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Author Topic:   idle mixture screws
rogracer2
Dirt Maniac

Total posts: 119
posted July 02, 2005 11:11 PM  
I have a 4 barrel Rochester. How many turns am I supposed to have on the 2 idle mixture screws? If I have these off, could that cause the exhaust manifold to glow red? If so, which idle mixture screw am I supposed to adjust, and which way - in or out?

Thanks for any information!

sc1 racing
Dirt Freak

Total posts: 419
posted July 03, 2005 01:12 AM  
in lean out rich. sounds like you are to rich or timing off. start with 1.5 turns out then adjust in 1/4 untill you get smoothest running and throtle response..

wermm
Dirt Maniac

Total posts: 107
posted July 03, 2005 10:41 AM  
I may be wrong, hey it happens, but aren't the mixture screws just for idle and if the manifolds are glowing wouldn't it be a jetting problem

rogracer2
Dirt Maniac

Total posts: 119
posted July 12, 2005 06:00 PM  
If you are too rich, could this also cause the motor to get hot quickly?


Steve Dupree
Dirt Freak

Total posts: 316
posted July 12, 2005 06:46 PM  
You are probally to lean not rich the idle screws only come in affect at what they are called idle. Put them at a turn and a half but up jets a little first and look at plugs and when you are done jetting then you can adjust your idle scews to keep your plugs a sclean as you can while at idle. You need to open jets,,,,,,,,

rogracer2
Dirt Maniac

Total posts: 119
posted July 12, 2005 08:08 PM  
Right now the screws are out 3 turns - and the car backfires after I hit the gas hard and then let off quickly. So I need to turn the screws in more (1 1/2 turns)?

I've run this carb for 2 years - never touched it - is something clogged maybe? Any tricks to cleaning the jets?

[This message has been edited by rogracer2 (edited July 12, 2005).]

Steve Dupree
Dirt Freak

Total posts: 316
posted July 13, 2005 04:47 AM  
I would just have someone put a kit in it if that is the case usually jets dont plug up but you could need a good cleaning so might as well put and kit in and call it good.

sc1 racing
Dirt Freak

Total posts: 419
posted July 13, 2005 11:43 AM  
setting idle mixture correct has alot more effect than just at idle. it comes in to play at all rpm, and does make a big differance.

backwoodsracer
Dirt Forum Racer

Total posts: 88
posted July 13, 2005 12:55 PM  
Sounds like a good one for KP.
mine are at 11/2 turns and no problems.I was told this is where to set them but dont know what the right method or spec is.

KPLugnut
Dirt Forum Champ
Total posts: 1019
posted July 13, 2005 01:35 PM  
Yes, guys, the idle mixture adjustment does affect the carb at other times besides just sitting at idle. Throttle response, acceleration, everything like that....
As for exactly where everyone should set them, a lot has to do with the position of the butterflies at idle and how much idle transfer slot is exposed, what the engine's needs are for fuel mixture at idle, and other things like air bleed sizes, etc.

In general, tho, a good baseline starting point for most of these situations is to start at 1 1/2 turns out at idle when there's only about .030" of the idle transfer slot showing underneath the closed butterflies. If ya can't get the car to idle at the right rpm without cranking open the butterflies a lot, then you need to drill bypass holes in the butterflies and size them so that you can close the butterflies down on the slot and get the right adjustment on the mixture skrews...

Hope that helps, it's a very brief summary and I didn't write it all out but that's most of the main stuff...

KP

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