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Post by benji on Aug 29, 2019 22:05:06 GMT -5
The 19mm carburetor has a skinnier bore than the 20mm allowing more air flow from the start. I'm really sorry to put up all these posts but I can't continue without clarifying this statement.
If the 19mm has a skinnier bore than the 20mm, how can it allow more air flow?
Again, I'm really sorry for so many posts, I'm only trying to keep up.
with more Venturi effect, for one. Skinnier not taller. |
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Post by dexameth on Aug 30, 2019 9:54:43 GMT -5
The 19mm carburetor has a skinnier bore than the 20mm allowing more air flow from the start. I'm really sorry to put up all these posts but I can't continue without clarifying this statement.
If the 19mm has a skinnier bore than the 20mm, how can it allow more air flow?
Again, I'm really sorry for so many posts, I'm only trying to keep up.
Sorry, I meant the larger 20mm carb flowed more from the start since the bore is larger... that's what this whole post is about. The 20mm carb flowing larger. Obviously this is going nowhere fast. |
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Post by benji on Aug 30, 2019 12:12:08 GMT -5
I'm really sorry to put up all these posts but I can't continue without clarifying this statement.
If the 19mm has a skinnier bore than the 20mm, how can it allow more air flow?
Again, I'm really sorry for so many posts, I'm only trying to keep up.
Sorry, I meant the larger 20mm carb flowed more from the start since the bore is larger... that's what this whole post is about. The 20mm carb flowing larger. Obviously this is going nowhere fast. ya I'm getting confused. Honestly, I think it would help if the difference was shown in pics. Or at least show the proper way to measure the bore. Take the mystery completely out of it and directly show everyone once and for all so this don't pop up again haha. |
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Post by dexameth on Aug 30, 2019 12:50:53 GMT -5
Sorry, I meant the larger 20mm carb flowed more from the start since the bore is larger... that's what this whole post is about. The 20mm carb flowing larger. Obviously this is going nowhere fast. ya I'm getting confused. Honestly, I think it would help if the difference was shown in pics. Or at least show the proper way to measure the bore. Take the mystery completely out of it and directly show everyone once and for all so this don't pop up again haha. I did just show pics of the butterflies measured, maybe 2 pages back, which is how they measure these CVK carbs. I don't have a telescoping gauge to get in the air filter side to get exact measurements but what jer said was right about the 17mm opening on the 20m carb, and it is round as if a bit did a straight pass to bore it out. The 19mm carb looks like it used a 15mm bit and elongated if the opening to 17mm tall, keeping it 15mm wide (my guess, since I can't actually measure it). 17x17 (in theory) is more flow than 15x17 (in theory). You can see this is if you look at the pictures. |
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jer
Scoot Member
Posts: 81
Location: Wyoming-Silicon-Genessee Valleys
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Post by jer on Sept 6, 2019 18:42:16 GMT -5
Alright my man, you obviously want to be angry about this... I've measured these, QC'd them personally and know they're good and marketed correctly... So yes, I'm entirely comfortable marketing these to the forum as a 20mm... Matt OK Matt, if you say I'm angry than I'm a dog, or a cow, or not angry...what did Abe Lincoln say? Oh, yes, in case you've forgotten so soon, If 30,000 people call a dog a cow, it doesn't make that dog a cow.
So I'm angry because Matt keeps saying it. Matt, et al, keeps saying 20mm carbs don't need to be 20mm all the way through. I look at that pic where the butterfly is removed and you call it what? a venturi step? I call it a 16.7mm carb. I've spent an afternoon conversing with Chad Thomson of Sudco, the master distributer for Keihin North America. He told me in no uncertain terms that the carburetor size is measured at the jet block and flares out from there, hence the advertised size is the narrowest dimension in the air passage. He did go on to say that say that some people manufacture dog carbs and market them as cows (I was paraphrasing there.)
And Matt, I have no doubt that you're entirely comfortable doing whatever you need to do to protect your enterprise and high-profile.
Anyone that wishes to verify this can go to the Keihin North America site (keihin-na.com) and put in a customer request for help sizing carburetors and it will be Chad Thomson of Sudco that gets back to you. But somehow I know this will never happen. You guyz just want to call your dog a cow.
