OB MAC21?

[ZIonOB75]

guys need ur input, first time to run a MAC21 motor and struggling to figure out what this motor set-up likes,,,btw, i'm using OS remote needle, motor will start by 3/4 turn only, more than that fuel will be spitted and won't start, then i need to turn bout 1 1/4 turn before throwing to water otherwise will stall,,thus this thing is intended for 3rd channel throttle control and not for remote needle,help! help! please......thanks in advance.

 

[RodneyPierce]

one question, you starting it in the right direction?

 

[Tim_Duggan]

guys need ur input, first time to run a MAC21 motor and struggling to figure out what this motor set-up likes,,,btw, i'm using OS remote needle, motor will start by 3/4 turn only, more than that fuel will be spitted and won't start, then i need to turn bout 1 1/4 turn before throwing to water otherwise will stall,,thus this thing is intended for 3rd channel throttle control and not for remote needle,help! help! please......thanks in advance.

How far do you have the carb open at idle with the needle at 1 1/4? Sounds like the idle is too low and it's flooding itself on the needle setting it needs to run at WOT. Also - what glow plug do you have in it?

 

[Jerry Wyss]

Its not necessary to have a 3rd channel needle control, a remote should work depending on which one your using.

 

[ZIonOB75]

guys need ur input, first time to run a MAC21 motor and struggling to figure out what this motor set-up likes,,,btw, i'm using OS remote needle, motor will start by 3/4 turn only, more than that fuel will be spitted and won't start, then i need to turn bout 1 1/4 turn before throwing to water otherwise will stall,,thus this thing is intended for 3rd channel throttle control and not for remote needle,help! help! please......thanks in advance.

How far do you have the carb open at idle with the needle at 1 1/4? Sounds like the idle is too low and it's flooding itself on the needle setting it needs to run at WOT. Also - what glow plug do you have in it?

once i turned it to 1 1/4 turn i have to rev- up or it will choke,,if i turn more it will flood,,,,, glow plug OS8 since i'm using 15% nitro before goin up to 60%.

thus the this thing is really tight at top end?

Rod, yup it's in right direction.

 

[Jerry Wyss]

The Mac O/B can be a tough challenge if you haven't run several motors before this. They're like a spoiled

brat, if things arn't just so so they throw a fit. Feeding it 15% nitro is like givin' a Shirley Temple to an alcoholic.

If you could, I'd suggest getting someone to help you (that knows this motor) run the thing. With a piston and sleeve

@ $110, I would definetly get help.

Just wanted to add that the last two Mac O/B's I delt with had .011" headspace with the aluminum shim in,,

with no shims you have .005". So a .003" head shim will give you .008" which is perfect with 60%nitro.

They like to run without water, essential in cold climates. Prop is gonna have to be small. Safe bet is X637 if

you don't have much else,, a X438 or an X440 thinned and balanced real well will get ya thru as well. These are

not the best props for this engine,, I just don't know what you have.

If you have a heat gun try and set the needle so you get near 200* shot in center of the head as soon as your

"helper" can pull it outa the water. Start rich and bring it back in soon to check to see if its getting hot enough.

You have to build heat in these things in order for them to run right, once you have heat, let'er breakin. A few tanks

and it will start to wake up,, and they can REALLY go.

JW

 

[Tim_Duggan]

Because the engine does not have a low end needle in the carb, and likes a lot of fuel up higher in the rev range, it needs to have a fast idle set and once it's running blip the throttle regularly to stop it fueling up on the shore.

 

[Richard Dahlheimer]

Because the engine does not have a low end needle in the carb, and likes a lot of fuel up higher in the rev range, it needs to have a fast idle set and once it's running blip the throttle regularly to stop it fueling up on the shore.

Here is a OS remote needle valve part number.......... 22281903. Try it at 23/4 turns out from closed. That should get you started, if everything else is ok. Hope this helps. Richard D

 

[MarkScott]

They like to run without water, essential in cold climates. JW

He will need some water cooling since a Irwin / Rex lower is in use..

 

[dougaldy]

guys need ur input, first time to run a MAC21 motor and struggling to figure out what this motor set-up likes,,,btw, i'm using OS remote needle, motor will start by 3/4 turn only, more than that fuel will be spitted and won't start, then i need to turn bout 1 1/4 turn before throwing to water otherwise will stall,,thus this thing is intended for 3rd channel throttle control and not for remote needle,help! help! please......thanks in advance.

Jul,

This sounds alot like my CMB too. I am using the Don F set up and I have that turned out about 3 turns. On shore it will spit so much fuel out that it floods itself. I found out that you have to have that thing at high idle and keep reving it to WOT to keep it cleared out and once you toss it in the water it will clear out and run like a striped a## ape.

Scott

 

[Joe_Knesek]

I had the same problem until Rod G watched the boat at last years Internats. He worked over the stock MAC carb and now it idles smoothly, throttles like it should and runs faster than ever.

 

[ZIonOB75]

I had the same problem until Rod G watched the boat at last years Internats. He worked over the stock MAC carb and now it idles smoothly, throttles like it should and runs faster than ever.

Hey Joe, so u think it's the carb is the culprit?

 

[ZIonOB75]

They like to run without water, essential in cold climates. Prop is gonna have to be small. Safe bet is X637 if

you don't have much else,, a X438 or an X440 thinned and balanced real well will get ya thru as well. These are

not the best props for this engine,, I just don't know what you have.

Hey Jerry, i noticed it too bout the prop runs better when small, or raised up.

 

[dougaldy]

Mine is on its way to Big Rod as well.

Scott

 

[Jerry Wyss]

Hey Jerry, i noticed it too bout the prop runs better when small, or raised up.

The power curve in the Mac is way up there much like some of the CMB's,, so your trading some torque

for more rpm's,, it would seem.

I never used the carb that came with them, I use the OS 20J on alota 21 motors.

 

[ZIonOB75]

i also noticed that MAC21 design has similarity to CMB OB,,,

Jerry do u have pic of OS carb u mentioned...

 

[ZIonOB75]

i also noticed that MAC21 design has similarity to CMB OB,,,
Jerry do u have pic of OS carb u mentioned...

nevermind i got one it's the carb for OS xm,,,,thanks a lot.

excited to test tom. hope will make a change..

Scott, CMB is my next target engine but i think i gonna hang around with this mac and rossi.

 

[SayMikey]

i also noticed that MAC21 design has similarity to CMB OB,,,
Jerry do u have pic of OS carb u mentioned...

nevermind i got one it's the carb for OS xm,,,,thanks a lot.

excited to test tom. hope will make a change..

Scott, CMB is my next target engine but i think i gonna hang around with this mac and rossi.

Hey bud i seem to rremember it having a low end needle in the carb, ya might back it out some. Makes sense lean slow idle and rich open huh

 

[Joe_Knesek]

I had the same problem until Rod G watched the boat at last years Internats. He worked over the stock MAC carb and now it idles smoothly, throttles like it should and runs faster than ever.

Hey Joe, so u think it's the carb is the culprit?

Without a doubt.

 

[Jerry Wyss]

If theres a low speed adjustment in the stock Mac O/B carb, I've never seen it :huh: