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Author Topic:   Rochester Carb Adjustments
Rookieracer76
Dirt Maniac

Total posts: 182
posted April 13, 2003 08:35 PM
I have a basically stock 350 with a rebuilt 2 barrel rochester carb. There two screws in the front of the carb. what do they do? and what would be a good setting? and also for the SC*** where the throttle linkinge conects to the carb. thanks

[This message has been edited by Rookieracer76 (edited April 13, 2003).]

KPLugnut
Dirt Freak

Total posts: 408
posted April 14, 2003 09:44 AM
start by hooking up a vacuum guage to read full manifold vacuum at idle. Adjust each ***** so that you get the highest vacuum reading.
Generally, 1.5 - 2 turns out from lightly seated is a good starting point.
You may need to drill/enlarge the holes in the butterflies so that you can close down the butterflies (using the curb idle skrew by the throttle). Your goal is to expose only about .020" of the idle transfer slot in the baseplate (the slot that the butterflies mostly cover up when they are shut). This adjustment will give you good response to adjusting the idle mixture screws on the front of the carb.
Also, before ANYTHING else, make sure you aren't running more than about 6-7lbs of fuel pressure at idle. Rochesters are quite finicky about that because of their small floats.

Hope that helps,
KPLugnut

avila
Dirt Full Roller

Total posts: 55
posted April 15, 2003 09:47 AM
KP, where on the butterflies do you drill the
hole and how big should the hole be?

Would you drill the holes on both sides and in the same place on both sides?

KPLugnut
Dirt Freak

Total posts: 408
posted April 15, 2003 10:41 AM
centered, side-to-side, and on the idle transfer slot side of the butterflies halfway between the shaft and the idle transfer slot. Yes, for this type of carb, it is best to do both butterflies the same way. Start with about a .090" hole and work up as needed. How large a hole needed is a function of the vacuum signal each particular cam/motor combo generates and how much bypass air is needed in order to let the motor idle at the target rpm and still have the butterflies closed the proper amount on the transfer slots.
So, basically, there is no absolute answer there, every motor is different. There are starting points but it varies.

Hope that helps,

KPLugnut

Rookieracer76
Dirt Maniac

Total posts: 182
posted April 15, 2003 07:52 PM
Kp, i adjusted the two front Sc**** 2 turns out and i couldnt seem to get the car to idle. is that due to not enough gas? or something else? thanks

KPLugnut
Dirt Freak

Total posts: 408
posted April 16, 2003 10:07 AM
well, keep in mind that EVERY carb and engine combo is different with that and you have to make sure you have the proper amount of the idle transfer slot exposed under the closed butterflies.
Just turn the mixture screws one way and then the other and find where the motor idles the best and has the most manifold vacuum at idle.
But to do the entire procedure correctly, you have to do more than JUST turn the screws.

Hope that helps,
KP

towmandan
Dirt Maniac

Total posts: 110
posted April 16, 2003 10:39 AM
You want to keep both sides fairly balanced .In other words, what you do to one side you need to do to the other. Give or take a little. example.. You dont want left side out 5 turns and right out 2 turns. You will have best results if you turn them both in..not to tight.. and back them out a turn at a time, but you got to keep it running while your doing this till it will idle on its own then fine tune your adjustment....towman

Rookieracer76
Dirt Maniac

Total posts: 182
posted April 17, 2003 11:32 AM
Going to try and fine tune the carb sometime today. Alos does anyone know of a web site that has diagrams of the carb? Thanks.

Rookieracer76
Dirt Maniac

Total posts: 182
posted April 17, 2003 05:45 PM
When i have the car in park and rev it the car runs fine, BUT when i put the car in gear and drive it up and down the road it breaks up and back fires out of the carb. Stalsl alot to. What is my problem?

teamstertom
Dirt Forum Racer

Total posts: 89
posted April 17, 2003 06:21 PM
Just off the beaten path,but I have to ask. Do you have a good ground? This problem may be electrical,rather than carb.

Some people never pay attention to a good ground from the motor to the frame,especially when the battery has been relocated.

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