My 540 is at 36degree with 9.3 compresion = 710HP . I was hiting a wall also. It ended up being not enough carb. I switch over to a CNC intake and a pro-system 1050 carb built for a 540 then it came alive. Just so happen I have the carb intake and 190 GPH mechanical fuel pump. Ill take 1150.00 for it all has about 10 HR of use. http://www.offshoreonly.com/forums/s...t-housing.html

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Thanks for all of the input. Heres what I have done so far.... Sorry for the lack of info. I seem to have a mystery motor. Knowone seems to know anything about my motor.... 86-92 gen 4 mercruiser 540 575hp N/A. I dont know the compression. I do run 93 octane in every boat ive owned. I know very little about this engine. What I have gathered is that some of the goodies ( intake, carb, ign, ect) that merc had on it was removed and some stock crap was put on it just to get it running. Its a super strong motor. Heres what I have done so far....


Went from a stock dual plane and a 750 to a single plane victor, 1050 dominator with 94 jets, went from 3/8 lines and adapter fittings with stock pump to a 110 gph holley pump and 1/2 in lines up to 5/16 carb line and bumped timing up to 35-36 ish and im only up to 4700 rpm. Now at 72.5mph gps with a 28p 4 blade. Best i had seen previous to changes was 4500 rpm at 71mph. Prop is still considerably too small. Ignition system is basicly stock 454. Could this be killing my rpms? Need to be around 5000. Motor rated from merc at 4800-5200. If not ign, does anyone have any other ideas? Rebuilt carb too btw.

What size jets were you using?

Are you sure your engine is a 540? I think the only 540 merc engine was a bulldog 500. That engine was not equiped with either a dual plane manifold or a 750 cfm carb.... Either way a 750 is WAY small for that engine. My little 439 is running an 830 cfm prosystem carb!

Also what boat and model is this engine in?

Yes mercruiser made a 540 575hp n/a motor in 86-92. This is why i say its a mystery motor. Only a few poeple on here know of the engine. It didnt come with the dual plane from merc. Someone before me put it on there. It came stock with a 1050 and a Gteam intake. Im trying to find out what timing, jet size, pump size, fuel line size into carb, ect. Its in a 01 25 ft sunsation. Heres a link.

http://www.mercruiserparts.com/selec...D8+1986%2D1992

I think you may just be over proped! My engine is around 525-560 hp in my formula 242. I'm going to be spinning a 23 pitch at 6000 rpms. Your sunsation has a more effecient hull but 72 mph is doing pretty good with a 4 blade prop! I'd go to the sunsation area and see what guys are getting for speed with equal hp you may be on target.

How do you know its really a genuine merc 540/575hp engine????? Also, what exhaust are you running????

How fresh is it???? If you bought it used, it could be used up and in need a refresh. It really sounds like it may have been cobbled together with parts from other engines.

If you want to do it right and know the correct specs, buy the Merc manual HP 1. They are expensive, but it could save you a lot of searching for answers.
http://www.mercruiserparts.com/shopManuals.asp

I was spinning a 24p 3 blade with my stock 310hp 454...

The engine runs very strong so there is nothing wrong with it. Its not "used up". Im just not sure what I need to do to get my rpms up. Stainless marine exhaust. No Its not cobbled together. Just some of the good bolt ons were takin off and used on the engine that it was swapped out for.

sorry, if you have not had it apart, or overseen it apart then you have really no idea what you really have ?? cam could be going away causing lack of RPM, springs as stated before, and yes the ignition could be giving you a issue..you can try dropping in a performance HEI they work awsome and are cheap and reliable...