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Old 10 April 2007, 04:12   #1
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yamaha wont throttle,just misfires

hi to all.

i have this outboard engine ( yam,40hp 3 cylinder,2stroke,electronic ignition ) but i start it and it will only throttle to a fast idle (at idle upto very low revs,all is well ) when i put the throttle up a little more or above ( say a third throttle ) the engine studenly gets very lumpy.(almost like the choke has been put on ) ive put a strobe gun on it to check the timing and i find that when it does start to run lumpy the fireing becomes very iratic.. (im 95%sure carbs are fine )

basicly as the enigine revs increase the strobe light gets faster as it should until i reach the revs/throttle ( aprox quater to a third of the throttle ) and on the strobe light the fireing of each cylinder becomes much less often.almost like it might only be fireing on aprox every 3 reverlution of the engine insted every of one revolution. plus ofcause each time the strobe lights up i can here the engine fire ( one stroke of power ) but its only fireing say aprox 3 times a second .

im almost cirtain its an electricle fault but i want as much advice as i can from any one who thinks they can help?????

i hope ive described it well enough

ive cleaned up the LT wires which were a bit coroded on all 3 coils,given the plugs a damn good clean ,im very sure the carbs are fine too,but still the problem persists..
could it be a simptom of the cdi ???
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Old 10 April 2007, 04:32   #2
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The point in the rev range that the problem occurs doesn't point to it being the LT side of your ignition(don't discount it though-CDI can replicate almost any problem. It's a bugger when it goes wrong)

From what you've described this is what I'd be checking first:-

Plug(s) breaking down under load (swap plugs into different cylinders and see if the misfire follows one plug),
HT leads, (run it in the dark and look for sparks)
Blocked main jet(s) or water in carbs,
Carbs waaaaay out of sync,
Very dodgy fuel pump (hold the tank high up and see if it improves with gravity feed)

Try using a different strobe light to make sure that your strobe's OK. I've had a faulty strobe do what you're describing and it's totally misleading. They need a good battery to work OK.
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Old 10 April 2007, 11:00   #3
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Check the overheat / low oil alarm wire is not grounded out, I thinks it's the pink one.
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Old 13 April 2007, 05:23   #4
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sounds like the carbs are gummed up, take the strobe lite point it down the carbs, this will let you see whats goin on when it runin, about that speed the main nozzel should be sprayin, the strobe lite will let you see it. sounds like the carbs need to be cleaned.
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Old 14 April 2007, 16:12   #5
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Check the overheat / low oil alarm wire is not grounded out, I thinks it's the pink one.
Agreed, Do these engines have a Safety system like the Johnson SLOW system to get you home if the engine is overheating then?

if they do then this sounds like a probable culprit.

Chris
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Old 14 April 2007, 16:20   #6
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Agreed, Do these engines have a Safety system like the Johnson SLOW system to get you home if the engine is overheating then?

if they do then this sounds like a probable culprit.

Chris
yep, restricts the rpm to 2000 or so.
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Old 16 April 2007, 04:04   #7
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Agreed, Do these engines have a Safety system like the Johnson SLOW system to get you home if the engine is overheating then?

if they do then this sounds like a probable culprit.

Chris
the problem still persistes.

when it goes into the safe mode and restricts the revs to about 2000 does the engine just stay at 2000 revs (unless you throttle down ofcause ) or will it sunt and vibrate like what my engine is when i reach the problem rev range ????

btw i want to get the cdi checked so where would i take it to check it ? ?
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Old 16 April 2007, 04:55   #8
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Jez, why not try disconnecting the temp sensor(s) first to see if they are giving a false reading and setting of the SLOW system?

Chris
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Old 16 April 2007, 05:00   #9
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With regard to the low oil safety system on my 1999 Yamaha 75....

When the oil went to 1 (out of 3) bars on the digital gauge, the alarm would sound and the RPM would be restricted to 2000RPM. However, if you tried to rev above this the engine would vibrate like crazy and sound very lumpy... almost as if it was trying to get above 2000RPM but the limiter wouldnt allow it.

Sorry I cannot tell you what wires are which as I sold that engine almost 3 years ago, but it could very well be your problem.

Hope this helps.
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Old 16 April 2007, 16:58   #10
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Can you try another fuel line? Symptoms like you describe can be caused by a dodgy primer bulb which allows enough fuel to run at tickover, but blocks as the fuel starts to be sucked faster.

John
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