OB MAC21?

[J_Moore]

i see where the mac likes heat & small prop to turn corr. r.p.m. my ? is which way does the heat go if you change to a smaller prop and/or raise eng? i know if you changed the needle the temp would change.lets say you didn`t have to adjust it (perfect world) which way would the temp. go? CARTER

my answer would be that moving to a smaller prop or raising the engine the temp would go up!

 

[ZIonOB75]

anybody dyno'd this powerhead?...anybody knows the hp and rpm claimed?

 

[AndyBrown]

well, Mac21 with OS carb start and idles great no problem, but won't rev high (scream). no more power coming out after half throttle, so half to full throtlle is the same...i'm thinkin it's not yet broke-in, i'm in 8-10 fuel tank, piston still really tight on top end...usually thus this thing takes so long to break-in..or the OS carb can't meet the Mac fuel/air required to run top end??

That is correct. the OS carb, won't meet the fuel delivery needs of the MAC.

BTW- I'm currently working on a much improved carb for the MAC OB as well as others.

This new carb will improve overall throttle response, power and fuel economy.

 

[Jerry Wyss]

"That is correct. the OS carb, won't meet the fuel delivery needs of the MAC."

Well, with all due respect, and I do respect you,,we've found that to be not correct at the last race for us.

The 20J is working extremely well on Jim Hills Mac/O.S. Lower.

We tried others but went back to the O.S. carb, it really hauls the mail,, rendering my new VZ-M O/B to 2nd place

from what I saw.

The only mod on the 20J (XM carb) that I did was chucked in the lathe and turned the "oval" carb exit to a round cone

but not all the way to the rotating barrel. I left the 1/3rd next to the barrel its stock slightly oval shape to allow room

for the air to move around the spray bar, simple mod.

 

[Jerry Dunlap]

Rod Geraghty did some modifications to my MAC carb and what a difference. I had basically given up on the engine. But, when I lost my CMB RS after the OB Championships in SC I gave Rod a call regarding the MAC I had.

Just returned from testing the MAC on a TS3 and it's running very well. Ran both a Silver Bullit and Cooper pipe. Both pipes worked. The Cooper muffles the noise better than a stock SB. However, I have an asprin bottle muffler on the SB and that helps reduce the sound.

JD

 

[Jerry Wyss]

Rod Geraghty did some modifications to my MAC carb and what a difference. I had basically given up on the engine. But, when I lost my CMB RS after the OB Championships in SC I gave Rod a call regarding the MAC I had.
Just returned from testing the MAC on a TS3 and it's running very well. Ran both a Silver Bullit and Cooper pipe. Both pipes worked. The Cooper muffles the noise better than a stock SB. However, I have an asprin bottle muffler on the SB and that helps reduce the sound.

JD

Hey, Thats good news,, about the simplest carb in the world, I guess I'll have to send a couple to him to find out

what the mod is. They're really startin' to stack up around here and maybe I can get a few of my OS20J's back :lol:

 

[Ed.R]

Rod Geraghty did some modifications to my MAC carb and what a difference. I had basically given up on the engine. But, when I lost my CMB RS after the OB Championships in SC I gave Rod a call regarding the MAC I had.
Just returned from testing the MAC on a TS3 and it's running very well. Ran both a Silver Bullit and Cooper pipe. Both pipes worked. The Cooper muffles the noise better than a stock SB. However, I have an asprin bottle muffler on the SB and that helps reduce the sound.

JD

JD , iS THERE A WAY YOU COULD EXPLAIN HOW TO MAKE A MUFFLER . kinda have an idea .

Thanks

No luck on your boats ?

That sucks , THAT SUPER SUCKS !

 

[Jerry Dunlap]

An asprin bottle muffler is made using brass tubing that slips tightly over the stinger, the bottle, and JB Weld Kwick.

1. Cut the tube sufficiently long to have at least 1/2" to clamp on the stinger and stick just a bit out of the end of the bottle.

