I'm having trouble keeping this motor running at full speed. I replaced the fuel line this spring and with new fuel thought the problem was corrected. Then yesterday it went back to the same old problem of running at half speed during full throttle. It's also getting harder to start. When out on the water, it tries to come to full speed but then boggs down to half speed again. Any suggestions?

Need to check compression and make it's ok. It could loosing fire on one cylinder or carb. could be dirty. You might try rebuilding carb.

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is primer bulb going flat?

HI Jim , did you find your problem? I have the same issue here. My 9.9 seems to run fine for 1 month every year then it causes me problems. I am wondering if it is because the water warms up here? I changed plugs. IT seems to run full in reverse and the odd time works full in forward then drops to 1/2 speed. It sound ok , starts alright just runs at 1/3 to 1/2 speed. Bulb is fine, hard not flat. gas new,
Does anyone know is a thermastat would do this ?? Water coming out is hot but not to hot to touch.
this is driving me nuts. ANY help would be appreciated.

Thanks Brad

Try the choke to see if picks up speed, if does then need to clean carb. It may have an ignition problem and lost fire on 1 cylinder.

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Sounds to me like you have a air leak before the fuel pump ,whats happening
is when its first run the leak isnt there then as things warm up the leak develops
and it starts sucking air instead of fuel thus the carb doesn't get the fuel it needs and it starts leaning out . ck fuel line from tank connection to engine
connection,also ck to make sure vent in gas tank is working .