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Topic: Idle RPM?

in Forum: C3 Fuel, Emission Control, and Exhaust Systems

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Idle RPM? (1/10)
6/14/09 12:46pm

bio11
Former Member
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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 have had the carb rebuilt and everything is running great except I have a question about the idle rpm. With it in drive and warmed up I have it set at 600 rpm according to all my manuals and that feels fine. But in neutral the idle is at about 1000 rpm, that just seems a little fast. Is there anyway to get these a little closer together. Does the fast idle screw just set idle speed when the chock is on?

Thanks for any enlightenment

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Idle RPM? (2/10)
6/14/09 1:10pm

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 forgot to mention that this is the original Rochester 4MV Carb.

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Idle RPM? (3/10)
6/14/09 8:31pm

Adams' Apple

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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

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The "fast idle" speed is set with the coke on, and by the screw on the choke side of the carb...waaay down on the bottom...not the easiest thing to get to.
The "curb" idle speed is set by the screw on the driver side of the carb, at the front. Once the choke has fully released, you should be able to lower/raise the idle speed with that screw.

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Idle RPM? (4/10)
6/14/09 8:56pm

VetteSpecialties
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Mounds View, MN - USA

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70 LT1 coupe, 69 350 HP coupe, 69 390HP 427 coupe, 71 LS5 convert, 85 coupe, 93 coupe

Joined: 5/24/2007
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Joel is right, but the big drop in RPM doesn't seem right. I suspect your mixture is off enough that it takes a lot more throttle to support the load of being in gear. My first guess would be too lean at idle. That is best checked with an air/fuel meter, but you can do it by ear, too.

Larry

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Idle RPM? (5/10)
6/15/09 9:45am

bio11
Former Member
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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

My idle speed with choke on and fully released at normal operating temp is ok, it's just that big rpm drop. I have the idle set at 600 rpm in drive. But when you shift into nuetral or park it goes up to 1000 rpm. I have the mixture screws set at 4 turns out. I'll play with this a little and see if I can make any differance. I also do not have either the throttle solenoid or the solenoid for the vacume advance on this engine. They were not on it when I bought it. My timing is set at 10 degrees instead of the recommended 8 degrees. So many variables Wits End Maybe I'll drive it to Duncanville.

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Idle RPM? (6/10)
6/15/09 11:44am

anips
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sonoma, CA - USA

Vette(s):
72 LT-1 AC coupe,69 l-36 coupe

Joined: 8/9/2002
Posts: 784

The best way to set the idle mixture is with a vacume gage, hook the gage to a manifold vacume supply port, not timed vacume supply port, 4 turns seems to be alot, should be somewhere around 2-2.5 turns, hook up the gage, in nutrual, engine warm, fast idle off, curb idle set at factory spec for auto trans, one at a time start to turn the idle mixture needle in watching the gage and try to achieve the highest vacume reading before the engine starts to stumble, then back the mixture screw out a 1/4 turn, then do the other side the same way, after you have done them both once, you may have to resent your curb idle again, then do the proceedure again, if you car has AC you should have the idle selenoid working to kick up eng idle when you turn on the AC, the Vac. Adv. selenoids job is to allow advance only in high gear, better for it not to even be on the car, also be sure that your vac. adv. line is connected to the "timed vac." port on the carb, not a "engine Vac." port. anips2009-06-15 08:47:00

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Idle RPM? (7/10)
6/15/09 1:05pm

Adams' Apple

Make sure there are no vacuum leaks somewhere. That would cause a high neutral idle speed, compared to in gear idle speed...

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Idle RPM? (8/10)
6/15/09 2:06pm

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'll do a little investigating when I get home tonight. I do have a vacume gauge...I'll try that and also look for that rare problem with an older Corvette...the dreaded vacume leak.

Thanks Guy's ...I'll let ya know how it goes.

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Idle RPM? (9/10)
6/15/09 2:12pm

dskopp
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Oak Creek, WI - USA

Vette(s):
1981 Great White Shark. Red Interior, 350/190hp. PS, PB (SS), A/C CC, T-Tops, Going to remain as Stock as possible. Served three years in Active Duty Army, then Retired Air Force after 34 years! Badger State Vetts Car Club. 175,000 Original miles!!

Joined: 5/21/2008
Posts: 1958

I am having my carb rebuilt as we speak!! Been in the shop now, three weeks. He may be slow but he is good.

Dan

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Idle RPM? (10/10)
6/16/09 10:03am

bio11
Former Member
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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

Well I got out my vacuume gauge last night and used it to set my idle mixture. It worked great. Runs a little smoother. With my idle speed in drive set at 600 rpm, my idle speed in park is now about 800 to 850 rpm. I also discovered my driver side headlight actuator relay has a very small vacuum leak. It doesn't do it all the time though. So I'll be pulling that off to play with it, probably have to replace. I temporarily block off the vacuum to the headlights and it runs much smoother. It just goes to show you it's always something. When my wife asks me when I'll be finished with it ....I just smile and say never.

Thanks for the info guy's

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