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Old 30 April 2012, 07:05   #1
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Country: UK - England
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Make: ribcraft
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Yamaha F50 missfire :(

I have a 1996 F50 with a persistent misfire. I originally took it to a Yamaha dealer who diagnosed a faulty carb on cylinder 4, from there it has cleared a high rpm issue but has a bad misfire upto 2400rpm, to the point that from initial start up drops onto 3 cylinders and only recovers to 4 after a high rpm run, from there on it pops and bangs at any rpm below the 2400 mark. So far, compression has been checked and is normal across the 4 cylinders, carbs have been stripped numerous times, cleaned (both with carb cleaner and ultrasonic bath and re-balanced. Plugs replaced, coils swapped out.
The fault seems to lie with Cylinder 4 which when the misfire occurs the HT lead can be removed with no obvious change of engine note, against the other 3 which are definitely firing.
Any assistance greatly received.......
Chris
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Old 01 May 2012, 09:40   #2
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indicates to carbs. bottom carb sounds like its flooding? is the plug wet when you take it out after a run.
usually if an ultrasonic bath does not cure a faulty carb it worthy of the bin. its a shame you cant swap the carbs over on these see if the problems follow.
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Old 01 May 2012, 10:23   #3
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Hi Tris,
Cheers for the reply, Yamaha have already replaced carb 4 which helped top end running has not cured this issue, it has been back to Yamaha several times but no progress as yet.
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Old 01 May 2012, 10:29   #4
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A while ago I had something similar happening on my yamaha 150 - have you tried changing the HT leads around?

Yamaha took several months to diagnose mine :|
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Old 01 May 2012, 10:31   #5
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Fitted a replacement coil and leads, also swapped coils from other cylinders
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Old 01 May 2012, 16:26   #6
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Dont know about your particular engine but have known problems with lower cylinders on some older engines having a missfire or not running quite right owing to running slightly cooler than the other upper cylinders owing to heat rising and or the flow of cooling water , i have cured some in the past by running a hotter spark plug in the lower cylinder.
You could always try running the engine in the dark with the hood off and see if there's any tracking /earthing or electrical leakage from any of the circuits or leads .
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Old 01 May 2012, 16:41   #7
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Check valve clearance/ broken valve spring.
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