I have a 1990 9.9 outboard motor. It is causing a lot of carbon deposits on the transom of the boat from the exhaust. I think it may need a carb adjustment. Would you please tell me what is needed? Thanks

Only adjustment it has is the low speed pilot screw, if it's running that rich then need to run a compression test, possible rebuild carb, check fuel pump diaphrams.

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Honda Outboard Parts

It sounds like you need to replace the thermostat and adjust the valves