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Discussion Starter #1
I have a merc 9.9 2stroke on my 16 foot deep vee. I was pleasantly surprised how well it performed eeven with 3 people. How much more ooomph is a 20 4stroke going to have on the same boat? I have been told 4 strokes don't perform like 2 strokes? Better idle, quieter, better on gas but not the power?
 

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I'd disagree. A 20 4 stroke is going to plane quicker and increase your top end. It'll weigh a touch more but overall it'll perform far superior to the 9.9.
 

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If you like the 2 stroke another option would be the carb swap on your 9.9 to make it a 15hp. The performance increase would be very good and alot cheaper then buying another motor
 

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It also depends on how you fish if you troll downwind with a 2 stroke you might as well smoke a pack of cigarettes. As far as oomph it might add one or 2 mph it isn't worth it if your motor runs well. Now if your buying new 4 stroke all the way quiet, no fumes and no mixing. When my dad did our 70's Johnson 9.9 carb to a 15 we thought 1/3rd more speed... nope just 1/3rd more gas.
 

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Discussion Starter #5
I'd disagree. A 20 4 stroke is going to plane quicker and increase your top end. It'll weigh a touch more but overall it'll perform far superior to the 9.9.
That makes sense to me. I'm probably going to get a new 20 hp merc, seems like it should make a good package. And legal in the lakes around here.
 

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I went from a 9.9 Johnson with a 15hp carb. to a 20hp johnson both 1980's two strokes.What a difference!Went from going 14mph(with prop guard) to 24mph no prop guard.The 15hp did go faster before the prop guard was on, but would not be as fast as the 20hp.
 

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A 20hp merc 4 stroke is a carbureted 4 stroke. The carbed 4 strokes are total junk. Hesitates, have not seen one that didn't and just a poor motor. Find something that is fuel injected or you will not be happy. Or find a 2 stroke 20 merc,much better motor.
 

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I will only peak about the 2 stroke vs 4 stroke motors.

I had 2 200 HP Yamaha's on my boat and changed them out for 2-200 Mercury Verado's.
I needed to watch the added weight the 4 strokes had since the Yammies would have been close to 200 lbs total heavier then the Mercs and would have partially submerged my self bailing scuppers on the deck.

Performance wise they move a 28' Grady through a 2' chop at 50 MPH, they have more alternator to keep battery's charged no matter how much electronics I am running and the fuel economy is what I was after.
With the 2 strokes I was getting less than 1 MPG but the 4 strokes are getting me 1.6 MPG.
At idle you cannot hear the 4 stroke run and there is zero emission smell.
Downside is much of the routine maintenance I did to the old motors I have the dealer take care of and it is not cheap.
Still when I look at my savings in fuel it still out ways the dealers costs.

The shape and design of the boat influences more the speed and way the boat handles than the power-plant on it.
4 Strokes are getting cheaper to purchase every year so if you need to buy a new motor go 4 stroke but if the old one is running well, wait till it wont run anymore before you upgrade. Maybe by then you will buy a complete package of boat, motor and trailer.

Good Luck
 

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Discussion Starter #10
I do want an alternator, thats one reason i'd like to upgrade to a newer motor. And although we're talking about a much smaller boat, improved feul economy would be nice, more for the convience than dollar savings. I can get a 20 hp merc or tahotsu for around $3k, anything feul injected will be a grand more, so i'll probably be sticking with a carb.
 

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If you can find a used 18 Tohatsu 2 stroke, that could be a good option for you. You'll absolutely see increased power, plane, and top end. I had one, went from a 9.9 on a 16 foot utility jon, to the 18. It will be very close to what a 20 hp 4 stroke will do for you also, and without the added weight.
If you can then get the motor set optimally on your transom (set back and depth in water), then experiment with a couple different pitch props, and between aluminum and stainless, you can surely dial it in to optimize performance.
 

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Discussion Starter #12
A 20 hp merc/tohotsu is around 125#, is that alot more than my 9.9? I really don't want to do too much experimenting, wouldn't the factory prop be adequate? I have a 16 foot mirrocraft deep fisherman, it only weighs 350 pounds. With people, dog,and gear i figure about 1100 pounds total. I'm just looking for a little more top end with a smooth trolling speed out of the same motor.
 

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Your 9.9 probably weighs under 100. I assume you are talking a 20 hp fourstroke weighs 125?
Would the factory prop be adequate? It should be.
What is the weight rating on your boat? What is the max hp it is rated for?
 

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Discussion Starter #14
Max hp rating is 25, max wieght rating is 1120#. It's just a plain jane aluminum boat, it has a floor but really nothing else. I did put a bilge pump, a livewell areator, a fishfinder, and some padded seats in it but that stuff didn't add much wieght.
 

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What is your floor made out of?

I had a few made with particle board and they were heavy.
 

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Discussion Starter #16
Plywood with a vinyl covering. Same boat without a floor wieghs 100# less.
 

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I have a 25hp 2 stroke on my 16'8" aluminum bass boat. Dry weight is 1000 pounds. Loaded for fishing with two adults i bet its close to 1800 pounds. average speed is 23-25mph. A 20hp on your boat with far less weight should run real good for you.
 

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A 20hp merc 4 stroke is a carbureted 4 stroke. The carbed 4 strokes are total junk. Hesitates, have not seen one that didn't and just a poor motor. Find something that is fuel injected or you will not be happy. Or find a 2 stroke 20 merc,much better motor.
I have a carbureted 15 hp 4 stroke. I wouldn't say it is junk but the finicky carb is a real pain in the butt. I have to take it off and clean it at least a couple times per season. I messed with it yesterday and couldn't get it running. It's a 2004 Suzuki. I got tired of messing with it and bought a used 15 hp 2 stroke.

I agree that the 4 strokes idle better and are much quieter. I had a 50 hp Merc with fuel injection that ran great and never gave me a problem. If I was going to buy a new motor I would get a fuel injected 4 stroke.

Now I have to figure out what to do with my Suzuki. I might spend the $300 for a brand new carb and keep it as a backup or sell it as-is.
 

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I have a carbureted 15 hp 4 stroke. I wouldn't say it is junk but the finicky carb is a real pain in the butt. I have to take it off and clean it at least a couple times per season. I messed with it yesterday and couldn't get it running. It's a 2002 (I think?) Suzuki.

Now I have to figure out what to do with my Suzuki. I might spend the $300 for a brand new carb and keep it as a backup or sell it as-is.

I have a friend who has a 15 HP carbed Suzuki. He has the same problem. Once we get it running it is fine until it sits for awhile.

Try running some Sea Foam in it. I use it in my outboards, lawn equipment and ATVs. I think you will find it makes a difference.
 
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I have a friend who has a 15 HP carbed Suzuki. He has the same problem. Once we get it running it is fine until it sits for awhile.

Try running some Sea Foam in it. I use it in my outboards, lawn equipment and ATVs. I think you will find it makes a difference.
Thanks for the suggestion. I picked up a couple cans. I took the Suzuki off and put an old 2 stroke on the boat for now. I have the Suzuki 4S on a cart and will try using the Sea Foam. The way I couldn't start the engine makes me think it was some kind of fuel delivery issue - possibly the carb but maybe even my old portable case can and hose.
 
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