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Author Topic:   Major Carb ????? Please help
Dirt-Tracker
Dirt Full Roller

Total posts: 60
posted March 03, 2002 09:48 PM
Im a newbie here and need some help....I bought a Holley 500 out of the Jegs magazine for around $200+. It is the competition one. Im seeing in other mags Holley 500's for a much higher price. This is making me wonder if my carb is made for what im doing with it. Are there several different 2 barrell 500's and what needs to be done to mine for maximum output?

KPLugnut
Dirt Freak

Total posts: 408
posted March 04, 2002 07:14 AM
Your carb is not modified in any way. The one Jeg's sells, or Summit, or most other catalogs "for racing" is just the stock 4412 Holley carb that is "ready for blueprinting". They are mass produced and always come thru with a basic jet/pv/bleed setup.
The much higher priced carbs many times are also mass produced BUT have additional mods that bring the performance level a bit higher.
Custom carbs, from carb shops that work with EACH customer individually, are the ultimate setup tho. The carb will arrive to you in a form that is much closer to the needs of your specific application. IMHO, that is the ONLY way to go if you are looking to put as much care into your carb as you would in the motor below it....
After all, you wouldn't buy a mass produced "crate" motor and expect it to be a race winner right out of the box, would you?

JMHO, hope that helps...
KPLugnut

Dirt-Tracker
Dirt Full Roller

Total posts: 60
posted March 04, 2002 09:14 PM
Thanks for the help....not what I wanted to hear but honesty I can accept!!!!!!

Machinah
Dirt Forum Racer

Total posts: 79
posted March 05, 2002 05:28 AM
Don't get discouraged. I don't know these carbs enough to tell you what to do by way of modification, but modifications are not as costly when done yourself.

Is there anyone who can help with what should be done to modify this carb?

Good luck.

KPLugnut
Dirt Freak

Total posts: 408
posted March 05, 2002 07:10 AM
I'd be more than happy to list things to do to your Holley yourself. First, based on your engine combination (cam, heads, compression ratio, rpm ranges, intake, etc) you'll have to alter some of this to fit your engine's needs....
Drill a pair of .090-.100" bypass holes in the butterflies, centered on the idle transfer slot side of them.
adjust the butterflies so that there is about .030" (give or take, depending on engine needs) of the idle transfer slot exposed under the butterflies when the throttle is closed.
The air bleeds should be about .073" for the outer (idle circuit) and about .035-.037" for the inner (main circuit).
As a baseline starting point, a 72-74 jet and a 6.5 power valve will get you started, but these vary GREATLY based on many things, obviously.
The boosters should be checked for looseness and if they move at ALL, use a good epoxy around the area where they come thru from between the air bleed bosses, but don't go overboard with it.
Next, install a power valve blowout protection check ball and spring in the baseplate, that is, if there isn't one there already.
Most racers at one time will fall for the "more is better" theory with regard to the accelerator pump. Almost all the time, a 30cc pump with a white pump cam usually, will be sufficient for a gas motor. Additionally, a .031" squirter will be fine for most stockcar classes as well.
let's see....ok, next the idle mixture screws most often will wind up in the 1 1/4 to 1 3/4 turns out range, but this needs to be set using a vacuum guage, preferrably.
I know there's a LOT more, but I've only had one cup of coffee so far. LOL!....

Of course, most carb builders will not give away ALL their secrets either. (wink).

This should get ya started tho.
Hope it helps...

KPLugnut

Machinah
Dirt Forum Racer

Total posts: 79
posted March 06, 2002 05:05 AM
Thanks KP... am I understanding you right in that these modifications may differ based on engine specs or are some not necessary? For instance, if my cam is not too radical and I have plenty of vacuum, are the bypass holes in the butterflies necessary, or do they help in any case? Maybe I'm just not understanding what they are for. Thanks

[This message has been edited by Machinah (edited March 06, 2002).]

[This message has been edited by Machinah (edited March 06, 2002).]

KPLugnut
Dirt Freak

Total posts: 408
posted March 06, 2002 07:28 AM
Yep, you're exactly right. Some motor's requirements will differ based on many things, vacuum, compression, etc to name a few.
And yes, some motors require very different sized bypass holes than others, and some may not require any at all and can pull enough at idle to allow the idle circuit to function just fine as is.
It's all basically gotta be done on a case by case basis, IMHO.....at least that's what I've had the best luck with so far. And that's what seems to have gotten our customers dialed in the easiest, too.

Hope that helps,
KPLugnut.

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