Carb Bore

[EatMyShortsRacing]

Well guess who just took delivery of a nice new MAC 3.5 outboard

So I noticed the carb bore is way smaller than a normal 3.5....

Now I would assume a slightly smaller carb would help in being able to throttle the engine but how much smaller is ideal? I am not second guessing Andy and I am sure the engine will go hard!!!! Just by my thinking somewhere between the normal carb (9mm) and the MAC carb (7.45mm) would be ideal... I was guessing 8mm would have worked fine....

Any experiences peole would care to share on carb bore for a 3.5 outboard???

 

[waterbug]

The stock .295 carb works fine...But some have used the .340 carb and said it was smoother power wise....

Butch

 

[james irwin]

Well guess who just took delivery of a nice new MAC 3.5 outboard

So I noticed the carb bore is way smaller than a normal 3.5....

Now I would assume a slightly smaller carb would help in being able to throttle the engine but how much smaller is ideal? I am not second guessing Andy and I am sure the engine will go hard!!!! Just by my thinking somewhere between the normal carb (9mm) and the MAC carb (7.45mm) would be ideal... I was guessing 8mm would have worked fine....

Any experiences peole would care to share on carb bore for a 3.5 outboard???

ARE YOU ASKING THIS ABOUT A SLIDE OR A ROTERY CARB. MAN WE JUST DID FIND A BUNCH OF HP BY SWITCHING OVER TO A SLIDE VALVE , I WILL LET YOU IN ON THE SPECS AS SOON AS WE COMPLETE OR DYNO FIGURES . TO GIVE YOU A TRUE FIGURE , I HAVE USED A.312 BORE FOR ABOUT 35 YRS NOW THIS BORE SIZE IS THE SAME ON THE SLIDE CARB WE ARE MESSING WITH . THIS IS FOR A 3.5 GOD BLESS THIS IS WHY I DEVELOPED THE SMALL SUMP TANK THAT WILL GLUE ON THE TOP OF THE COWEL , TO GET A SMALL AMOUNT OF FUEL UP AT THE CARB . CENTER LINER , SO WE COULD RUN A LARGER BORE CARB,

 

[Joe_Knesek]

Well guess who just took delivery of a nice new MAC 3.5 outboard

So I noticed the carb bore is way smaller than a normal 3.5....

Now I would assume a slightly smaller carb would help in being able to throttle the engine but how much smaller is ideal? I am not second guessing Andy and I am sure the engine will go hard!!!! Just by my thinking somewhere between the normal carb (9mm) and the MAC carb (7.45mm) would be ideal... I was guessing 8mm would have worked fine....

Any experiences peole would care to share on carb bore for a 3.5 outboard???

I have been running a MAC on my son's Lynx for the 05 season and what I'd say you are on track. We started the season with the .300 MAC carb then went to the .350 MAC and finished with a .340 D&J. With the .300 MAC, the boat had fuel draw problems because the spray bar is so close to edge of the bore. It uses the same spray bar as the .350 MAC. The .350 MAC did not have as much throttle response but the boat was faster on top end. When we changed tothe D&J the boat throttled well and was actually faster than the .350. The D&J has a spray bar that runs to the center of the bore. I think this is more about spray bar design than ID because I made a spray bar like the D&J for my MAC 45 and found the same results. I also got the same results with the inboard MAC 21 on my daughter's boat running the D&J.

 

[waterbug]

Joe

Where can I get a D&J carb ??

Butch

 

[Joe_Knesek]

Joe

Where can I get a D&J carb ??

Butch

I get mine from Speedmaster. It's actually for a Nova Rossi but the OD is the same. You will just need to cut the length down some.

 

[XtremeRcBoats]

WIll a D&J carb bolt straight on the the MAC or do you have to mod it? If the performance is better i'm thinking about buying one for my MAC21.

 

[Geraghty]

area to that of the MAC carb.

