rpm/prop questions

[hadvisor]

I just bought a 1987 larson 17ft citation - had a few questions(I did order a service guide from mercury for the record...) The boat has a 4cyl merc 180 serial ob702300 transom ob759124. The boat runs great had all the major items replaced/repaired - we have a lot of fun with it so far..My major question is max wot rpm on motor states 4400-4800. The boat came with two props one of them a 21p I think the other one is a 19p. I like how the boat runs with the 21p - we don't pull anything yet - general crusing and speed I guess. I can get the boat to 50mph - the motor is not bogging or sounding poor or overworked - sounds strong. The tach goes to 6000 - have easily gone to 5500 before. Fill me in on the max wot stamped on the motor - Does it sound like I have a wrong pitch prop? Am I gonna blow this thing up? Also, the motor does not trim up as far as it did when I purchased it - 3/4 the way up - low trim fluid probably? The res. looks a bit low. Sorry to ramble - I don't want to wreck anything....<br />Don

 

[Bt Doctur]

Re: rpm/prop questions

compare the reading using another tach first.Never seen a 470 turn that much and stay in 1 piece.13 3/4 X 21P is the normal size for this motor.IF the rpms do go that high Id check the gear ratio of the drive. it may have 1.98s instead of 1.84s installed in it.

 

[Hog Wild]

Re: rpm/prop questions

Probably low on trim fluid and can just refill it. Do so before it starts getting air in the system as that can be a real pain. But it shouldn't be low so I'd advise keeping track of if it's leaking to become low?<br />Your engine should run like you said 4400 - 4800. Mostly you want to cruise at like 3200-3600 if you want long life out of your engine.

 

[fastimz]

Re: rpm/prop questions

I have a 1989 Larson that the tach said I was running 6000 at WOT. I didn't believe it so I got a new set of gauges. When I pulled the old set out the Tach was set on a 6 cylinder setting for my V8. I would start by checking that. Not sure if Larson has some wise-guys setting up the tachs.