40 Merc
 

I know I'm about to open a huge can of worms. I bought one of the good old Mercury 40's that everyone claims runs anywhere form 40-99 MPH. I put it on a Southfork 16x46 flat bottom with a jackplate. Same set up with my old 25 Suzuki would run 30 +/-. With the 40 it's running 35+/-. According to everything I hear, it's hard to get those motors to run under 40 MPH on anything. I have
my jackplate set up so that I'm on the verge of blowing out so I don't think I can come up any higher. It is a stock motor with stock carbs. I had the carbs cleaned a couple of months ago when I bought the motor but could it be possible they are not in sync, adjusted right, etc.? I have a tach and I'm only turning up about 5200 RPM. The prop is stamped 14 pitch and does not appear to be altered.

is it the 3 cylinder? most people run 60 carbs in those motors

maybe try lowering the jackplate, also before someone asks which size prop?

5200 is not bad. I think they have a rev. limiter on some of those 40's.

I would drop the pitch to a 12p and add some extra cup if you want to run it high on a jackplate. Is it a 3cly or a 4cly?

This is a 3 cyl and it's a 14" pitch prop.

I got the 3 cyl with 60 carbs, jack plate - 16' alweld w/ trolly...I think I have a 14 pitch prop I run two people and gear 36 to 37 mph...

That sounds like an honest man. I question that some people are running as fast as they say. Reality is that a lot of these "40 MPH" boats don't really run 40 on a GPS. I'm running mid 30's on a GPS and maybe that's all it gonna do without 60 carbs. I just wanted some input.

I'm sure I could hit forty if I would change the prop and mess with the jack plate....but frankly its fast enough for me...

some folks have alot more mods that could push them well over forty but I'm not a engine buff so I'm not gonna get these mods done and then f something up and not be able to fix it myself.

I had a 20 X 36 Weldcraft with a tunnel hull and a 40 HP with a 14P prop. with some cup, that ran in the mid 30's.
I carried pretty heavy loads when alligator hunting and duck hunting. I didn't want top end speed as much as I wanted to get on top fast and carry a load.

Those 3 cylinder 25 Suzuki's are darn near bullet proof.

The 3cyl merc is a tough motor too,look at how many you see on crawfish skiffs.

That's what I had before I got the 40 Merc.
It wasn't built as good as the Merc.
I broke off several parts and the tiller handle broke off also.
It ran good, but didn't stay together well.

in freshwater


carbs, reeds, and exhaust tuner won't really decrease the life. When you get to shaving metal it does.

anyone have any suggestions on where I can find some 60 carbs? I would t mind doing that but I'm not gonna do anything that's gonna tame life off of the motor. I also don't want to spend a bunch of money for a couple of MPH. It's just not worth it to me. I was mainly looking to get everything I could with what I have. I didn't k ow if someone held the secret to that.

I've got a 40 Merc on my tank! 17.5'x 48", all 3/16th aluminum and it pushes plenty for me. The only thing that has been changed is the prop.

By GPS, it ran 32mph with two men, shotguns, a dog and 3 dozen decoys on Wax lake. Since I can't aim for crap, I'm sure there were also several hundred rounds in the boat as well!:*****: While I don't know the speed, it would still get up on top (took a while) with 28 sacks of crawfish, two men and 150lbs of bait!

I don't see me worrying about spending $600 or so on new carbs to gain a few MPH. BTW, I don't know that they cost that much but, I'm sure they aren't cheap!

NLK, I also have a Merc 40 on a 18x42 tunnel, 14p wheel and run 38 on GPS with scary hole shot. Is your wheel cuped? How much? Tuning that motor is going to cost you a grand minimun, do you really want to spend that much for maybe 10 mph? Don't forget you will need a fuel barge to follow you since Sulphite says you can see the level fall wide open. I'd suggest LaAnglers fix, reeds,exhaust tuner.

The best part for me is that we would run a minimum of 7 hrs a day and never used more than 5 gal of fuel in it! It's a beast in the trees and fuel efficient!

yes she does drink gas like margaritas!!

You should be turning more like 5800 RPM. You need to play with the prop. I have the same engine with CMC'd carbs and a new reed plate. It dinos at 65.5-66hp. The rev limiter is gone. You can safely run it at 6000rpm. The best I can get is 5900RPM and that is by myself with no load. I run a heavy boat. 1648, all 100gauge aluminum, steering console, floor, 3 batterys, trollin motor and 12 gallon tank. I can get er up to 40mph.

Cmc'd carbs? Do tell. How much and where. I totally agree with the RPM statement. I would like to turn a little over 5500 and I'm not there.

also, power trim adds a few mph

What is a reed? Can you do all the same things to Yamaha motors, I have a 50 on a 16 ft that doesn't push the boat much over 30mph.

Reed is like an intake valve on a 4 stroke.
It lets air/fuel mixture into the engine but doesn't let it flow back into the carb.
It is used an a carb'ed engine for sure, but not sure about the newer fuel inj. engines.

Where do you find performance mods for boat motors?

You can get the carbon fiber reeds for almost any motor.......2strokes aren't all that efficient. .......they're supposed to help.....

want to trade a 07 camo 1648 vflat with a tunnel for a narrower boat

my boss had a 1442 jon with a 40 merc with tilt trim, jackplate, 2 people 53mph! boat rated for 25 had reinforced transom thats crazy for a small boat,
If I had a 40 wanted more speed, I'd get a prop with a progressive rake and some cup on it, some disadvantages is can't haul as much weight which doesn't matter if your bass fishing with a few rods and a few pounds of tackle,

Yo!

what up dink

my uncle has one on 15 ft alweld and it runs about 35!!!!

NLK, I reread your post and I think you are trying to run to high on jack plate. Try lowering jack plate 1/2" and GPS, mark location with a sharpie. Make notes of speed at each new mark, when you are done you should have a pretty good idea of where the motor will preform best on the boat.
40 mph should be an easy number on that setup to achieve without mods.

Them engines run great, but the tiller handle and motor mounting brackets break too easy.

I have a stock 06 4stroke 40merc on a 20x36 tunnel hull that runs 36mph on gps. That is with just me in it and I have no jack plate. Not sure what pitch prop is on it.

I pull 30-32 mph with my 40 Merc with two men onboard. No jackplate and my boat is a 17.5'X48" tank. I'd like to get more but, it gets me where I need to go and I think my boat would be unsafe if it went much faster!

37 Loaded
 

I had all the bolt on upgrades on my 3 cylinder 40 Merc...ie, 60 carbs, boysen reeds and I had a jackplate that i spent many hours setting up so the motor would run as efficiently as possible. It was on a 17'x48" Weldcraft.

Loaded to fish it would run 5800 rpms and 37mph (on GPS)...if I took everything out just to go run around, I could get 40 - once I got 41 with a favorable wind and light chop.

After I heard my buddy go on and on about how his would do 45 loaded to fish I boarded with my handheld GPS and broke his heart...he was doing 36.5...he swore that I needed to get my GPS fixed!

I guess my point is that 36/37 mph loaded to fish is pretty fast for a comfortable jon boat. I mean, a tin can with one of these super hopped up 40s will get you a few mph faster, but a couple of mph versus a reliable, biggger and more comfortable ride just wasn't worth it to me.