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Old 27 August 2007, 14:33   #1
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Yamaha 50hp missing rev range

chaps,

got a wierd one here, went out today and the engine would not rev past 2300rpm, no alarm sounded or anything so don't think it was that. But it was like the motor was 'bouncing' at those revs and would not go any higher no matter what. I had a good look at the manual and I am thinking maybe a dodgy charge coil (high speed) or suspect CDI unit maybe. Got home and had a go at it in a test tank and low and behold the fault had disappeared but it has done this before! I earthed out the pink alarm wire and that brought on the buzzer and the engine seemed to limit itself to just under 3000rpm but no 'bouncing' as such.Anyone got any thoughts on this? Does my motor have a low and high speed charge coil? Anyone know the wire colours, as the manual is a bit inaccurate!
cheers
Jiz
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Old 27 August 2007, 15:35   #2
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Fuel delivery problem? That may show up while running, but not when revving with no load. Usually won't trigger an alarm.

Could be electrical/electronic, though, so you may be on the right track as well.

jky
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Old 27 August 2007, 15:39   #3
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Is it a 3pot 2 stroke if it is I'd check the wiring loom from the throttle I had this problem changed the loom all ok
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Old 27 August 2007, 15:44   #4
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Yep, 3 cylinder, 2 stroke, do you mean the loom from the 703 control box?
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Old 27 August 2007, 16:16   #5
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I do I really do. you are welcome to bring it round to our unit and we'll have a go at it. FOC
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Old 28 August 2007, 16:54   #6
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Quote:
Originally Posted by Rogue Wave View Post
I do I really do. you are welcome to bring it round to our unit and we'll have a go at it. FOC
Bloody hell stu, I would not have thought the loom would do that! Cheers for that info, not too dear to replace either. I may well take you up on your offer in the near future.

Sods law tho', the fault is not there anymore! Ran it in a test tank, in gear at 2000rpm or so, for an about an hour and the prob did not return. It ran as sweet as a nut and I had to stop it in the end as the tank water was getting very warm! The only thing I did find was that the flywheel had worn through one of the crank position sensor wires, so I repaired and re-routed that. I dis-connected that as a test and the motor ran a little rough but not too bad. I also put the bare wire to earth to see if it created the fault but i didn't so no luck there anyway.
Couple of things I am not sure of:
Why does my motor have a crank position sensor?
Is it to govern the timing of the spark as the motor only appears to have one pulsar coil not three as I expected?
Does my motor have a high and low speed charge coils as there are only 2 wires (brown & blue) when I was expecting three, unless it grounds through the motor?
Why do the lighting coils have a red link wire between them as none of this really corresponds to the schematic (seloc manual) I have?

thanks
Jiz
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Old 30 August 2007, 16:40   #7
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Just in case change the plugs and check that all leads are making contact with the plugs.it can be so simple you could miss it!
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Old 31 August 2007, 10:37   #8
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Just in case change the plugs and check that all leads are making contact with the plugs.it can be so simple you could miss it!
Agreed, Just picked up a set of plugs as a matter of course.
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Old 01 June 2008, 16:52   #9
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This has reared it's ugly head again today.
Thought I'd cured it, obviuosly not! Anyone got any ideas?
(while I try to source a control loom at a reasonable price, as per stu's suggestion)
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Old 02 June 2008, 06:23   #10
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i had a same problem a couple of years ago with a 30hp yam 3 cyl 2 stroke
turned out to be crap in one of the carbs there was a piece of material bloking the throttle needle i think thats what you call it this was cleaned and it sorted it out but as you say ran fine in the tank as soon as under load no power

the way the chap who fixed mine found it was to run the engine with air intake off and squirted in some "quick start" in to the carbs to identifiy which one was faulty

Andy
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