The spraybar "outlet" design also has a ton of effect on the fuel draw capability of a motor.I don't know why you had fuel draw problems with the MAC carb but it wasn't because the spraybar was to close to the carb bore wall.I suspect the very tip of your spraybar had a problem and that was reducing your fuel draw on the .300" MAC carb.Difficult to say without seeing it.

If you were to make a spraybar for the MAC .300 carb,splay the outlet to 100 degrees inclusive and place the tip of it about .060" to .090" inch off the carb wall,the motor will run even better yet.The motor will be very user friendly and the throttle response will be very linear and predictable.

Now you can go to work on your set-up[prop diameter and pitch],now that you have a motor you can work with.

 

[Joe_Knesek]

WIll a D&J carb bolt straight on the the MAC or do you have to mod it? If the performance is better i'm thinking about buying one for my MAC21.

The neck is a little long. You can run it as is if you use a thicker o ring or you can cut down the length a bit. Bill sells them with the OS needle built in or you can get it without a needle.

 

[XtremeRcBoats]

Joe,

is this Bill from speedmaster (www.rossisales.com)?

 

[Martin Lovsund]

" If you were to make a spraybar for the MAC .300 carb,splay the outlet to 100 degrees inclusive and place the tip of it about .060" to .090" inch off the carb wall "

Hello Rod , I can't picture what this means " splay the outlet to 100 degrees inclusive " do you have a photo of a spray bar like this. Do you mean to angle one side of the spray bar , on the intake side , back 100 degrees. Also " .060" to .090" inch off the carb wall " is this the far side carb wall or the side the spray bar protrudes from.

Martin

 

[james irwin]

Joe,

is this Bill from speedmaster (www.rossisales.com)?

HEY ROD , HOWZ IT GOIN . PLEASE EXPLANE TO THESE PEOPLE WHY , YOU NEVER PUT A SCREW DREVER ON A HIGH SPEED NEEDLE AND RUN IT IN TILL IT BOTOMS OUT , AND I SEE GUYS DO THIS EVERY DAY , IT WILL SEND COLD CHILLS UP MY SPINE WHEN I SEE THEM DO THIS . YET THEY WILL ALWAYS BE THE ONES WITH THE TOP STARVING ENGINES I AM HERE TO TELL EVERYONE , THIS GUY BUILDS THE BEST CARB EVER , OF ALL THE CARBS WE HAVE CHECKED . HIS WILL ALWAYS COME OUT FRONT , THE BEST FUEL DRAW / AND THE MOST PROPORTIONAL OF ANY . I GAVE ROD A CALL ABOUT A YR AGO , I ASKED HIM IF HE HAD A PROBLEM WITH ME COPING ONE SMALL DETAIAL OFF HIS CARB, FOR MY ENGINES THAT ARE COMING IN . HIS WORDS TO ME , AND I WILL QOUT , HELL NO JIMMY DO ANY THING YOU WOULD LIKE . I DO NOT HAVE A PATTEN ON IT , NOW THATS THE KIND OF GUY ROD IS , HE IS SUPER MAN / SPIDER MAN . AND ALL THE GOOD ONES . O YA ROD , I DECIDED NOT TOO DO IT . I TOO HAVE A REPETATION I MUST MAINTAIN . ANY WAY . JUST WHY DO YOU CALL ME JIMMY ? YOU KNOW I DONT LIKE THAT . HE HE AFTER ALL I AM YOUR ELDER, SHOW SOME RESPECT ? OK / TALK TOO YA LATER . GOD BLESS

 

[Geraghty]

" If you were to make a spraybar for the MAC .300 carb,splay the outlet to 100 degrees inclusive and place the tip of it about .060" to .090" inch off the carb wall "

Hello Rod , I can't picture what this means " splay the outlet to 100 degrees inclusive " do you have a photo of a spray bar like this. Do you mean to angle one side of the spray bar , on the intake side , back 100 degrees. Also " .060" to .090" inch off the carb wall " is this the far side carb wall or the side the spray bar protrudes from.

Martin

 

[Martin Lovsund]

Perfect, thanks Rod.