Need more speed,,1986 Johnson 90Hp.

I just picked up a 19 ft Spectrum Aluminum with a Force 85 hp,which I removed and put on my 1986 Johnny 90 with trim & tilt.I would like to run a bit faster with this set up..The props that I tried tonite,,with 3 pretty big guys in boat are,,,OMC Alum prop 14 by 17 cupped ran 35 mph at 4800 rpm's....12 3/4 by 21 cupped ran 35 mph at 4400 rpms...Older stainless (not cupped) 13 1/2 by 15 ran 35 also at 5200 rpms. I liked the power of the stainless but would like a bit more speed to keep up with my brothers boat.Any advice on what brands are best and on what recommdations on pitch and diam.I do not mind sacraficing some low end to pick up the top end.all props were 3 blade.

Re: Need more speed,,1986 Johnson 90Hp.

You missed out on the 13x19 pitch prop.<br /><br />TMD

Re: Need more speed,,1986 Johnson 90Hp.

MR...<br /><br />Your top end RPM is 5500rpm.<br /><br />Take a look at the engine set up on the boat. Where is the anti-cavitation plate compared to the keel of the boat. Just a rule of thumb for standard mountng, the anti-vent plate should be no more than 1 inch below or above the keel. 1 inch above is desireable. <br /><br />What ya' need to shoot for is around 5300rpms. This is a' ok range for the 90's. Gives good speed and longer life to the engine which I might add is more desirable than keeping up with the neighbors boat.

Re: Need more speed,,1986 Johnson 90Hp.

OBJ is right.<br /><br />5300 RPM puts your configuration of boat with a 15 pitch and 35mph.<br /><br />Expecting more out of the engine might be asking too much.<br /><br />TMD

Re: Need more speed,,1986 Johnson 90Hp.

Actually those motors will live longer and perform better at a slightly higher rpm. The target rpm, with an average load, should be around 5800 rpm. <br /><br />As suggested, work with the set up and X dimension (mounting height) to obtain maximum performance (and therefore efficiency) with each prop. Be sure to use props of the same brand for accuracy in your comparisons. Generally speaking, props with larger diameter can utilize a higher X dimension.<br /><br />Sounds like you have a fair baseline. Its kinda odd that they all ran the exact same top speed. Was that the top rpm for each or was that the rpm it took to run 35 mph? How was speed verified?<br /><br />Let us know if that was top speed and rpm for each prop and we can calculate slip % for you.<br /><br />Good luck!

Re: Need more speed,,1986 Johnson 90Hp.

Hey Marine Doc,,what do you mean I missed the the 13 by 19 prop.My brother has a 14 by 19 OMC prop that I have yet to try.Outboard Jim,the cav plate is about dead level with the bottom of the boat...dhadley,,,speed was just checked with a regular speedo,actually tried 2,and they read the same,which seemed to be about right,seat of the pants feel.All props were running wide open,,but like I said,we are all pretty large dudes.I realize that this a lot of boat to push with this motor.What are some recommdations on prop brands,or are they all created equal.I actually hit 38 with just myself in the boat,with the 12 3/4 by 21,but I very seldom run alone.I have also heard that it is possible to put carbs from a 115 on this motor to gain performance,is there any truth to this.

Re: Need more speed,,1986 Johnson 90Hp.

Yes, the 115 carbs will fit. With the port timing of a 90 it will burn more fuel and have a slower holeshot. Besides, at 5200 rpm and less you arent getting the most out of your 90 hp. <br /><br />Try raising the motor on the transom, especially if you end up with a prop with decent cup and / or large diameter. remember, youre looking for 5800 rpm with an average load.<br /><br />Good luck!