Thread: New 383 Stroker engine will not rev.

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Try a prop with less pitch, let it rev past the flat spot. Not a real fix, just a test.

Cam comes in at 1350 -5500. It seems that we are at an impasse. Motor builder says now. ( I guess thats all it's gonna do)
5200 on a shop tach is the best that we have seen. I'm thinking that although the boat runs much stronger than it did before. Much quicker to plane, and I can stay on plane at a lower RPM than before. Will plane off with 4 adults in it now as compared to last year when you had to move everybody up front to get the front end down. Boat design is rear heavy and all the scorpions I've seen lay low in the water in the stern. I guess I'm just gonna wait for the cupped and vented stainless 19 pitch to come back from the prop shop and see how it works then. My son blew the upper gearcase on his boat last week end , so any extra funds are going in that direction right now. A Friend's Scorpion will run out to close to 70 mph. but his motor was dynoed at close to 600 Hp. Has to run 93 and the friggen thing has no idle. Docking the beast is almost impossible unless you paddle it in.

Installed a new stainless 19 pitch and had it cupped. Boat runs very strong and quick for it's size up to about 4700 and thats all there is. Same stuff. A little faster with the new prop, but still won't rev past about 4700.

Get your engine builder to re degree the cam.

What does the manufacturers specifications say? You need to talk to the factory about this and have them send you the specifications in writing if you don't have them already. Don't ask the local guy that rebuilt the motor because according to your postings he is telling you two different stories. First he tells you that you should get 6000 rpm then tells you that it is set up for max of 4000. Well, which one is it?

You are talking about two different things here, rpm's and speed. In the boating world these two things aren't necessarily related. There is no transmission except F-N-R and no rearend gearing to consider. There is a gear ratio consideration in the lower unit, but it shouldn't vary that much. Speed has to do with trim position, how clean the hull is, weight, and of course the prop. The aluminum prop is going to perform differently than the SS prop because of flex in the aluminum. Anyone can make your boat hit 6000 rpm, just drop down in pitch. But you won't have the top end speed and you'll spend a ton of money on fuel. Boats are kind of like running a car with one gear (prop). You can prop it for low end performance like skiing of prop it for top end speed.

If you want speed go up in pitch and use a ventilated prop to get you out of the hole. Get one with different size plugs (usually a set of three) and test them. Also, most dealers worth their salt, have test props and will let you try them if you plan to purchase from them.

I say, check for fuel restriction. Line is either to small, kinked or pinched, or filter needs to be one of higher capacity That small filter needs to be pitched and the fuel tank pickup should be inspected. Also, is this motor broke-in yet?

You have a lot of excellent suggestions in the replys to your question. I have one more. Don't make too many changes at one time without testing the boats performance as you go. Change one thing and test and so on.

Good luck!

Engine may have 5 weekends of running on it. Out of that maybe a total of one hour WOT each week end. Fuel pump now delivers at 5 lbs. WOT. 3/8 Fuel line. I was thinking fuel problem , but plugs don't read lean. I've tried direct ignition jumper, in case of wiring problem, but it made no difference. I tried a new 14 X 21 pitch aluminum, but it didn't work as well as the 14X19 lost RPMS and speed. The engine just seems to run right up to about 4700 and then thats it. Runs strong at WOT , just won't rev any higher.
I've fiddled with the trim. With the boat trimmed up just to the point of starting to porpoise just a tad and then a very small trim back. Frigen thing seems to be flying , but the speedo and GPS say something different. Does anybody know if a dolphin on the lower unit will slow me down ? It certainly helps getting up on plane in this tank. This boat also has anti-fouling paint on the bottom from it's days in the salt. If it would rev to 5500 it would go 55mph. Thats all I was ever looking for. My friends Scorpion will run out to over 70 mph ( radar clocked) Same Boat, Same hull, Maybe 500 lbs. lighter. But I'm at around 400 Hp and he's close to 650. He's also running a 14X19 Cupped stainless, but his motor will rev to 6000.