Boat won't plane with 2.5L merc

[murphy1]

Hello, I bought an 18' Renken bow rider without an outdrive recently. I found a used alpha one with the 1.98 ratio that I needed for the 2.5L 4 cyl. The engine runs great out of the water, but I took it for it's maiden voyage and it wouldn't get up on plane. I didn't know what kind of boat the unit came off of, so the first thing I thought was that the prop was too big. Anyone know roughly what size this prop should be? It would run above 2 grand, so I'm wondering if it could be overproped. Or, any other ideas what it could be, timing? or something else that would hinder the engine when under a load?? <br /><br />thanks in advance

 

[Wart T]

Re: Boat won't plane with 2.5L merc

I think a 17 pitch would be a good starting point, that should allow it to plane.

 

[murphy1]

Re: Boat won't plane with 2.5L merc

thanks for the respose. 'IF' it's not the prop, do you think it could be carb related or something else that adjusts when a load is put on the engine?

 

[Wart T]

Re: Boat won't plane with 2.5L merc

I doubt it. If the choke is stuck shut that will cause it. Stuck shut exhaust flapper will cause it too. Retarded timing can cause low power also. Low compression is another possiblility. You don't say that it is running badly, ie: backfiring, missing, etc. So I assume it runs okay just doesnt get on plane. If it runs badly then you need to fix that first. If it doesnt run badly then it is not a fuel delivery problem or ignition problem other than timing.<br />But check the simple things first. What is the current prop pitch? Check the choke flap and make sure the throttle plates are opening up all the way. You DID spin the drive and check the gimbal before you put it on?

 

[murphy1]

Re: Boat won't plane with 2.5L merc

I did say it runs fine out of the water, but it does backfire when trying to start sometimes. But when it is started it runs pretty smooth. I did notice at idle that it surges. it will idle at 800 rpm and then rev to about 1200 and go back down. it does that a couple times, but don't know what that means. I replaced the gimbal bearing, so I don't think it's that. when out of the water and in forward I can rev it through its range and it's sounds fine, maybe a little stutter at midrange, but I'm sure the carb might be due for a little cleaning? I'll come back next time with the size of the prop before I waste too many peoples time

 

[Rhadley]

Re: Boat won't plane with 2.5L merc

The carb on that engine is famous for bogging during acceleration. I have the same engine and a 17 pitch prop, and at WOT I get 4800 rpm.

 

[murphy1]

Re: Boat won't plane with 2.5L merc

what did you do to fix the bogging problem? is it just in need of a good cleaing?