OB MAC21?

[Tim_Duggan]

JW,

The MAC O/B carb I have on my Bench has no low end either! The CMB O/B carbs I've seen had low end needles on 'em and had a larger bore.

Joe - still running the Gadget pipe on the MAC OB?

Tim.

 

[shadow]

There isn't a low end on any of the recent MAC o/b or inboard motors. And I'm trying to jump in the stomping ground, because I don't know which one came out first (the mac o/b or the cmb o/B). Has anyone really noticed that they are the exact same motor other than the carb, water jacket, and the anodizing of the case. I'm not trying to start up the age old war, but why can the cmb (the same motor as the mac) run without total restriction of water, and the mac can't. That just doesn't make sense to me. I have run both o/b motors, and this totally stumps me. The mac runs really strong as long as it has no water cooling, and the cmb runs really strong with very little water restriction. Honestly I think the mac is a much faster motor, but the temp difference between the two stiffles me.

Could someone please explain this to me without bringing in the cmb/mac war........

Thanks,

Cliff

 

[Jerry Wyss]

Hey Cliff, I run alot of my 21 O/B's (5) with the water head on (no water) or with a severely cut down airhead. I never really

knew how they were running the CMB cause I haven't seen that many. All I know is that the more heat I can make the more fuel I can use.

The Mac has a .18cc bubble volume with a dish in the piston (on some) which would surely make it a .19cc at least.

Do you know what the CMB's have in the bubble?

Another thing the Mac induction timing is what most would call over the line,, its the loudest 3.5 o/b I've ever heard.

An extra amount of noise seems to be coming from the carb and I think thats a product of the earlier induction opening.

It also closes later than most, beyond that 65* ATDC that some consider the limit.

Could be that the early opening is cooling things more than we might think,, this is just conjecture on my part though.

 

[Richard Dahlheimer]

Hey Cliff, I run alot of my 21 O/B's (5) with the water head on (no water) or with a severely cut down airhead. I never reallyknew how they were running the CMB cause I haven't seen that many. All I know is that the more heat I can make the more fuel I can use.

The Mac has a .18cc bubble volume with a dish in the piston (on some) which would surely make it a .19cc at least.

Do you know what the CMB's have in the bubble?

Another thing the Mac induction timing is what most would call over the line,, its the loudest 3.5 o/b I've ever heard.

An extra amount of noise seems to be coming from the carb and I think thats a product of the earlier induction opening.

It also closes later than most, beyond that 65* ATDC that some consider the limit.

Could be that the early opening is cooling things more than we might think,, this is just conjecture on my part though.

I believe Rod Geraghty told me the MAC outboard has more crankcase volume than the green head CMB outboard. If i remember, the first CMB outboard was the TT. The TT was made before the MAC. Richard D

 

[kesso2118]

Following on from Cliff's question and Jerry's comments on the MAC induction timing, maybe we need to start a separate topic, but I'd be really interested if anyone can provide the Timing Numbers for the MAC and CMB RS Outboards.

I know Tim Duggan was given the following timing # for a CMB RS by CMBrunner in a Post last year:

188 exhaust

126 transfer/boost

199 atdc rotor open

54 atdc rotor close

215 duration

I need to rebuild my "broken" CMB RS and was considering using a MAC crank. I would like to know whether the MAC crank would work with the Transfer and Exhaust timings on the CMB sleeve???

Thanks,

James

 

[ZIonOB75]

I finally able to run the Mac21 with OS carb, idles good, no problem starting, though engine won't scream yet at least a lot better,,,thanks a lot guy for the input, most to u Jerry W. for mentioning the OS carb... more fun to come....

jul

 

[Jerry Wyss]

Most of the fun is still ahead,, sensitive motor,, a real brat but worth it

Another area is the pipe, I've had good luck with the Silver Bullit "HP" with some of these longer stroke motors.

You have 183 degree Ex port timing, a pipe length of 8 5/8ths is a good spot to move it around from.

That's measured from the center of plug thru to the starting of the pipe getting smaller line.

JW

 

[Joe_Knesek]

JW,The MAC O/B carb I have on my Bench has no low end either! The CMB O/B carbs I've seen had low end needles on 'em and had a larger bore.

