Hi, I have a 90's model 18hp nissan. Since I got it, it would never take full throttle. I cleaned the carb by taking all apart and found some trash in the main jet. Put back together, no luck. I got fed up with it and took it to a trusted boat mechanic a county away. He again rebuilt carb, changed fuel pump (diaphram), used his gas tank, new plugs (ngk), replaced coil, replaced cdi module, replaced all gas lines, has great compression, de-carbon cleaner in the plug holes, still will only take just over 1/2 throttle in neutral, when I put it in gear it barely has power enough to move the little 14' fiberglass boat. All it does above 1/2 throttle is uuuuuhhhhhh, And it dies alot. What in the world can it be? The only thing left is the flywheel and stator. This is a tiller handle, I was running 50:1 then read 32:1 was also good so started running 32:1 but at either ratio it still does it.
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