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carb porting
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Author:  icaneat50eggs [ Sat Dec 16, 2006 1:44 pm ]
Post subject:  carb porting
How do you port a carb? Has anyone here done it? Or had it done. Does it increase the cfm? Thanks for any replies.
Author:  slantvaliant [ Sat Dec 16, 2006 3:27 pm ]
Post subject: 
This is one of those situations where people will rightly question whether it's worth it. What do you want from the car? What kind of carburetor? Why not go to an XYZ (bigger, more barrels) carb or EFI? On the other hand, if you have a little time, a spare carburetor, and a steady hand ...

You can clean up the airflow a bit, but be very careful - it's easy to ruin the carburetor and/or the airflow. If done correctly, the carb will flow more air (CFM). Done wrong, you have an expensive paperweight. Proving the change in flow would take a flow bench.

It's the same principle as porting a head or manifold. Find the things that unnecessarily restrict the flow, and reduce their effect without creating new problems. Mind the venturis/jets and the butterflies/bores.
Casting flaws (make sure they are flaws and not details) are the obvious place to start.
Some racers mill the choke horn off. Notice that some performance carburetors have nicelly rounded edges at the throats. That helps get the air in.
There may be a restrictive step in the air flow where the parts of the carb body mate. Stepping to a slightly larger size is not a big deal, but stepping to a smaller size is. The step can be reduced or eliminated by careful metal removal. Carb walls are often thin, so watch out.
Knife-edging the butterflies can help, as can using a half-shaft and countersunk screws. Just don't screw up the bore.
A little light polishing won't hurt.
Don't forget the gaskets in and under the carburetor, and the manifold itself.
Author:  BigBlockBanjo [ Sat Dec 16, 2006 5:59 pm ]
Post subject: 
While on the subject.....
When looking down the throat of my 1920, there is a "I" shaped cluster of metal (3 bars) almost midway down. It's hard to explain....If you have one, you know what I'm talking about. What is it's purpose? Is it like a fuel pull-over or something?? Looks like it would cause turbulance...
Just a thought.....
Author:  SlantSixDan [ Sat Dec 16, 2006 6:40 pm ]
Post subject: 
Its purpose is to channel the air so as to break up larger drops of fuel into smaller droplets. It's definitely supposed to be there!
Author:  argentina-slantsixer [ Sat Dec 16, 2006 9:22 pm ]
Post subject: 
Quote:
While on the subject.....
When looking down the throat of my 1920, there is a "I" shaped cluster of metal (3 bars) almost midway down. It's hard to explain....If you have one, you know what I'm talking about. What is it's purpose? Is it like a fuel pull-over or something?? Looks like it would cause turbulance...
Just a thought.....
that piece is actually the fuel dosificator unit. 1920's doesn't have boosters such as other carbs, they work by means of just creating the pressure differential with a very rough and primitive square machined "restriction" (reduction in throat diameter that decreases pressure and increases velociti facilitating the air to draught fuel from the bowl wich has more pressure than that area) you have there 3 parts: one large wing shaped ridge that has a passage that connects from the brass casketed hole inside the bowl, and that passage branches up to a T on each side. The "top stroke" on each T are just flaps and you can live withouy them for certain applications, or can improve your signal strengst and low rpm torque by carefully bending the sides with a duckbill long nose pliers. Hard to understand but I'll snap some pictures as soon as I can get out my home (sick and home now) and that primitive, "we don't care $#!+ about this" shape can be greatly improved by reshaping the underside of the venturi (under this "flapper-fuel deliverer") to just above the idle transfer slot. removing of that piece would be a great safety and plus you'd be able to remove airflow offending protrusions on that area with an emery flapper and a dremel tool. Removing of this piece is as follows: tap in a screw extractor on the brass retainer that holds the piece to the fuel bowl. previously apply heat on that area. dont try to break the piece loose by spinning as it ain't acrewed but press fit. A little too much tapping and twisting and you can actually screw up the piece. With steady hand, pull the piece with the most linear action possible. (don't try to work it wiggling to the sides) then insert a rod of DELRIN or other hard but not brittle plastic, or wood, and carefully drive it off from the trhottle blade side with a mallet. port and have a festival inside, polish it to a mirror finish, and reinstall that piece. Before reinstalling, enlarge all holes to 1/8 or better yet 3.5mm - 5/32" with a REAMER. Don't use a drill here. Install the piece, press fit the retainer, ream up the retainer and reassmble. You'll feel a lot of diference.

I don't use the search feature often mainly because of doesn't completely understaiding how it works, but I have plenty of posts about heavy carb modification. I'm sure Dan can dig up that posts for you...:wink: eh? dan?
Author:  mpgmike [ Sun Dec 17, 2006 7:26 am ]
Post subject: 
If you're serious about getting the absolute most out of it, I found this for you:

http://racingsecrets.com/holley_carburetors.shtml

Mike
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