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5/22/14 8:31am - Original Message: 'Idle screw adjusting for Rochester'
bio11
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Oklahoma City, OK - USA

Vette(s):
1972 LS5 Convertible,Auto.Targa Blue with Black Interior.

Joined: 4/1/2007
Posts: 432
I'm not happy with the way this thing idles sometimes...  the timing is all good but to adjust my idle screws should I do it with the car in drive under a load or just at idle in park? It is the original carb that has been rebuilt about a year ago. Sometime when i'm at a stop light its idling fine but then it will start to kinda roll a little and I'll kick it in neutral or just use 2 feet....   drives me crazy...   and when adjusting the idle screws.... slowly in until it starts to roll and then back out 1 full turn on the Rochesters?

I hate carburetors!!!!!!!!


Thanks
Lonnie


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5/22/14 11:23am - Reply: 'Re: Idle screw adjusting for Rochester'
Adams' Apple Lifetime Member
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Duncanville, TX - USA

Vette(s):
#1-1974 L-48 4spd Cp Med Red Metallic/Black deluxe int w/AC/tilt/tele./p/w-p/b/ Am-Fm/map light National/Regional/Chapter NCRS "Top Flight" #2-1985 Bright Red/Carmine Cp.L-98/auto Member: NCRS, NCRS Texas, Corvette Legends of Texas

Joined: 11/8/2003
Posts: 19457
What is the idle RPM in Park/Neutral, to start with? If it's idled to high to begin with, it will want to continue moving while stopped.
There should be a sticker on the firewall above the brake booster that tells you what the idle RPM should be in gear. Don't know without checking, but I think idle rpm, in gear, should be somewhere around 650/750 rpm. To set the idle speed, get it to operating temp, apply the park brake, block the wheels/tires to prevent accidents, and turn the idle adjustment screw IN to raise the speed, OUT to lower the speed. Make sure the choke plate is fully opening, also, before adjusting the idle speed.
If the carb has an anti-diesel solenoid on it, you will prolly also need to adjust that, or at least check to see if it is working as designed. The anti-diesel solenoid is supposed to close the throttle completely with the key Off. Key ON, the solenoid should kick the throttle open to idle speed.
If the car has A/C, the solenoid operates differently. With A/C, the solenoid only operates with the A/C ON, to raise the idle speed to compensate for the added load on the engine.

 A vacuum leak can also cause the idle to be erratic when in gear....

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Joel Adams
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5/23/14 8:48am - Reply: 'Re: Idle screw adjusting for Rochester'
bio11
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Oklahoma City, OK - USA

Vette(s):
1972 LS5 Convertible,Auto.Targa Blue with Black Interior.

Joined: 4/1/2007
Posts: 432
Sorry Joel... the idle is at 700 in drive. The idle mixture is what I think may be my problem. Dont know if its to lean or to rich... 


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5/23/14 9:23am - Reply: 'Re: Idle screw adjusting for Rochester'
F4Gary Gold Member
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Grapevine, TX - USA

Vette(s):
1972 LT-1 convertible with factory air.

Joined: 8/26/2006
Posts: 869
Joel, that's not how the idle stop solenoid works with a/c on a 72.  It is only an anti-dieseling solenoid.  Has nothing to do with a/c.  Many of us have wire it to work with the compressor, but that is not what it is designed to do.  In fact it doesn't have enough strength to bump up the idle, you have to bump the accelerator to help the solenoid come all the way out.

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5/23/14 11:47am - Reply: 'Re: Idle screw adjusting for Rochester'
Adams' Apple Lifetime Member
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Duncanville, TX - USA

Vette(s):
#1-1974 L-48 4spd Cp Med Red Metallic/Black deluxe int w/AC/tilt/tele./p/w-p/b/ Am-Fm/map light National/Regional/Chapter NCRS "Top Flight" #2-1985 Bright Red/Carmine Cp.L-98/auto Member: NCRS, NCRS Texas, Corvette Legends of Texas

Joined: 11/8/2003
Posts: 19457
Wasn't 100% sure what years had what function for the solenoid. The wiring diagrams don't even show it until '73, as an "Idle Stop"....hot anytime the key is on. Thumbs Up

As far as adjusting the idle mixture screws, the best way is to hook a vacuum gauge up to manifold vacuum, and with the engine idling at the correct RPM, turn the screw(s) in a little at a time. You're trying to get the best vacuum reading you can. You can also use a good tach, and set it by rpm....using both is the best.
 If turning in lowers vacuum/rpm, turn them out. After each small adjustment, give it a few seconds to compensate, then adjust a little more. It's a good idea to rev it up and clear it out ever so often, too, during the process. If adjusting the idle mixture raises the rpm, adjust the idle speed back down to spec, and start over on the mixture screws.
Of course, if you have an exhaust gas analyzer sitting int he corner of the garage, you could use that, too....LOL


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Joel Adams
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             NCRS

"Money can't buy happiness -- but somehow it's more comforting to cry in a CORVETTE than in a Kia"
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