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Old 04-28-2010
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Cal 39 maximum RPMs

I just put a rebuilt Perkins 4-108 engine in my Cal 39. I have a Martec feathering prop, and I adjusted the pitch so that the engine tops out at about 3000 rpm. My mechanic says this is about where the engine should be. Problem is that I get the engine rpm up to about 2300, I get quite a bit of cavitation which makes the rudder and wheel shimmey and shake due to the current coming off the prop. I had the old engine topped out at about 2500 or so and she clipped along real sweet at about 2000 or so and 6.5 knots, with no cavitation. The theory is that to develop maximum horse power, i should be at 3000, but it doesn't make much sense if I get so much cavitation that I'm stressing the rudder. Just wondering where everyone else's engines top out in rpm's. Thanks.
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Old 04-29-2010
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As a general rule all marine motors need to be able to reach there full RPM if NOT

It will cause problems because the injector will PUMP in extra fuel trying to reach the RPM it is set at and if it cant reach it the extra fuel causes cloged exaust elbows and oil dulition issues

I would think you have some type of balance issue causing the problem
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Old 07-14-2010
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cal 39 rpm

Okay, I did some more adjusting to the martec prop. I had the engine topping out at about 3400, so I increased the prop pitch so it now tops out about 3000 (I didn't have it there before). It runs real sweet now at 2400-2500 rpm, boat will go over 6.5 knots in a chop and headwing towing a dinghy. No vibration or cavitation at all!
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