merc 90hp wot

[chipzinger]

My boat manual says that wot for my 90 horse is 5200 to 5500 rpm, while on the lake yesterday at wot I could only manage 4600 rpm at 36 mph...even though the boat seemed to run fine and it sounded good to me. Can anyone tell me does that sound normal, I did have my boat at the marina where they installed 2 new throttle cables, Boat is a 18 Fisher,

 

[achris]

Re: merc 90hp wot

Before the cables were replaced, did the engine make 5200-5500? If so, take it back and get them to get the adjustment right! If not, tell us the prop pitch.<br /><br />Chris.........

 

[chipzinger]

Re: merc 90hp wot

I,ve only had the boat in the water twice and I didnt do a wot on it cause I was having some issues with the throttle cable, I dont know the pitch of the prop its a factory prop that came with the outboard if thats any help..

 

[achris]

Re: merc 90hp wot

Chip,<br /><br />On engines 30 hp and above Mercury did not supply 'factory' props. It would have been fitted by the dealer who fitted the engine to the boat. The pitch is stamped on the hub of the prop, between 2 blades. It will be a number starting with '48-' then 6 to 8 numbers then a number which will be 15, 17, 19 or 21 followed by a 'P'. This last number is the pitch of the prop in inches. That's the number we're interested in. <br /><br />Have you checked that when you push the control lever to the full throttle position it's actually opened the carb right up? You can check by removing the engine cover and looking at the throttle levers on the left hand side of the carbs.<br /><br />Chris...............

 

[chipzinger]

Re: merc 90hp wot

Thanks I will do that, and get the prop pitch..

 

[chipzinger]

Re: merc 90hp wot

The number off my prop is 48-77348-415-21P<br />if I read this right its a 21 pitch from reading previous posts higher the pitch less rpms at top speed....so 4600 rpm at wot might be correct,can someone correct me if I'm wrong....Thanks

 

[achris]

Re: merc 90hp wot

Chip,<br /><br />Good work. Yes, the prop is 21" pitch. The full number is almost right too. It's 48-77348 A45.<br /><br />That prop sounds a bit big to me. My brother has a 17 foot boat with a 90hp Merc and he's turning a 17" prop. The thing is, that running that prop is seriously overloading the engine (and not just a WOT) and will lead to serious piston damage. <br /><br />See if the marina can let you test a smaller prop, with a view to buying the right one.<br /><br />Chris...............

 

[chipzinger]

Re: merc 90hp wot

Thanks achris your information is greatly appreciatred, I will look into purchasing a smaller pitch prop.....Thanks again