I've got a 1988 mercury 9.9 that has been giving me trouble since I bought it a couple months ago. Here's what I've got going on. This motor is the easiest starting and smoothest idling outboard I've owned however when I give it throttle it starts running rough and doesnt reach the higher end rpms. Under load the top cylinder is not running right. The bottom plug is nice and tan and the top plug is black oily and wet.
I had some extra parts for this engine so I've tried changing out the switchbox, coils and stator with known working parts and the engine still runs exactly the same. The engine never dies and when the throttle returns to idle is back to running smooth as can be on both cylinders. The compression is 110 on both cylinders. I've also cleaned the carburetor and inspected the reed valves so I'm puzzled. I'm wondering if the top cylinder is somehow drawing some water into it from somewhere? I have the water jacket cover removed as well as the exhaust cover and baffle waiting on new gaskets but the old ones looked good. What should i check for next?
Thanks
I had some extra parts for this engine so I've tried changing out the switchbox, coils and stator with known working parts and the engine still runs exactly the same. The engine never dies and when the throttle returns to idle is back to running smooth as can be on both cylinders. The compression is 110 on both cylinders. I've also cleaned the carburetor and inspected the reed valves so I'm puzzled. I'm wondering if the top cylinder is somehow drawing some water into it from somewhere? I have the water jacket cover removed as well as the exhaust cover and baffle waiting on new gaskets but the old ones looked good. What should i check for next?
Thanks