Yamaha 1995 150 HP, losing RPM's

[jmchach]

I am having a problem with one of my motors in a twin engine setup. I am losing 300 rpms on my motor, it runs fine for a while than it acts up with the rpm drop. I have had the identical problem before and have cleaned and soaked my carbs and this has always fixed the problem, not this time. I have had my carbs cleaned and soaked, I have changed my spark plugs, changed the primer bulb and have inspected the fuel hoses and none of these fixes has worked. I have had the same problem many times but a good carb cleaning has always worked but not this time. The only clue I found is when i changed the plugs, one plug was much dirtier than the others and had a bunch of build up around it. I thought it may have been the problem but after changing all the plugs, the problem has continued. Does this mean that there was no fire on this cylinder sometimes which would have caused the dirty plug? Could it possibly be a coil failure or possibly a bad spark plug wire or could it be fuel pump related? Any suggestions or questions are greatly appreciated.

 

[12Footer]

Re: Yamaha 1995 150 HP, losing RPM's

Welcome to iboats. The dirty polug is the working plug. The new-looking plug may be indicating water in the combustion chamber.First, drain the associated carb bowl and see if there is any water in it at all. If so, chack the tank for signs of water.<br />Rulling that out, I'd start looking into the switchbox or the coil powering that clean plug.<br />A healthy 2cycle will blacken plugs in one outing,and should be considered normal. <br />I don't have the manual for Yamaha, but they are available at the top of the forum page.<br />Kinda pricey, but they ALLWAYS pay for themselves.<br />But you are on the right troubleshooting track' so far.

 

[seahorse5]

Re: Yamaha 1995 150 HP, losing RPM's

If that one plug had a buildup that looked "crusty" or had little chunks of carbon, you may have a small water leak most likely in the head gasket. A larger amount of water will wash the plug and the piston clean, but small amounts remove just tiny bits of carbon, and they show up on the plug.

 

[jmchach]

Re: Yamaha 1995 150 HP, losing RPM's

Actually 5 of the 6 plugs are normal looking a little dark but not overly clean, sorry for not being clearer. The dirty plug had a lot of black crud built up on it. Can a switchbox problem just affect one cylinder or would it shutdown a whole side? How can i test a coil?