21 CMB Valvola Carb question

[Samuel Hagan JR]

I bought a carb with a low need built in for the Sport 20 to help with milling.

My question is that the fuel bar inside diameter is a lot smaller then the one I had on it previously. What should the fuel bar diameter be.

Motor is a CMB Green Head Valvola. It's not a wrap around exhaust. Thanks.

 

[Mike Betke]

What kind of carb is it that you bought?

 

[Samuel Hagan JR]

I'll have to post a picture. Give me a second.

 

[Samuel Hagan JR]

The Black one is the new one.



The Silver one came with the motor no low remote needle.

 

[Alan Elzer]

Sam on my outb motors I just ran them stock

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[moparbarn]

The black looks like a CMB carb, silver may be off a Mac. What are the bore sizes? CMB used 2 sizes that I know of in that version, the Mac carbs I have are all .350". Need to measure my CMB stash, CRS kickin' in.......

 

[Samuel Hagan JR]

It just need the fuel bar spec.

The Carb bores are the same.

 

[Alan Elzer]

My cmb carb is about 0.345 if I remember right. Can check in about 5 hrs & let you know

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[moparbarn]

I for sure haven't seen all the .21 CMB carbs out there (I think....), but the only ones I have that are .340" have a different intake horn. They are more bell shaped, not a straight taper like that one Sam. The ones I have with a straight taper inlet are either .320ish or .330ish. The spray bar in a Mac carb is much larger than any CMB carb, so the smaller size on the CMB carb shouldn't be an issue. The respective carbs were developed & sized for the engine design they were intended for. I have tried Mac carbs on my green head CMB's, & was happier with the CMB carb. The size differences are why Mac's need a larger fuel tank & "generally" don't mill at slow speed as well as other .21 engines, IMHO. The green head LS's used both the .330ish & the .340 bore carb, smaller on the silver case & the .340 on the black case/evo2 LS. Most of my engines are crank induction, but I do have a couple LS valvolas I will check this evening & post some sizes for bore & spray bars.

 

[Tom Foley]

Normally fuel goes in the needle from the other direction than shown in the photo . Nothing to do with your low speed problem but I just noticed it .

 

[Alan Elzer]

Sam my cmb black carbs came off a 21 outb cmb, bore is 0.335..hope that helps. If you need one of these I'd probably part with one...

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[Samuel Hagan JR]

Should I open the spray bar up to match the size of the MAC carb.

 

[moparbarn]

No, the spray bar is matched to carb bore/ flow. Too large a spray bar will alter the response to the vacuum signal generated by the bore size & the flow of the engine's design. Think "total package" (-;

 

[Samuel Hagan JR]

Okay. I get what your saying now.

 

[dave roach]

Sam

The answer to your question is the carb with the low end needle the spray bar Dia.is .055 and the carb without the low end needle is .060 Check and see what your got.

Dave Roach

 

[Samuel Hagan JR]

Sam

The answer to your question is the carb with the low end needle the spray bar Dia.is .055 and the carb without the low end needle is .060 Check and see what your got.

Dave Roach

Thanks Dave.

 

[Mike Betke]

Should I open the spray bar up to match the size of the MAC carb.ed

Run the black one the way it is. Opening it up will pretty much give you what you already have an issue with. You can always open it up later. The black carb is the cmb carb t that came with that style motor. I had one just like it.