Changing from single carb to twin carbs

[tyst67]

Hello! I have a 40hp twin mercury single carb and I was wonder if I can make it a twin carb and would it be a benefit? The 40/50/55/60 3 cyl 3 carb mercury are the same except one more cylinder. If I bought a intake manifold and reed cage from the 3 cyl and chopped off one of the cyl intake I would be able to mount 2 60hp carbs on my motor. Has anybody ever done this and is it a benefit ?

 

[jsimms724]

Re: Changing from single carb to twin carbs

Hello! I have a 40hp twin mercury single carb and I was wonder if I can make it a twin carb and would it be a benefit? The 40/50/55/60 3 cyl 3 carb mercury are the same except one more cylinder. If I bought a intake manifold and reed cage from the 3 cyl and chopped off one of the cyl intake I would be able to mount 2 60hp carbs on my motor. Has anybody ever done this and is it a benefit ?

You can do just about anything if you have the time and the money.Just gota ask why?

 

[ondarvr]

Re: Changing from single carb to twin carbs

buy a bigger motor, it will be easier and cost less.

There are things you can do though.

 

[Chinewalker]

Re: Changing from single carb to twin carbs

Using a different intake doesn't change the fact that there is still only one reed block on the crank of your 40. Adding two carbs will put the intakes away from the reeds and the motor absolutely will not run. At all.

Bottom line - if you want more horsepower, get more motor...

 

[Frank Acampora]

Re: Changing from single carb to twin carbs

This argument comes up all the time, especially when people want to put on a bigger carb to increase horsepower, like increasing an OMC 9.9 to a 15 to get a bit more speed on limited horsepower lakes. "They are the same."

In fact, they are NOT the same and with almost ANY outboard, a simple (or in your case complex) carb change will do nothing for performance. In fact, in your case, in one breath you stated they are the same and in the other you stated they are not the same---one is a two cylinder and one is a three cylinder version.

Bottom line, if you want more horsepower either do MAJOR engine modifications to the one you have and risk damage, or as stated above buy a bigger engine.

 

[tyst67]

Re: Changing from single carb to twin carbs

The reason I want more out of my motor is do to weight! I race very light flat bottom boats and having a heavier motor is out of the question! And frank I will argue with you on that one! My motor is a 30hp mercury and I put the 40hp parts on it and gained about 15 mph! They are the same! The only reason that mercury does this is due to hp specifications on certain bodies of water ! Instead of designing a new motor they just detuned some of there motors! Like the 40/50/55/60 hp mercurys, they are all 59 cubic inche engines except the air/ fuel components are different! And the reason I want 2 carbs instead of 1 is less restriction and more air equaling a few more rpms! And the racing I do 100rpms can mean the difference between winning and losing

 

[Frank Acampora]

Re: Changing from single carb to twin carbs



I hear you about weight! This hull has a single carb 50 on it and I would really like to mount on the twin carb 60 in the last photo. BUT----Without me sitting in it, she floats at rest right at the rubrail. I had to glue the back half to stop water intrusion whiile I was getting ready to run. So, the extra 40 pounds of the superior twin carb engine simply will be too much.

Instead of going for all the fuss of two carbs with the possibility of one going lean and ruining the engine, why not modify the existing manifold to accept a bigger venturi carb? A carb like the old Delorto type has no butterfly so there is also no restriction and turbulence at WOT (Now I am showing my age) However, remember that reeds are the limiting factor in airflow. A bigger carb still will not give much added performance unless you modify or buy aftermarket reeds.

Another thought is that since time and work involved is not a factor for you, Even if your engine is a loop charged one, with study--the literature is all out there, especially for Asian 2 cycle bikes---porting can make major gains in racing applications.

BTW: The hull is a Cougar Cub tunnel hull designed for class racing with Merc 25 HP engines. There are some reports of speeds as high as 47 MPH with a Merc 25.

 

[tyst67]

Re: Changing from single carb to twin carbs

Thanks for the reply right now I have a 30hp sea pro on my flat bottom and last time out I GPS it at 55.1 mph but I'm running out of little secrets that make me faster lol. If I put a bigger single carb on it and ported the intake, do your honk I could actually burn the extra fuel ? Maybe I should make a cold can and cool my gas to -20? to let more air and fuel in! Do you think this will work?

 

[JB]

Re: Changing from single carb to twin carbs

Power is developed by your engine because a series of things work together, in balance. Changing one of them usually does little to really change the result.

Think of airflow, fuel/air mix, reed valving, scavenging, port timing, compression ratio and spark timing as a chain of events. If you put one bigger link in a chain it doesn't do much for the whole chain's strength unless that link was weak to start with.

 

[Frank Acampora]

Re: Changing from single carb to twin carbs

Porting the engine will usually give significant gains in horsepower. BUT---You need to do your homework. You can not just port the intake, you must port the intake, exhaust, and (bypass or boost ports) depending upon whether the engine is crossflow or loop charged.

As I said, you must do your homework: You can not just take a mill or Dremel to the engine because improper porting WILL decrease horsepower production. YTou need to "map" the ports first to see where you are then use well established formulas to increase port area or raise port timing. Look up a book called 2 cycle tuners handbook.

Note also that porting develops increased horsepower at a higher RPM, usually with less torque developed so a smaller diameter prop of higher pitch is necessary. Again, here is where homework is necessary.

As far as fuel, remember that air is the limiting factor in horsepower developed and usually it is easy to get excess fuel delivery. Thus: much more will be gained by increasing the air pumping efficiency of the engine.

And, of course there is always the nitrous bottle or exotic fuels (nitromethane/alcohol) but that requires special jetting and oil and gets to be expensive for longer runs. (Ahh! but nitro and castor smell great when burned)