Carb venturi questions/testing

Lucky_7

Member
So I was looking at some carbs yesterday and noticed a difference in the venturi. The exit or head side has a different angle and the i.d. is smaller than the other. So this got me thinking. Which one flows better? What would the smaller i.d. carb do to overall engine performance? Well after some testing. The larger i.d. carb has advantages. It can handle a larger idle jet. And it had around 200 more rpm. The smaller I.d. seemed to get lazy towards the higher rpm ranges (takes longer to gain rpm).

I'm I making any sense here?

P.S. : Whats the AKRA rule on machining the carbs?
 

1fasttiller

Member
I'd suggest getting an updated version of the AKRA rulebook....
 

Lucky_7

Member
Ive wanted to pick one up for a while now. Just havent got around to it. My main concern is that both of these carbs were purchased as they are now. Why the big difference in the venturi. I understand that castings are different and get updated a lot. I just figured the rules would allow these carb/engine builders to machine these to the proper dimensions.
 

Kart43

Member
You ask why the difference in components that are manufactured in China, in dozens of factories, by the 10's of thousands a year?
 

DynoDon

Moderator
The smaller bore carburetors are good for restrictor plates and the larger bore carburetors are good for unrestricted. AKRA rule is .615 maximum there is a minimum without looking at the book I think it's .609. you may bore to size. There are a few new tech tools to identify the interior of the carb for 2020.
 

Lucky_7

Member
So the max venturi is .615 in both. My question is about the ending taper from the .615 i.d. to the end of the taper on the head side. One is longer and smaller....
 

Lucky_7

Member
You ask why the difference in components that are manufactured in China, in dozens of factories, by the 10's of thousands a year?

Oh, I get that. Been in the manufacturing industry for 15 years.

But, since the castings are so different. Why not allow machining of those tapers to even out the carbs. You can still have the max .615, and the max bores front and back. But the taper can be smoother, and larger in overall i.d.
 

DynoDon

Moderator
Everybody in the country would be sending carbs back or forced to get new ones because there would be a carb of the month. It wouldn't be fair to those who don't have that type of work done
 

Lucky_7

Member
Well the fairness or legality of this is secondary. I just need to know if there would be a performance gain in the larger tapered bores. I want to know what to look for next time a buy a carb...
 

flattop1

Dawg 89
Unrestricted I would want bigger , longer and smoother taper .
 

Lucky_7

Member
Alright, took some fancy measurements tonight. The carb we were running was a .615 to .670 taper. The carb we found in the pile is .611 to .700, which one would you run??? I think the .030 is going to be a much bigger gain than the .004....
 

flattop1

Dawg 89
Agreed , flow results would be handy .
With head even better .
 
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