1988 mercury 9.9hp 2 stroke won’t get up to gig rpm

Hi, I have a mercury 9.9hp 2stroke and will not get up to high rpm while in the water.. I can rev it up in neutral and it will rev high but when in forward gear seems to only get up to 2500rpm or so ( just a guess don’t have a tach). I’ve done a lot of work to it but nothing has fixed it, please help with any input on to what I should try next. Here is what’s been done so far:

-ignition wires, sparkplugs replaced
-carb pulled apart and cleaned( nothing replaced but I’m pretty sure I did a good job cleaning and everything looks good)

- then took to outboard shop and they replaced the black box, they said I have no thermostat( previous owner must have pulled it out) they say it’s still off and maybe the exhaust port has carbon build up..but they said it should work... well still not moving very well.

- then I tested stator ohms and 2 readings were off so I thought yes found the problem, replaced stator and now seems to run a little better but still no speed.

compression; both cylinders at 98psi ( not great but should be ok)
- spark is nice and blue and will gap a 3/8 jump.

pull spark plugs off and top cylinder Plug is wet and fouled and bottom is dry and tan coloured

not sure what to do next, seems I don’t have much left to replace and it’s still very sluggish and the guys on kayaks are passing me on the lake.. very very frustrating. Any suggestions of what to try next would be greatly appreciated.

thanks

These did not have a thermostat from factory, it was an "option".

Sounds like they replaced everything they could and didn't bother to clean and/or rebuild the carb which sounds like maybe part of the problem from the symptoms you describe.

However the one wet plug/one dry plug could also be an upper or lower crank seal that is leaking, a bad transfer cover gasket or a bad powerhead base gasket (all somewhat common issues on these models as they age).

Feel up under the flywheel. If you find an "oily/gloopy mess" you have a bad upper crank seal. A similar mess in the lower cowl could be a bad power head base gasket. If the mess is on the right side of the motor (standing looking at the prop), just forward of the water jacket cover (what most think of as the heads) then that would be the transfer cover. I don't know an easy way to "see" the lower crank seal.

If any of those are "leaking" you lose power, engine may sneeze or backfire and the 50:1 mix gets "blown out" of the (bad seal). The gas evaporates and leaves the oil behind as an "oily/gloopy mess".

Thanks for the response. I decided to pull the flywheel and i have Found some oily substance on top of seal and around stator so I will be replacing the seal.. I also thought I would pull of the the power head and take a look at the exhaust tube. Holy molly the carbon build up in the tube had completely sealed up the 1in exhaust port.. so now cleaned all that out and figure I’d replace the lower seal while I’m this far.

now my question would be how come this would plug up with that much carbon like that? How to prevent from happening again? And Would this cause the massive lose of power?

Thanks

Somebody may have used the wrong oil.

Jeff

Or it has been running very cold (without the optional thermostat).

They (thermostats) are about 20 or 30 bucks with a new cover gasket (aftermarket is fine).

Worth putting in - helps keep the motor at operating temp and improves the idle somewhat, particularly in colder water...

The synthetic and semi-synthetic oils sold now are a world away from what I used 35 years ago, burn a lot cleaner and mix better with the gas but like anything there is still the cheap stuff to be had. Pays to mix in the proper ratios also, eyeball measurement is risky.