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Unread 11-25-2011, 01:38 PM   #1
Oily
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Is It Possible ? An Unasked MC2100 question ?

Just when you think you have seen it all, along comes Oily, .
It is said, that we need the MC2100 with 1.08 stamped on the side. That the 1.21 is too big because it gets ?too many CFMs of airflow.
What makes it a 1.08 or a 1.21 ? The venturies, correct ? I ask because I got a 1.08 and I do not think it is the best rebuild candidate. I just got four 1.21s for $25.00. I am thinking I can just swap in the 1.08 venturies into the 1.21 body. The upper air intake (air horn) is the same. I do though have a choice of bottom butterfly valve sizes. The 1.21 carbs (three) have dual 2 5/8 and the other has dual 2 1/2 butterfly valves. The 1.08 has dual 2 1/2 butterfly valves. I suppose that if we are restricting the CFMs for ideal compatibility with the 258 6cyl, I will use the smaller butterfly valve setup.
If I have not confused you enough, it seems as though the bottom of the base plates has different styles. I'll have to get pics up to explain that.

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Unread 11-25-2011, 05:11 PM   #2
scrapman
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I have thought the same thing, but if you use the 1.21 base, then the holes will indeed be larger and you will have to use the larger butterflies. I would assume the venturi cluster is built to work best with the correct cfm that it is usually mated with, so I am not sure how the 1.08 venturies would act with the 1.21 base.

I am in the same situation, I have a 1.08 that has throttle shaft play, and a 1.21 that is in better shape. Please post your findings.
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Unread 11-25-2011, 05:43 PM   #3
MoC
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I have a 2100 1.08 I am selling if you are interested. Will PM you some pictures.
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Unread 11-26-2011, 05:30 AM   #4
Oily
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Quote:
Originally Posted by scrapman View Post
... if you use the 1.21 base, then the holes will indeed be larger and you will have to use the larger butterflies. I would assume the venturi cluster is built to work best with the correct cfm that it is usually mated with, so I am not sure how the 1.08 venturies would act with the 1.21 base.
I would have been confused trying to understand what I wrote.
I have one 1.21 base with the same sized venturies as my non candidate 1.08.
With regards to what you said, a 1.21 works with different sized butterflies. Why not a 1.08 ? Granted only one 1.21 has the smaller butterflies, so perhaps a 1.08, if different sized butterflies work, they may only work with a smaller sized set of butterflies.
I believe with what I have, I can replicate a 1.08 with the 1.21 body by swapping in the venturies from the non candidate 1.08 that I have. What I need is a carburation physicist's opinion. Dozens, maybe hundreds, have posted on this topic before, .
When are CFMs measured ? At idle, wide open throttle, a median application ? When this forum's carburation physicist's caution to not get anything larger than a 1.08, what type of operational issues are they cautioning against ? Too many CFMs of air/fuel mixture causing a rich condition ? Lack of ideal idle and/or preferred operating conditions ? Poor gas mileage ? I have not yet disassembled a 1.21 yet and compared the venturies to each other. I would think the butterfly valves determine CFM.
It is also suggested that intake air flow and exhaust air flow be somewhat balanced, with an optimum back pressure quotient The head also factors into this and some opt for the 4.0 head for better flow. Feel free to join the "you think too much Oily" bandwagon, .
Quote:
Originally Posted by MoC View Post
I have a 2100 1.08 I am selling if you are interested. Will PM you some pictures.
I got your PM. I could never pay you for what it is worth, considering you got it from the EBay guy, (just ribbing 'ya). I admire his capitalist spirit but he charges $300.00 for what we can build for about $85.00 which includes a new V8 throttle cable (eliminating fabricating new bracketry) and a new air cleaner. $35.00 is the most I have ever paid for a 1.08.
As I mentioned, all of the MC2100s have different ... vacuum chambers cast into the underside of the base. Would you post a pic of the bottom of yours for reference. I am gonna cut/paste one from another thread. I will also try to get pics up of all of mine.
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Unread 11-26-2011, 05:32 AM   #5
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Unread 11-26-2011, 07:50 AM   #6
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Quote:
Originally Posted by MoC View Post
I have a 2100 1.08 I am selling if you are interested. Will PM you some pictures.
MoC, will you measure the diameter of your MC2100s butterfly openings please, ?
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Unread 11-26-2011, 08:47 AM   #7
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Will be this afternoon but I can post up a picture and measure.
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Unread 11-27-2011, 07:19 AM   #8
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Did I stump the forum , ?
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Unread 11-27-2011, 12:30 PM   #9
Oily
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This is my 1.08. Notice the four plugs just above the butterfly valves. Also, you can't see it well, but there are openings on the base of the butterfly valve chamber which provide vacuum to the differing vacuum ports and/or enable other carburator internal functions. These openings supply vacuum to very different sized/configured vacuum chambers. Also, vacuum passageways differ in that some have more/less/or are different.

