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Author Topic:   Carb ??s and flow mods
Dirt Maniac

Total posts: 173
posted October 26, 2002 05:05 PM
What type of boosters work best? I've seen a few carbs with annular boosters and I'm just wondering what advantage, if any, there is?

I'm thinking about throwing some epoxy on an old carb body to try and get some flow improvements. I've already milled the choke horn and I'm wondering what I could tdo to fill/smooth the valleys around the horn between the airfilter ring and the venturis. I'm wondering what kind of epoxy to use for this.



[This message has been edited by HRT187 (edited October 26, 2002).]

Dirt Freak

Total posts: 408
posted October 27, 2002 05:50 AM
There are many different styles, types and sizes of booters. Annular discharge ones are meant to further aspirate the fuel mixture as it enters the venturi, but at the expense (usually) of restricting a bit of the airflow in there and also vastly changing the signal thru the venturi.
Booster height, size, shape, angle, position longitudinally, and several other factors all have a great affect on whether it will improve performance, do nothing noticeable, or greatly lessen the throttle response and performance of a particular carb.
Also, what works for one carb body and style in one application, may be the totally wrong thing to do for another.
Some carbs are designed by their builders to benefit from the use of annular discharge boosters, yet others, will suffer greatly.
The same goes for the difference in performance when changing from straight leg ones to downleg/dogleg/stepped boosters. Some carbs (like 390 4bbl's, little 2bbls, and sometimes 600cfm vacuum secondary carbs) can actually suffer in performance from "over-boostering" as I've come to call it.
An educated decision CAN be made as to what booster style will be the best for your application, but it almost always takes a significant amount of flow bench time, or a ton of trial and error.
Basically, it's something that I'd recommend not just blindly trying. It also requires special tooling to properly install boosters safely. There is a GREAT risk of a stuck throttle and injury or worse, if a booster falls out and into the throat of the carb.

Next, as for epoxy....It's been our experience that a small gain can be seen with a little epoxy in the corners just above the narrowest part of the venturi. This can aid (but only slightly) in improving laminar airflow and reducing air turbulence (which messes up a carb something fierce). Any over-enthusiastic use of epoxy, however, in the areas above this and outside on the top and around the area where the air horn used to be, probably will do nothing but introduce the possibility of a piece coming off and passing thru the motor.
Not a good thing, IMHO.

Probably a more noticeable gain would be seen simply by bolting on a K&N Flow Control air cleaner setup for your application.

As for what epoxy works, NAPA makes a good fast setting clear epoxy that lasts a while if the surface is prepped well. JB weld won't always work but if done right and mixed right, it can last a while. It's kinda one of those "take yer chances" things with that stuff. We try and stay away from it, generally.
Moroso makes a 2 part A-B epoxy that is very good in internal engine environments (head ports, etc).
A popular stuff that many carb builders seem to use is a clear acrylic-like, very hard epoxy (similar to what NAPA sells).
But in general, you'll probaby have the best results overall with the NAPA stuff. It's easiest to find, easy to use, and stays put. Just don't use it on a cut float in an alky me, that don't work...LOL!

Hope that helps, sorry it's so long...


Dirt Maniac

Total posts: 173
posted October 27, 2002 04:37 PM
No need to apologize... thank you!

I have used a K&N flow control on this carb before, but I would like to smooth a few areas where the carb meets the filter-base. I would like to keep the filter as low as possible which the K&N does not do... but oh well.

As for boosters, I guess I'll leave them alone when I rebuild this carb. Planning on going to a good aftermarket basplate. This carb has worked well before but wouldn't work at all this year even after a rebuild.

thanks for the help.

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