What's wrong with my Yamaha 25hp 2-stroke outboard?

Question

It starts and runs ok at idle, but when you add throttle, it shakes and smokes instead of building rpm's. I put new plugs in, but it didnt make any difference. I checked the spark, and Im pretty sure it is getting fuel, because it smokes and smells like gas. Bad coil?

Answer 1

sounds like the carb is gummed up a simple rebuild will clean it out . Note always run the fuel completely out of the lines on any small engine after use.

Answer 2

I AGREE CLEAN YOUR CARB & ADD AN INLINE FILTER++++

Answer 3

More likely a bad carburetor. When it idles it runs, when you increase the throttle setting, it misfires, the other possibility is the spark advance is bad.  Take it to the shop and have it serviced. It could be something serious like a bad powerhead.

Answer 4

Why check spark?  To rule out bad coil (among other things).  :D

If it's getting spark & fuel, that just leaves compression -- did you check compression?

Fuel in the carbs isn't the same as fuel in the cylinders, as these other guys pointed out.  Could be getting too much fuel -- through worn/leaky float valves.  Carb rebuild is the fix for that too.

Remove, disassemble, soak overnight in carb cleaner (I use B-12), blow through passages with compressed air (or aerosol carb cleaner), reassemble with new kits,reinstall, link & sync, adjust slow speed mixture screws (if equipped).