump pressure which is just above the bottom reed valve assembly after I replaced the reed valve assembly, the hiss was coming from here. I have not replaced the reed plate gasket and wonder if the issue could be here? It just has the old gasket on right now.

I do not know how to rewire the alarm system, I'll try and find a diagram online so I know where to connect up. Thanks for all the tips by the way, I need all the help I can get because I've only just started using outboards and working on them.

 

[tommarvin]

Just to be clear, the boat would throttle up under load before, now it won't throttle up under load.
I would hook the alarm back up so you can eliminate that off the list.
Sure sounds like fuel delivery.

 

[Orky]

Thanks tommarvin, yes, when I first bought it the motor would give me enough power to plane but soon started to vibrate at 3/4 throttle and lose power then stall. Now it is starting fine, throttling fine and idling a low revs very steadily but that is just in neutral, put it in gear and it almost immediately vibrates then runs like junk and wants to stall. Thanks for the pm also I will look at that. I'm planning to clean and check every electrical connection and inspect every fuel line plus I will check jets in the carbs with soft wire for obstructions. Only other things I can think of is - There is a visible gap in the gasket between the crankshaft and cylinder head. I have a new gasket which I should have replaced, the one between the intake and the reed valve plate. One of the coils looksw like it has been replaced before I got the outboard.