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Mystery solved. Dealer took it out and had the same thing could not get over 5100. He said it sounded like it was boging down on the top end. Brought it back in for inspection, thought it might ne a reed valve, looking around finds a washer sitting on top of the plenum----- whats that doing there, got to looking and discovered the factory installed a plenum marked for 150 use only... Ordered the correct one, almost 1/2 bigger, and now hits just about 5500, which is fine on a 5700 max wot.

Still looks like your motor is too low.
The anti-cavitation plate on the foot of your motor should be about even with the water's surface while on plane. You might see even better performance by raising it up a bit (less drag). It will definitely improve the porpoising if that continues to be a problem.[/quote]

I measured it and the cav plate is level with the keel.. Will have to see what happens.

But your keel will not be level with the water's surface while on plane unless your running a nitrous system and getting about 90 kts out of that merc!

OK splain, if I already have a trim issue what will raising it do..

Raising the motor won't affect your trim angle appreciably, it will reduce your drag (less or the foot in the water) and help with porpoising. Right now your anticavitation plate is even with the keel. Next time you're out, have someone drive while you look over the transom. Unless your boat is extremely light and fast (like a bass boat) your keel is probably under several inches of water, as is your anticavitation plate. Most people recommend running the engines at a height that places the plate at the water's surface while you're on plane and trimmed out. Too low and you've got a bunch of the motor's foot dragging through the water and slowing you down. Too high and the props can break free and also can result in chine walking. My boat was almost undrivable due to porpoising and inability to trim with it's original set up. I had the motors raised to the top hole and essentially fixed the problem.

Guess I check it out as soon as I can. Thanks all.

Don,
Sometime back I learned to forget the way it's suposed to be and try till it works right. I'm with Tom on this one.

I have a very heavy 18' Custom Craft CC. A while back I had the 90 horse Johnson off to repair steering cable. I raised my motor at the highest hole on the mount when I put it back. The plate runs 3 inches above the bottom of the center keel while all the way down. When running, the plate skims right on the surface when its trimed out.

I picked up 4-5 miles per hour on cruise and WOT, plus I can get up on plane and run in skinner water. It catches a lot less floating grass while running on plane.

Opposite of that thought. I was under the impression porpoising was due to you prop loosing bite. Raising it you'd lose more bite it seems.

CSMarine wrote:I was under the impression porpoising was due to you prop loosing bite. Raising it you'd lose more bite it seems.

Thats what I thought, so I am not sure what to do..

Don, raising the motor will cut down on drag. You might need to have the blade tips on the prop cupped to cut down on the porpoising. That will also cut down on the prop blowing out and losing it's bite.