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Author Topic:   Carbeurators
Dirt Newbie

Total posts: 4
posted June 01, 2000 07:55 PM
Any thoughts on how to set up the carb (either 2 or 4 barrel) on a street stock type car with a 350 and 500 lift cam. Mixture normal (max vaccuum), rich, or lean?

Earl Parker II
Dirt Roller

Total posts: 17
posted June 03, 2000 10:37 AM
Originally posted by NewGuy1:
Any thoughts on how to set up the carb (either 2 or 4 barrel) on a street stock type car with a 350 and 500 lift cam. Mixture normal (max vaccuum), rich, or lean?

There are some many things to consider that you could write a book on the subject (which I did). Below are some of the things I would do to begin with:

) Make sure you have no more than 6 psi of fuel pressure at idle. If you have a high-performance mechanical or an electric fuel pump, you very likely have too much fuel pressure, which will create all kinds of problems. Put a gauge in the line, at least temporarily, and check your pressure. If it's too high, get a Holley 12-704 fuel pressure regulator, or quivalent, and plumb it into the line as the last thing before you get to the carburetor, after all filters. Set the pressure on 6 psi as per the enclosed nstructions. You will be surprised at all the little (and some big) problems that will go away if you do this. It's really that critical.

2) Set the float level(s). You want the fuel level right at the bottom of the sight plug hole but not dripping out. Do this after you've set the fuel pressure.

3) Make sure the butterflies are in the correct position at idle. If you remove your carburetor and turn it over, you'll see a small hole in each throttle bore- the curb idle discharge hole. Open the butterflies a little and you'll see a slot- the transfer idle slot. If, at idle, the butterflies are set to where a lot of this slot is exposed to manifold vacuum, the engine will never accelerate properly. The details of why are a little long to go into here, but here's how you fix it. Close the butterflies down until about .020" of the slot is exposed to manifold vacuum. Set the carb back on the engine, attach a manifold vacuum gauge to it and start it. If it won't idle, open the primary butterflies with the idle speed screw, counting the turns and fractions of a turn, until it idles. When the engine is warm pick one of the secondary side idle mixture needles (assuming four-corner idle, otherwise work with the two primary side mixture needles) and turn it, in 1/4 turn increments (letting the rpm stabilize each time), first one way then the other, until you find the "sweet spot" where the manifold vacuum (or the rpm) is the highest. Since you've presumably raised the rpm, you can close down the butterflies somewhat. Repeat the process with the other idle mixture needle. Then move to the primary side of the carbureor and repeat the process (again, assuming four-corner idle). The whole idea is to wind up with a smooth, stable idle and have the butterflies returned to the ideal position right at the bottom of the transfer slot.

If you'll do the above items you will have a good basic setup that will allow you to tune the carburetor effectively. Be sure to come back to us when you have some specific questions and you'll get plenty of help.

Of course there is a ton more that can be done to improve performance. Feel free to give me a call and I'll help all I can.

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