low rpm

[gator-24]

hello..i have a 17 ft aluminum fish ski model deep v..with a 78 merc 70 hp with a 13x15p prop..i am only hitting 4200 rpm @ 28 mph @ wot am i way over propped or could it be somthing else..engine seems to run great..if i go down in pitch i will lose mph..maybee this engine is just to small for this boat..these old mercs are rated hp at the flywheel i think.. thanks

 

[steelespike]

Re: low rpm

I dont know your max rec. wot rpm but I would guess your about 1,000 rpm low.there is surely<br />room to drop 2 inches in pitch and you may actually pick up some speed,but:<br />I would start with a compression check then be sure the carb butterflies are opening all the way.<br /> Do a check on timing and syncrinization.<br /> Be sure the motor is trimmed right especially the virticle height.As little as an inch can mean <br />a 3 mph difference with a corosponding change in rpm. Cavitation plate should be about even with the bottom of the boat.<br />When running wot give the choke a hit if you get a burst of speed then your running a little lean<br /> usually a carb problem.

 

[Dhadley]

Re: low rpm

I would agree to look at the motor and set up. To gain the 1200 to 1400 rpm you need you would have to drop 6 to 7 inches of pitch. Somethings wrong with that picture unless somebody has put some type of really big gear ratio on and I doubt that.<br /><br />A 15" prop on a 17' aluminum boat sounds about right depending on the load. We use 19" on 18' Starcrafts with 70 hp and a light load.<br /><br />Good luck!