Sport Jet 120 high compression low top speed

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MercFamily

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Hello all, glad to be here. Thank you in advance for any help :)

Fixing my fathers boat up so my family can have a ski/pleasure boat for the summer. Its a Sea Rayder F16 with a Mercury/Force Sport Jet 120(not XR) I got it running well enough to try on the lake. The boat doesnt have the rpm/top speed that it should. The engine runs perfect it just sort of hits a wall at 35mph and 4200rpm(if i remember right). Manual says 5000rpm at WOT. I have checked everything on this engine. Carbs cleaned and checked for WOT, Oil pump checked, fuel pump checked, timing checked. Checked compression today and got from cylinder closest to flywheel down, 160 - 170 - 170 - 170. I have the Mercury repair manual and didnt see an exact psi reading but from what ive read should be 140 - 150 - 150 -150. My question is, doubtful but, could a plugged exhaust cause the high comp. reading? It would certainly explain lack of rpm. If not exhaust then what...?

And before anyone asks, impeller to wear ring is .013" ride plate seal is good and the reverse gate is up all the way. Nothing to do with engine rpm but I figured someone would ask.

Thank you all!
 

MercFamily

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Jun 20, 2014
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Double checked impeller this morning, was thinking someone may have changed to a different impeller but its the one that should be on there
 
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