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Mercruiser 120 sterndrive with a 165 sterndrive.

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Slash8

New member
My question is will a Mercruiser 120 hp work with a Mercruiser 165 leg. The gear ratio difference is 120 = 1.98 and the 165 = 1.65. I have recently put a 165 leg on to a 120 hp. The boat is a 1975 Glastron 17'. It will not get past 2000 rmp at WOT. Should it? Will that much of a gear ratio make a difference. The prop that is on it now is a 19. i know i should got to like a 16 or 17 but why won't it open up when the the throttle is wide open? Is it a carb issue or a gear ratio problem. All I can get out of it is 20 mph and it takes forever to get there.
 
First, a 120 in a 17 footer is marginaly sufficient under any circumstances. A 19" prop may have been too much pitch even with a 1.98.
Going to a 1.65 ratio could make a significant difference. Assuming (assuming!!!!) everything is good with the engine (compression, timing advance,
fuel flow, accurate tach, etc.) you need to reduce the prop pitch to the point you can achieve the max recommended wide open throttle rpm, trimmed out for max boat speed. Dont know exactly what the max rpm is for that age engine, probably around 4200. With each 2" reduction in
prop pitch you should increase max rpm by about 300 - 350, and will vary boat-to-boat. 2000 to 4200 only using pitch would mean you need
to reduce the pitch by at least 6". Try a 15" (alum props are in 2' increments), but you may end up with a 13.
First thing to verify is that the tach is correct.
 
I'm pretty sure it is running fine, it idles fine, in neutral it revs out fine as well, it just won't seem to gain speed when you crack the whip. The tach is correct.
 
its just like putting your car in 5th gear with a trailer on the back.the WOT for a 120hp is 4200-4600 RPM,so you need to start at say a 15 inch pitch and see if yr revs go up.your spinning the prop faster with a lower ratio.my 18ft boat with a 120hp motor will crack 30 knots with a 17pitch,so theres enough power there.the correct prop for each situation takes a bit of experimentation
 
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