90 Johnson

[erikleppe]

After a couple years of storage due to jack plate problem on boat I will start using my Johnson 90HP v4 from 90' again. But I would like to have it function perfectly before the prop get wet. Wich rep. manual would you recomend, and wich parts usually have issues on these engines?<br />And how can I get more power out of the engines? I've heard something needs to be done with the manifoils....

 

[Dhadley]

Re: 90 Johnson

Do the usual - carb rebuild, water pump kit, head gaskets and deflectors, and t-stats. All new fuel and oil.<br /><br />Make sure the prop is in good shape. Go test with your average load to get your baseline including water pressure, speed and top rpm's. Thers no since tearing into the motor if you arent getting the full potential out of what you have.<br /><br />Did you get the jackplate fixed / replaced or going without it?<br /><br />Keep us posted!

 

[erikleppe]

Re: 90 Johnson

Well actually the whole stern is replaced and strenghtened to handle more power. the target is above 50 knots. There is now a 20'' Raker fitted on the engine that gives the boat 46 Knots on GPS. But now the stern is larger, and the engine higher , so this year I'll try with a 24'' Raker and see if I reach 50 knots.<br />I have replaced 1 coil, as it was broken. The dealer adviced me to change all coils at once, but I believe that's just to make more money! Or is he right? The coils are rather expencive here....<br /><br />Le boat:<br /><br />

 

[Dhadley]

Re: 90 Johnson

You can change only one coil if thats all thats needed. <br /><br />Good looking ride. Let us know how you make out with it!<br /><br />Good luck!

 

[Spidybot]

Re: 90 Johnson

Nice - good ol' Shakespeare design. Watch out for chinewalking... raised motor should make it less, though.

 

[erikleppe]

Re: 90 Johnson

Yup, both my shakespeares are originals. There is a lot of semi-manufactured Shakespeares out on the market!<br />What is Chinewalking?<br /><br />This is my other Shakespeare:<br /><br />

 

[Spidybot]

Re: 90 Johnson

When a V-hull gets on plane it lifts itself out of the water. The more speed, the higher lift.<br /><br />From a certain speed the boat is running on a very narrow portion of the hull and the stability (sideways) decreases. As the prop is turning in one direction, the grip of the prop in the water makes a counterforce which pushes the hull in the opposite direction. When the hull turns the side down it gets more lift from the water and is forced upwards. Once up, the tourque from the prop pushes it down again and so on. Feels like sideways porpoising. When you raise the prop it will produce less torque.<br /><br />Some boat designs have a flat pad centered on the rearmost part of the bottom to improve stability under high speed.

 

[erikleppe]

Re: 90 Johnson

ahh, I see. And yes, the boat is rather unstable in high speeds in tight turns. And Shakespeare has a rather Deep V hull, but is pretty flat at the rear anyway!<br />But what about a surface propeller? Would that decrease speed or increse it in this case? I'm very happy with the raker prop. but with engine raised it get less resistans and therfore higher RPM at WOT. So I think a 24 pich Raker would decrease RPM at WOT to 5500. 5800RPM at WOT is whats ideal for these engines right?

 

[Spidybot]

Re: 90 Johnson

Max rpm range is 4500-5500 according to my list. <br /><br />When you raise the motor and the prop starts surfacing, it will get higher rpm due to less drag/resistance. You'll have less blade area in the water and so, theoretically, have much less grip - but still sufficient for your motor/boat. It has to be a prop designed for running an air/water mix, and I think the Raker is pretty good for your use. You could look for another pitch (1" = 200 rpm), but I'd make sure that I had a chance to test it before paying. Special racing props and lower units are available but they cost a lot.<br />You have to take care when raising the motor as the water intake on the lower unit must have sufficient water at all times.<br /><br />If you like, go here for a free online book on propellers.

 

[erikleppe]

Re: 90 Johnson

Then I'll have to do something like this:

<br /><br />But I don't think it'll be nessesary, but off course I'll check that the engines gets enough coolant water...