Sledge Hammer

It sure doesn't seem you should see a 1300 rpm drop going from a 21 Mirage to a labbed 25 Mirage plus. 5700 to 5900 seems like too much rpm for that comination. My old engine was a 468 with the 731 cam and a 750 carb. It ran great in the 5300 - 5400 range. I am guessing that a 23 M+ is going to be closer to the right prop once all the bottom paint is gone. You didnt say if you changed exhausts. If you haven't, you are choking the engine. I think your boat is going to be to heavy to pull a 25.

endeavour32

iTrader: (4)

I'm running stainless marine exhaust manifolds. For the most part the paint is off the bottom and I picked up just about 5 mph. The current 21 I'm running is in fair shape and I know I'm loosing a few mph by using it. I always thought that a labbed 25 would be equal to a non labbed 23 as far as what the engine sees? Am I wrong on this?

Thunderstruck

Slip is high? Did you check you numbers carefully? Are you trimmed out too much??? Slip number is also affected by more than the prop as you figured out with the paint. Do you have a hook in the hull??

endeavour32

iTrader: (4)

Yes the formula 242 hull has a hook. My last 242 also had the hook but the slip was closer to 13% which IMO is normal. The prop is only in fair condition so that is adding to the slip. I'm going to try the 25 again tomorrow along with a fresh fuel/water seperator and see what happens.

Formula311-SR1

I went through some of the same growing pains with my 242. The boat was very sensitive to prop setup and motor setup. I had added 50 HP in theory and slowed down from 68.7 to 66.7 with an aluminum headed 502. What I found was the boat liked a lot more fuel, spend some time out with the GPS doing fuel pressure and jetting checks. In my case I just added fuel till it quit going faster and stopped. Other thing was I tried 21 mirage pluses to get the RPMs up and the boat would not run any better, my best results were with a stock or labbed 23 mirage plus. With the hook in the hull we found adding cup for bow lift was the ticket. Tried 25s and always slower even if RPMs were in range. Test and test and don't give up

endeavour32

iTrader: (4)

Well I got out today with the labbed 25 and a fuel pressure guage! The boat will now pull the 25 without much problem up to 4000 rpms, then the engine will stall out unless I pull back. I hooked up the pressure guage and I'm only getting 2.5 lbs from idle to 4000. At 4000 the bowls are drained! So that tell me one thing the fuel pump is junk!

jmrapp

Looks like that is your issue or at least part of it.

endeavour32

iTrader: (4)

Ok after a ton of testing here is what I found.
The fuel pump was not the problem not even the wimpy 2 psi pump was a problem. I repleaced with another stock pump got 4 psi and same speeds. Replaced that with a 7 psi/120 gph and still same speeds. I then tore into the carb, there were bits of rubber hose in the secondary needle and seat. Removed and problem was solved but then it happened again. I have a feeling that the fuel rail hose may be junk.

I was still not able to pull the 25 pitch labbed mirage plus, 4400 rpms max vs the 5800 on the 21 mirage that is banged up. I'm going to get a 23 Mirage plus today. I'll report the results.

PatriYacht

Based on your engine specs, you have a high winding engine. The cylinder heads you have flow more air than iron Chevy rect. port heads. The cam has similar timing to a HP525 cam which has 62 cubic inches more so your engine will want to rev higher.. Your builder advised you to run it at 5900 rpm's. Hopefully he set up the valvetrain correctly because you are running on the edge of valve float. He is correct that the power peak is up there. After looking at the Bam (www.go-fast.com) speed and slip calculators, I doubt if you will be able to pull more than a 23 pitch Mirage or a 22 4 blade. Any prop that slows the engine below 58-5900 will probably slow you down. You might even be faster spinning a worked 21 above 6000 if you can control the valvetrain. There's no substitute for cubic inches and you are a little low on them.

P.S.Fix the fuel system. More high perf. engines have been ruined from running lean. Get a good fuel pump and make sure you have a steady 5 lbs of pressure at all times.

ezstriper

as far as the fuel issue is concerned look at the anti-sih valve at the tank inlet...very restrictive...but also why would you build a 427 ? great engine but not great torque...cubic inches are key to torque in boats...if I were spending $$$ on doing a engine would have been a 468 min...the longer stroke 454 much better in a boat. now I agree with griff on the props. the other issue with that engine is the RPM thats needed to make power..upper 5000's and the drive is not going to like that all that well....