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View Full Version : '93 25hp merc hop-up



naughtybynature
07-30-2002, 08:37 PM
has anyone done mods to this motor? can i get a bigger carb and would you recommend drilling the leg to relieve the exhast? any suggestions would help. the manual sez 5000 rpm - 6000 rpm, is it safe to spin it closer to 6000? the stock 13' prop only spins it to 5000rpm. would you recommend knocking it down to 11' to bring the rpm closer to 6000? thanks for your help http://free.***boat.net/gallery/Reader_Rides/Outboards/tn_jvn05.JPG

HavasuDreamin'
07-31-2002, 05:56 AM
I would leave the prop alone for right now and drill out the exhaust. It will be the best bang for the buck and you will not lose any reliability. Adding a larger carb without drilling out the exhaust does not make sense to me. You are just flooding the engine with more fuel without opening up the exhaust to let the fuel out and to let the motor breath. Drill the exhaust first, my guess is you pick up a few hundred rpm with the same prop. Good Luck :)

nhbuoy
07-31-2002, 10:16 AM
HEY NAUGHTY, I know from experience that you won't gain the kind of rpm you're looking for by relieving the exhaust,but it will give you a little bit more and it sounds really cool so do it. Since you'll have the drill out you can also relieve the prop W/small vent holes. If U have a manuel it explains the procedure. (It's mostly for slow planing boats...it helps aerate the prop to create rpm...)If you are going to rework the prop,do it a little at a time. I would suggest cutting it by a half to 3 quarters of an inch and at the same time adding "some" cup to help W/bow lift to get rid of all that trim you're using to carry the bow and send the thrust in a more mph/direct route. As for the carb,I know the OMC35 will bolt to 25 but for max power you have to replace the reed cage and intake and the carb. With any luck MERCS are the same story. If so you now have pretty much "factory" 35 hoss. (FYI>>A guy at IBOAT>COM is spinning his 25 OMC(bored .030 over) at 10,000 RPM and supposedly thats the only mod!!!???) That's where I got the info on the carb swap....Hope this helps.........VELOCITY BOATS RULE!!!!!!!!!!!!!!!!

naughtybynature
07-31-2002, 01:03 PM
i talked to a local hotroddr and he spins his johnson 25's (twins on a 14ft aluminum.....don't ask) to about 7000 rpm! you should hear them scream. they are bored and that's it. as for the merc, he told me that they can't be bored because they have a cylinder sleave or liner. interested in the venting of the prop though. i have read about that with the bigger boats. i just finished fabricating my new billet aluminum transom plate that includes a wedge to get more negative trim. that helped alot with the planning. i did notice however that by my measuring mistake the motor now sits 1/4 ++ higher on the transom and the stock prop seams to ventilate with much easier. i was conscerned at first but will give it more seat time to see if i need to bring it back down. i have not had a chance to see if it will plane with two people in it. a good thing though is that i seemed to pick up a couple miles an hour (good for consistent 35 on the speedo, where i was only managing 32 in most conditions before) due to less gearcase in the water. i am really curous to know what i cam safely spin this motor to. i just doesn't seem to be "gettin on song" like it should. i was foolin around last weekend with it and over trimmed the stock prop to make it spin closer to 6000rpm and man did it sound sweet! just a note that the pics with the big rooster is only from the 18" prop and not the 13. i get no rooster with the 13. i have to overtrim just to get the rpm to 4500. imagine if i could get it spin 6000 without the trim! i should be good for 40+
keep the suggestions coming. oh ya... need to make a fuel tank, should i put it in the bow or leave it in the back where the plastic one is now. the battery is is in the back in the middle.

nhbuoy
07-31-2002, 07:12 PM
HEY NAUGHTY, I'd keep as much weight in the back as possible. (tank & battery). You won't need as much trim to carry the bow,although it won't plane as quickly...so what,top ends what U want!!!Vent holes in the prop is explained in my SELOC OMC manual but not in the MERC one...GO FIGURE! I would explain it here but I think U should see the diagrams to really "understand" where to drill the holes. If you have a SELOC for OMC look in the first chapter under propellers/safety. If U can't get your hands on one let me know and I will fax a copy to you......VELOCITY BOATS RULE!!!!!!!!