THE TRUTH IS NOT ALTERED BY YOUR PERCEPTION (OR DISCRIPTION) OF IT. a Dios
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Post by ryanqzqz on Sept 19, 2019 22:32:47 GMT -5
HOKAY; I'll wade in here, as an ASE L1 Master Tech, and an air-cooled engine junky, and one of the rare breed of younger technicians who still understand, build, and re-build carbs. I had loads of fun determining optimum venturi sizing, and calculated throttle bore when I started setting up dual carbs for my 1776cc daily driver, highway capable setup in my '71 Super Beetle. I asked old timers for tips, I studied books even  ... and I learned a lot about carb sizing, and relationships to inlet bores and intakes, etc etc. Venturi diameter, is different from carb sizing. Remember this fact. Venturi diameter is merely a measurement of the side to side distance and top to bottom distance, of the venturi. ( What is a venturi? Glad you asked) Literally, you are just measuring the distance between the sides of the throat, or narrowest spot, on the carb. MANY MANY carbs will not have circular venturi. ( Using a telescopic gauge to properly measure carb venturi size) Just like the one in the video link. In this case, you need to measure the AREA of the shape created by the venturi, in order to properly size both the venturi, and usually, the carb. Most carbs of this type, will be classified by the area measurement, as that is the most valid 'flow' amount you can expect. ( Correctly measuring a 'non-round' Venturi) Most carbs with circular venturi; that are SINGLE VENTURI or PLAIN VENTURI type ( Yes, there are multiple types of venturi), will be classified by the plain diameter of the venturi, because that is an accurate measurement of the flow amount you can expect, as far as calculations are needed for venturi size to throttle bore to intake inlet size ( See how to actually calculate how much carb you really need) Now, when you get into calculating size based on compound Venturi, you start having to figure in math that I have a hard time with, because you can get into vane styles and variable venturi styles, that vary the area.... Strangely enough, our CVK carbs would technically count as a variable venturi carb, because the slide moves to open up the venturi or close it, changing venturi area based on engine intake needs. So to properly calculate actual carb size, you would need to factor in fully closed slide area, by measuring the overall venturi area BARE, then subtracting a measurement of the 'covering' slide area... and then you would need to do the same math, but factor in the 'open' slide area that protrudes into the airflow. And then you could classify the carb with a size RANGE. So if we REALLY want to get technical, NO 20mm carb is a 20mm carb in the case of our CVK's. To sum up. This thread LOOKS like it started as a warning to valid prospective buyers, in regards to carbs out there sold with 20mm OUTLETS that were not 20mm all the way up into the throttle bore/throttle plate. NOT as a way to determine what is truly considered a 20mm carb. At least if we want to do the math and be technical about it. Sorry to be smarmy, but I deal with young guys at work all the time who are either scared shitless of carbs in general, or who think they know, and then screw stuff up that I have to fix. So it's a tender spot for me. ***Disclaimer---- I am not the worlds foremost expert on carbs. I'm merely taking about 15 years worth of tech work, experiments, and asking more knowledgeable people for pertinent information, and trying to apply it. So take any and all words in the keyboard vomit above with as many salt pills as you would like. |
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Post by dexameth on Sept 20, 2019 6:13:05 GMT -5
Great stuff right here ^^^
Evrything you explained is completely understandable, and most of which I already knew (how to measure and whatnot) EXCEPT the idea of the carb being a variable venturi. Light bulb totally just went off.
That slide IS in fact the venturi which changes its size from the vacuum of the engine, which is controlled by the user manipulating the butterfly with the throttle.
Awesome explanation!
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jer
Scoot Member
Posts: 81
Location: Wyoming-Silicon-Genessee Valleys
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Post by jer on Sept 29, 2019 0:58:25 GMT -5
OK, I give...but only because the local carb shop put me in the right place. When I asked him to tell me which carb had a higher flow rating, he just looked at them both and poked his telescope gage in one, then the other and handed them back to me and said this one (the stock 19) flows more air than this one (the aftermarket 20.) He then announced to the shop that because he just saved me $70 ($35 per carb on the flow bench,) that I was buying lunch for everyone as he busted out laughing they all cheered. While we were eating lunch, the whole story about bogus carbs came out and he put it into the proper perspective: MASS PSYCHOSIS! He didn't even need to put them on the flow-bench. And this guy does a dozen carbs a day (or more.)
So I will leave you all with this advice: Don't listen to anyone that actually works for Keihin or anyone that actually makes his living operating a flow bench for carbs.
Do what the vendors here say to do: 'Listen to the people that are selling the (counterfeit) carbs to you.'
Oh, and that one about "bores aren't round," helped bring me to naught.
So keep jabbering on, convincing yourselves that dogs are cows or whatever. I won't bother you anymore with all my sense-talking and (real) experts.
a-dios amigos
over & out
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Post by dexameth on Sept 29, 2019 9:16:01 GMT -5
Okay, later! 😂 ✌
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Post by magoconnor on Oct 3, 2019 22:39:46 GMT -5
I found this on another forum, they are also talking about it : motorbicycling.com/threads/measuring-carburetors.60032/The question is really, should the carb be measured by its throttle body or the venturi ? People tend to think its the throttle body and that's what most carbs are sold as. But I think the venturi is the real size of the carburetor. So if a carb that has butterfly valve that is 20mm but the venturi is only 18mm. Then the carb must then be a 18mm, regardles of what size the throttle body is. After all the venturi is where the fuel and air are mixed. It would be pretty hard to find the right carb for your setup, if you went with the throttle body size as the correct size for the carb. Cause it would not show how much fuel and air the carb can mix. But I could be wrong  |
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Post by magoconnor on Oct 13, 2019 23:29:58 GMT -5
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Post by scooterted on Oct 14, 2019 8:14:32 GMT -5
Definitely a better way to measure carbs. Doesnt change the fact that the naming conventon of using the tb diameter to identify these carbs is already established though. |
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Post by Mech Warrior on Jul 10, 2020 9:26:07 GMT -5
You should always measure a Carburetor from the rear where it connects to the intake manifold atleast thats how us old school do it.
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Petro
Scoot Enthusiast
Posts: 149
Location: Gothenburg, Sweden.
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Post by Petro on Jul 21, 2020 7:03:37 GMT -5
You should always measure a Carburetor from the rear where it connects to the intake manifold atleast thats how us old school do it. That's exactly what the chinese noname cheap manufacturers expect us to do... And they get away with it too often... |
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Post by dexameth on Jul 21, 2020 7:52:20 GMT -5
You should always measure a Carburetor from the rear where it connects to the intake manifold atleast thats how us old school do it. What's funny is some manufacturers will mill the spigot bigger on the inside but only around an inch deep. This gives a false reading. True 20mm are the same bore to the venturi. |
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... and I learned a lot about carb sizing, and relationships to inlet bores and intakes, etc etc. 