2. Drill a hole in the bottle the size of the tube.

3. Drill 1/8" holes in the tube that's inside the bottle.

4. Glue the tube into the top of the bottle and glue the cap.

5. Stick some JB into the part of the tube that extends out the bottom of the bottle and set it down on wax paper so the JB will form a plug in the end of the tube.

6. Drill 3/16" holes in the bottle

7. cut a slot in the end of the tube that attaches to the stinger so it can be compressed onto the stinger with a wire tie or small hose clamp.

I'll post a photo of the one on my SB pipe later this evening.

JD

 

[Ed.R]

Thanks JD .

Ed.R

 

[Joe_Knesek]

Rod Geraghty did some modifications to my MAC carb and what a difference. I had basically given up on the engine. But, when I lost my CMB RS after the OB Championships in SC I gave Rod a call regarding the MAC I had.
Just returned from testing the MAC on a TS3 and it's running very well. Ran both a Silver Bullit and Cooper pipe. Both pipes worked. The Cooper muffles the noise better than a stock SB. However, I have an asprin bottle muffler on the SB and that helps reduce the sound.

JD

It's like night and day difference isn't it?

 

[Jerry Dunlap]

Here's a photo of the asprin bottle muffler on my Silver Bullit HP.

JD

 

[Jerry Dunlap]

Rod Geraghty did some modifications to my MAC carb and what a difference. I had basically given up on the engine. But, when I lost my CMB RS after the OB Championships in SC I gave Rod a call regarding the MAC I had.
Just returned from testing the MAC on a TS3 and it's running very well. Ran both a Silver Bullit and Cooper pipe. Both pipes worked. The Cooper muffles the noise better than a stock SB. However, I have an asprin bottle muffler on the SB and that helps reduce the sound.

JD

It's like night and day difference isn't it?

Unbelievable difference!!! I'm calling it "Mondovi Magic."

JD

 

[ZIonOB75]

after long break from running this engine somehow i able convinced myself to run it to my surprise engine runs lot better from my last winter test, using stock carb, 50% nitro, OS remote needle, it start right up, idles good, and noticeable power though i didn't push hard coz still in break-in, well, after 1 tank, steering acting weird so i call the day off before worse happen.

i do no what's really goin with this motor, the only difference from last time is the cold weather and using 20% nitro...hope engine temperemental is gone now :lol:

 

[Joe_Knesek]

after long break from running this engine somehow i able convinced myself to run it to my surprise engine runs lot better from my last winter test, using stock carb, 50% nitro, OS remote needle, it start right up, idles good, and noticeable power though i didn't push hard coz still in break-in, well, after 1 tank, steering acting weird so i call the day off before worse happen.
i do no what's really goin with this motor, the only difference from last time is the cold weather and using 20% nitro...hope engine temperemental is gone now :lol:

Going from 20% to 50% nitro is big.

 

[ZIonOB75]

ok guys, bout a gallon of fuel consumed and the motor goin stronger,,,,got a quick q's on tuned pipe, it won't work on my salisbury carbon fiber tuned pipe but it's better with rossi tuned pipe and i'm still not satified, so what pipe u guys run with it??

jul

 

[Jerry Dunlap]

I'm running a Silver Bullit HP. The Cooper pipe also works well.

What props are you running? I have to run a small prop(Y537) on my MAC to get it on pipe.

JD

 

[ZIonOB75]

Thanks Jerry, i started running with x438 but i found ABC 40x53 mod, works better, but there's a speed lag, it takes bout 1/4 of the course straightaway before speed will kick in so i'm thinkin' of my pipe.

jul

 

[Tim_Duggan]

Is the motor getting hot after a run?

 

[ZIonOB75]

Is the motor getting hot after a run?

not really, temp. runs 127-145 oF.. with partially restricted water pickup.

 

[k&b guy]

jerry- i`m putting together the same combo. you have. i was going to start with a 438. did you find this prop to be too small? tnx. CARTER