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Author Topic:   how to set the carb.....
Dirt Full Roller

Total posts: 53
posted September 12, 2005 03:14 PM  
I've got a hobby stock 9:1 355, pretty much stock. I bought a new and supposedly good 2bbl rochester. The motor runs all right, but it seems rich to me. We couldn't get it to pull any vacuum to set it by gauge (pretty sure that's a cam thing). Here's the part I don't get. I turned the mixture screws all the way in, and it kept running. It almost seems like it didn't change a thing. We threw a rag on the carb, and it killed the motor, and we sprayed carb cleaner everywhere, no sign of a leak. If you haven't guessed yet, I'm totally new to racing, so any advice is appreciated...

Dirt Full Roller

Total posts: 53
posted September 12, 2005 07:44 PM  
I got to playing with it and took the mix screws all the way out, nothing happened. Is it possible that it's only adjustable by jetting?

Dirt Full Roller

Total posts: 53
posted September 12, 2005 07:48 PM  
Just from my short experiance i find that all cars tend to run rich at idle. You need to run the car and see what the plugs look like. every car i have ever tuned for power on the gas has turned out to run rich at idle.

sc1 racing
Dirt Freak

Total posts: 419
posted September 12, 2005 08:38 PM  
i always idle car up to about 3500 rpm, and start with screws out 1.5 turns and adjust until i get the smoothest run, and best throtle response..

Dirt Forum Champ
Total posts: 1019
posted September 13, 2005 06:15 AM  
If it's "idling" at 3500, you're not using the idle circuit anymore and the mixture screws won't do a thing.
The procedure has been posted here in the past, but in a nutshell, start by setting your butterflies so that only about .035"-.040" or so of the idle transfer slot is showing underneath the closed butterflies (when looking up from under the carb). Assuming the float level is OK, make sure there isn't more than about 6 lbs of fuel pressure at idle (never more than 7lbs is needed for a 2G or the lil' float can't hold it back).
As long as those butterflies aren't open too far at idle, the mixture screws will show a response when adjusted.
Then, place a vacuum guage on the intake or baseplate of the carb (if there's a port), and start (as stated above) with about 1.5 turns out on each screw and do one side at a time until not only the idle sounds the smoothest (shoot for about 1200rpm), but also that the vacuum guage shows its highest reading possible. Then move to the other side.
If you find you can't get the car to idle at 1200 or it seems to want the fast idle (aka curb idle) screw turned in so you are opening the butterflies a lot, then you need to enlarge the bypass holes in the butterflies some. We usually see a need of about .125" or so, sometimes up to .160", in oval track applications. This will allow you to keep the butterflies closed enough so that there's good response from mixture screw adjustment and you're not getting too much fuel from the transfer/mid-range circuit.

Keep us posted on results and we'll walk ya thru the rest of the tuning.


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