99 Yamaha GP760 randomly losing 1 cylinder?

sptjet90Reflexx

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My GP760 feels like it is dropping a cylinder randomly while running at WOT.. it will slug down then pick back up right away. anyone have a place to start?
 

sptjet90Reflexx

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Sorry for the late reply. I pulled the carbs apart and cleaned them and I see no improvement. It is still randomly dying..
 

sixfigures

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probably a coil or plug wire issue or perhaps a ground issue at the ignition coil
 

sptjet90Reflexx

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the coil on these are single coils though... wouldnt that mean if the coil had an issue it wouldnt run at all?
 

rogerwa

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Be curious what a compression test shows.. Also when you disassembled the carbs, did you readjust them according to the Mikuni guide? The other thing is to look for potential cracks in the fuel lines where is could suck air.

I could be wrong, but I think both plugs fire at the same time on each up stroke.
 

sptjet90Reflexx

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Rogerwa, sorta correct. This type of coil setup is whats known as a wasted spark ignition. Even though one cyl in on a power stroke and the other is not they still both get spark at the same time. Ill double check compression but it only has 80 hours on it and we bought it brand new in 2000.
 

sptjet90Reflexx

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Also I should note that before I pulled the carbs off myself, I paid to have the whole ski "refreshed" which basically included emptying the fuel tank, replacing the fuel lines and cleaning the carbs.

I thought possibly the carb screens were gummed up but they were very clean as is the fuel filter.. To me it has to be ignition related cause I can see no reason why it could be fuel.
 

sixfigures

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Check each plug wire , the cores can turn to dust, remove the boots and pull on the inner wire using needle nose pliers if it is good it won't move if it is bad the wires will pull out.
 
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