1990 90HP Force prop...

[Beamerman1]

Currently im running at 4200RPM WOT, in the previous posts I was told that it should be around 5200. Everything else (from what I can tell) sounds fine. I checked the prop and it is in good shape, but it has some play (I can pull and push it). The ONLY number that I can find on the entire engine was this "908F90B", it was next to the top spark plug. Can anyone suggest which prop size I should get to increase those RPM. Examining current prop I could not find ANY numbers on it. Also, would anyone know what the original prop size was on that year/size engine?

 

[The Marine Doctor]

Re: 1990 90HP Force prop...

Your engine is a 1990. There was no standard prop, it was whatever the dealer figured would work best for its application.<br /><br />If you need to pick up 1000Rpm...you will need to drop 4p to increase by that amount.<br /><br />If you are currently running a 21 drop down to a 17.<br /><br />If you are running a 19 drop down to a 15.<br /><br />Also..run a compression test on the engine, before purchasing a prop.<br /><br />TMD

 

[Beamerman1]

Re: 1990 90HP Force prop...

Thanks M.D.<br /><br />Is there an easy way to figure out a pitch of a prop when there is NOTHING stamped or engraved on it? Also, how will speed change with the difference in pitch? Currently I'm only travelling ~12-15Mph at WOT. By the way, the engine is on a 24' Pontoon.

 

[The Marine Doctor]

Re: 1990 90HP Force prop...

The speed may not necessarily change as you will be gaining revolutions. It all comes down to hull design and prop slippage. ie...how efficient the prop is.<br /><br />There is hopefully a number under the water that retains the prop. If not its on the side between the blades.<br /><br />TMD

 

[rickdb1boat]

Re: 1990 90HP Force prop...

Beamerman1 <br /><br />On a toon, you should be running either a 13" pitch or a 15" pitch prop. Personally, I believe a prop in the area of 14" X 13" will get you close. You should expect 25-28 MPH if you're propped correctly and your motor is running good. Mine does 24 MPH with a 70 HP rude on it. Thats with the 14 X 13 prop at 5500 RPM's. I gonna try an 11" pitch just because those rudes like to run around 5800-5900 for best performance. Sounds like you are way overpropped. Take it to a prop shop and they can tell you the pitch and recommend the correct prop. But as was suggested, make sure your motor is in good shape by doing the compression test.<br /><br /> Discount Props

 

[backdraft]

Re: 1990 90HP Force prop...

Beamerman, Depending on the weight of your boat, you might try a 4 blade 13 inch pitch. If this allows to many rpm , have a double cup {the tips are bent over} put on by a prop shop. If you go over 52 to 53 hundred rpm, the life of the engine will be dramatically shortened. This type of prop has more surface area for greater push off the line and yet will give you sufficient speed. nevjb