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Mercury 115 2 stroke mod from 2+2 ... to...4

R

Roland J

Member
I want all the carbs to work all the time.

I'm not sure this is possible so i need some help sorting this out. I really like my 1995 17' CapeHorn but just found out why my 1995 mercury 115 wont run between 1800 RPM and 2300 and it's because it only runs on 2 cylinders below 1800 RPM then the othe cylinders kick in and it's 2300 rpm plus. I need better control on my low end speed for towing. i'm pulling foil-boarders.

The Idea is to buy a extra set of carbs, use the top 2 carbs from that set, and put them on the bottom of my current set on my motor.
Probable not a DIY job for me, as im not sure i could swap the linkage on the butterfly shafts.

1. Does anyone know if this has been done?
2. does any thing else nee to be addressed to do this?

Thanks
 
racerone said:
I have seen posts where it says the attempt was a failure.------Others will possibly confirm this.
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Good to know. I haven't seen that yet I'd like to see the details. I've read where someone tried to re-jet the carbswith 125 jets which understandably failed. But that's as close as I could find.
 
Ping Faztbullet, business owner, certified mechanic, boat owner poster on here and elsewhere for years. He has a one word solution based on facts. He can save you a lot of money and grief.

I just bought a 2002 this time last year. Off and on it has taken me a year of tinkering to get the performance I want and I really like it....the OEM setup engine.....the conversion rpm area isn't where I boat. My advice for the area in which you want to dwell is to buy another engine of a different type. I think the engine was built to do 2 things: troll efficiently, and get a boat on plane....nothing in between. Wink!
 
Texasmark said:
Ping Faztbullet, business owner, certified mechanic, boat owner poster on here and elsewhere for years. He has a one word solution based on facts. He can save you a lot of money and grief.

I just bought a 2002 this time last year. Off and on it has taken me a year of tinkering to get the performance I want and I really like it....the OEM setup engine.....the conversion rpm area isn't where I boat. My advice for the area in which you want to dwell is to buy another engine of a different type. I think the engine was built to do 2 things: troll efficiently, and get a boat on plane....nothing in between. Wink!
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Sorry I'm not sure I understand.
Did someone try and swap carburators the way I suggested or something else?
 
faztbullet said:
Drop pitch on prop 2 sizes but watch WOT RPM...only way around this for what you are doing...
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That's an option. I didn't think about that.
It won't give the control on rpm but maybe the slower speed might work. Thanks

Do you know if anyone tried to swap the carbs?
 
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Get yourself a 4 bladed Turning Point brand, Hustler alum prop..iboats sells them for about a hundred bucks. I have a couple myself and know how they run. It's ported. Between the ports and the 4 blades, especially if you drop pitch like Faztbullet said, you ought to have a winner. You will be up around 3000-3500 rpms for the speed you want to run and with that setup, boat control (staying on plane at a slow speed, all 4 cylinders working for you) should be excellent. Surely better than the cost of another engine.
 
Texasmark said:
Get yourself a 4 bladed Turning Point brand, Hustler alum prop..iboats sells them for about a hundred bucks. I have a couple myself and know how they run. It's ported. Between the ports and the 4 blades, especially if you drop pitch like Faztbullet said, you ought to have a winner. You will be up around 3000-3500 rpms for the speed you want to run and with that setup, boat control (staying on plane at a slow speed, all 4 cylinders working for you) should be excellent. Surely better than the cost of another engine.
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I'll look at that one.
The control speed area i want the boat to travel at is between 12- 20 mph depending on who im towing and which hydrofoil they are on. It's not like pulling a skier because the hydro foil has next to zero drag on the boat which makes it basically being able to control the boat speed playing with the throttle and the trim.
 
Not pulling skiiers just makes it easier. Do what's recommended and if that doesn't work, add a Whales Tail to the engine AV plate. You see a lot of them around and the purpose is to aid in keeping the stern up during slow speeds since they are just a big flat surface sticking out behind the boat with water pressure forcing it up. As you mentioned, the trim advantage makes a great deal of improvement in slow, controlled "just at/on" planing speeds.
 
A whale tail wont work as we don't want boat on plane, just bow high and large wake so they can surf curl on wake. You can try a trolling plate to slow you down and add a wake shaper
 
Texasmark said:
Not pulling skiiers just makes it easier. Do what's recommended and if that doesn't work, add a Whales Tail to the engine AV plate. You see a lot of them around and the purpose is to aid in keeping the stern up during slow speeds since they are just a big flat surface sticking out behind the boat with water pressure forcing it up. As you mentioned, the trim advantage makes a great deal of improvement in slow, controlled "just at/on" planing speeds.
Click to expand...

I just learned about my 2+2 motor last week, before that I never knew that was a thing on outboards. While the 2+2 motor explains my boat speed issues, it's now hard for me to get my head around whether i want the engine to stay in the 2 cylinder mode, below the kick in RPM (1800). Or do i want the boat to operate at a higher RPM where the all the Cylinders are working. The latter would need the lower pitch prop like Faztbullet suggested

So now that i know whats going on with the engine i have to pay attention to... planing + speed,... RPM + speed,... and tilt +speed, then sort out what RPM i want the motor to tow at.

I thought about the whale tail. but like i said it's hard for me to wrap my head around whether being on a plane with a Whale tail would allow me the slower speed capabilities 9-15 mph ( not 12 to 20 as i said earlier) or use the lower pitch prop with all 4 CYL running. It might just be a matter of experimentation.

Seeing what the whale tail's effect on speed while keeping RPM low enough not to engage the lower carbs may be worth determining.The whale tail is not expensive so I think it is worth a try.
 
faztbullet said:
A whale tail wont work as we don't want boat on plane, just bow high and large wake so they can surf curl on wake. You can try a trolling plate to slow you down and add a wake shaper
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A wake would be nice for sure. The trolling plate is another thing to consider. Cool thing is with all these ideas i'm pretty sure the 9-15 mph will be doable:)

Now it's time to get on the lake
 
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