Joe - still running the Gadget pipe on the MAC OB?

Tim.

Yep.

 

[Jerry Wyss]

I believe Rod Geraghty told me the MAC outboard has more crankcase volume than the green head CMB outboard. If i remember, the first CMB outboard was the TT. The TT was made before the MAC. Richard D

I remember hearing the same thing from him Richard now that you've mentioned it. But I thought they

shared the same case with different labels,, don't know for sure.

JW

 

[Richard Dahlheimer]

I believe Rod Geraghty told me the MAC outboard has more crankcase volume than the green head CMB outboard. If i remember, the first CMB outboard was the TT. The TT was made before the MAC. Richard D

I remember hearing the same thing from him Richard now that you've mentioned it. But I thought they

shared the same case with different labels,, don't know for sure.

JW

Yes Jerry, they sure look like the same case from just eye balling. Maybe Rod G. will have a reply to make? I know the pto"s interchange.

 

[Tim_Duggan]

There seems to be two different versions of the RS21 O/B cranks. The early ones had crazy timing and the one I worked for someone had 0.008" sub piston induction. The latter ones are more useful.

The green head LS21 is older than the MAC O/B and had issues in the early days with PTO's and upper rod bushings.

The TT is even older and had a gold head.

As for the MAC - yes they definately are a temp sensitive motor - especially with a pipe that has noticable staging. A low end carb totally changes the way the engine behaves also, as Murph found out yesterday when he tried his with an OS carb.

What is the weight difference between the MAC and RS flywheels - if any?

 

[ZIonOB75]

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??

 

[Joe_Knesek]

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??

Our MAC Outboards were 2 staging by the 3rd tank. I'd be willing to bet that the carb is not flowing well enough to turn the RPMs your looking for. When it comes on you should be able to hear it over any other boat on the water.

 

[Tim_Duggan]

What prop and pipe are you using?

 

[ZIonOB75]

What prop and pipe are you using?

i'm using salisbury cf pipe and x438,,,bigger prop slows the rev of the engine...HSN set is limited to 1 3/4 - 2 turns, more or less with this either will stall or running rich.

Joe, that was my thinkin' too.

 

[Tim_Duggan]

What you are describing sounds like it's not on the pipe. When it comes on it needs a lot of fuel!

The Mac can make some serious rev's with the SPP pipe, but you have to restrict the water down to next to nothing and tune the needle based on a rich launch. As engine and pipe warm up it comes into tune. usually after a lap or two. Then Hold on!

My personal experience with this pipe and engine combination was on a lynx, and that thing hauled with a 40x53 on 50% nitro but needed 10 oz. of fuel per run! That was with a CMB low speed needle carb fitted.

 

[Joe_Knesek]

What you are describing sounds like it's not on the pipe. When it comes on it needs a lot of fuel!
The Mac can make some serious rev's with the SPP pipe, but you have to restrict the water down to next to nothing and tune the needle based on a rich launch. As engine and pipe warm up it comes into tune. usually after a lap or two. Then Hold on!

My personal experience with this pipe and engine combination was on a lynx, and that thing hauled with a 40x53 on 50% nitro but needed 10 oz. of fuel per run! That was with a CMB low speed needle carb fitted.

I'd agree with Tim on the fuel. We lost a few races early on when the boat ran out 6 feet before the finish line.

 

[Tim_Duggan]

Do you know what the CMB's have in the bubble?

The RS21 I measured had 0.19cc in the head alone. I dropped it to 0.15cc and flattened the squish angle out of it to zero, set to 0.008" clearance. Runs very well now.

 

[dwb2620]

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.

If it is running better now, I would stop using the 15% nitro. Start using the 60 % you mentioned. It will run a lot better! I always break in a engine on the fuel that I plan to race with. 21 engines love the NITRO! Just be sure to richen it up a little when you do.

Just FYI, in the past when I ran 15 to 20 % nitro to "break in" the engines they were very, very hard to stay running on the low nitro. They would gurgle and pop and load up no matter what I had the mixture control set at. When the engine would die Oil would just pour out of the exhaust pipe. When I started using the 50 to 60% nitro to break them in, it was a lot easier to keep them running out on the water.

David

 

[k&b guy]

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