naughtybynature
07-31-2002, 07:27 PM
hey thanks bouy, your input has been very helpful. still looking for some input on the rpm thing. no one i know has a 25 merc or even cares how to get the most out of it. i live in a very small looging town full of fishing boats and big trucks. hmmmmm.....

nhbuoy
07-31-2002, 07:38 PM
HEY NAUGHTY, What are you saying, you don't have access to a fax or a SELOC manual??? Do they deliver mail???!!?(HAHAHAHA) JUST KIDDING! Isn't there a helo skiing outfit located "near" you? Are you a skier? You definitly have some of the best in the world!!!VELOCITY BOATS RULE!!!!!!

naughtybynature
07-31-2002, 08:30 PM
my buddy sez he can take care of the prop vents. i was until recently the sales manager for mt. washington alpine resort... so that makes me a skiier, and snowboarder, and wakeboarder, and barefooter, and ......

naughtybynature
07-31-2002, 08:31 PM
hey bouy, in your profile it sez you like fast cars.... what ya got?

snappertapper
07-31-2002, 08:39 PM
I know a guy who has a stock 25 merc and it runs about 6500 i would think you can run almost 7000 safely, i used to on my 35 johnson!

nhbuoy
07-31-2002, 08:43 PM
HEY NAUGHTY, I'm a LOON MTN boy from WAY back!!!I'll bet you know a good friend of mine that I think is the GM at attitash and works the MT WASHINGTON buses in the summer...Tom Chasse...?I just sold my 20th anniv 84 1/2 mustang GT convert that I bought my wife new.(needed the $$$ and we both needed new wheels so....) Do you live in the cranmore/intervale area now?

naughtybynature
07-31-2002, 08:49 PM
hey snapper, how does he do that? my stock 13 pitch prop only spins 5000? does he hav a smaller prop. and if so, would the higher rpm bring more speed with a smaller prop?

naughtybynature
07-31-2002, 08:51 PM
back up the bus bouy... this mt. washington is on the west coast of canada... not that mt. washington on the east coast.. do you know how many times i got calls from people with an accent asking for info on MT. WASHINGTON!
hehehehe. kills me. small world eh?

naughtybynature
07-31-2002, 09:51 PM
hey bouy, i just sold my 92 miata that was done to the nuts, now have a sebring convertible. had to get a back seat cause we now have 2 kids.

snappertapper
08-03-2002, 12:42 AM
he has it set so the cavitation plate is above the bottom of the boat by about and inch or two and he is runing 13 pitch also i think , with ym 35 johnson i could run almost 6500-6700 with an 11 pitch

naughtybynature
08-03-2002, 08:19 AM
i think i am right on the line with my hieght, if i go any higher i think the prop will be slipping too much. i know this because i can't trim it up very much anymore befrore the prop ventilates and loses grip, the boat really slows once this happens.when i made the transom plate i added another 1/4 inch. keep in mind that my hull is far from conventional being a mod-vp with a notched transom. what works for others may not work for me. i guess my biggest questions is that if i go smaller on the prop, say 11, will the gain in rpm give me more speed or will i loose speed with the smaller prop? hmmmmmmmmm. noeone in my redneck town can help with this. here a 25 is used on the back of your fishing boat to trawl with. HEHEHE

snappertapper
08-03-2002, 09:09 AM
im not too sure if the smaller prop will help or not but if you have vent holes cut in your prop you should be able to go higher!!!

naughtybynature
08-03-2002, 01:41 PM
just got back from the lake, if i trim it right to the point where the prop wasn't breeking loose, i was at 5000rpm exactly. if i went any more (wich wasn't much) the prop would break loose and speed would drop drasticly) i'm thinking i need less pitch and more cup to carry the nose like my 18 chopper will. then i think i will have the right setup. anyone at all with prop knowledge to help me?