Now, here is a 1.21.

Another ...

Another ...

[and my last one ...

Ok. On preview, I see that my pictures do not show detail, too much light. Let me play with the camera, PhotoBucket, etc.
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Unread 11-27-2011, 01:34 PM   #10
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Quote:
Originally Posted by Oily View Post
I am thinking I can just swap in the 1.08 venturies into the 1.21 body. The upper air intake (air horn) is the same. I do though have a choice of bottom butterfly valve sizes. The 1.21 carbs (three) have dual 2 5/8 and the other has dual 2 1/2 butterfly valves. The 1.08 has dual 2 1/2 butterfly valves.
Ok, this is where I get confused. I went out and measured the holes for the accelerator butterflies on my 1.08 and 1.21. The 1.08 is 1.5 inches and the 1.21 is 1.675 inches. Are your holes an entire inch bigger than mine? I believe the hole size is what makes the carburetor able to flow a certain cfm. The venturies are matched with the cfm size. I also have a 1.02 mc2100 and the holes for the butterflies are even smaller. Am I missing something?
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Unread 11-27-2011, 10:07 PM   #11
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Picture test.
Why did my above images get removed ?
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Unread 11-27-2011, 10:15 PM   #12
Oily
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Quote:
Originally Posted by scrapman View Post
Ok, this is where I get confused. I went out and measured the holes for the accelerator butterflies on my 1.08 and 1.21. The 1.08 is 1.5 inches and the 1.21 is 1.675 inches. Are your holes an entire inch bigger than mine? I believe the hole size is what makes the carburetor be able to flow a certain cfm. The venturies are matched with the cfm size. I also have a 1.02 mc2100 and the holes for the butterflies are even smaller. Am I missing something?
I will double check my measurements tomorrow. I think I got them right. Good to know that you have a 1.02 MC2100. Your CFM /hole size theory sounds logical. Yet I have a 1.21 with the same sized openings as the 1.08 and three other 1.21s with larger openings.
MoC, can you get us your butterfly opening size ?
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Unread 11-27-2011, 10:31 PM   #13
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Reference only, copied from this thread (a good read)
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Unread 12-01-2011, 05:07 AM   #14
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Quote:
Originally Posted by scrapman View Post
Are your holes an entire inch bigger than mine? The 1.08 is 1.5 inches and the 1.21 is 1.675 inches.
No, I was wrong. I used a tape measure and did not subtract an inch to compensate for my starting point (as the solid flat starting edge is not a good starting point for a circle). You must have used a micrometer and are correct.
Quote:
Originally Posted by scrapman View Post
I believe the hole size is what makes the carburetor able to flow a certain cfm. The venturies are matched with the cfm size. I also have a 1.02 mc2100 and the holes for the butterflies are even smaller.
I do have a 1.21 with 1.5 inch butterflies and the other with 1 5/8 inch (tape measure) butterflies.
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Unread 12-01-2011, 07:35 AM   #15
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The venturies are machined into the carb body. If you look down the throat of the carb you will see it is tapered to a point then becomes wider again. this constriction is the ventury. The taper speeds up the air to make it draw fuel through the jets better. If the venturie area becomes to large for the amount of air you are pulling the air speed drops and you don't get as good of vacuum sinnal at the boosters. At this point throttle response gets worse.
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