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Topic: Idle suddenly dropped from 1100 to 500 rpm

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Idle suddenly dropped from 1100 to 500 rpm (1/6)
 8/7/22 3:15pm
Christian
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Houston, TX - USA

Vette(s):
1971 LS5 Convertible 4 sp


Joined: 2/25/2003
Posts: 193

The idle speed dropped from 1100 to 5 or 600 rpm, barely holding it. Not progressively, no; suddenly. From one traffic light to the next. I checked all connections between spark plugs and distributor. Everything looks fine.

In order to drive the car, I re-adjusted the idle to 1100 rpm with the idle screw at the carb but I know I am just compensating for / hiding a problem.

I checked the vacuum line between the carb and the distributor. When disconnecting it (and plugging it with my thumb), the idle speed drops a little, which tells me the canister works.

At this point, I am puzzled. Any suggestion?



|UPDATED|8/7/2022 1:15:11 PM (AZT)|/UPDATED|
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Re: Idle suddenly dropped from 1100 to 500 rpm (2/6)
 8/7/22 4:19pm
manchestersharkLifetime Member
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Equinunk, PA - USA

Vette(s):
1972 conv, 4-speed, 350, 200hp, numbers match, rally wheels, war bonnett yellow w/white top. good condition, nice driver.


Joined: 10/31/2007
Posts: 2129

5 might be a tad low. 7 would be nice. 11?? Quite high. Why? 



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Re: Idle suddenly dropped from 1100 to 500 rpm (3/6)
 8/7/22 5:52pm
F4Gary
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Grapevine, TX - USA

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1972 LT-1 convertible with factory air.


Joined: 8/26/2006
Posts: 1159

Spark plug wire, different vacuum hose, something in the distributor/condenser.



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Re: Idle suddenly dropped from 1100 to 500 rpm (4/6)
 8/7/22 6:45pm
Christian
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Houston, TX - USA

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1971 LS5 Convertible 4 sp


Joined: 2/25/2003
Posts: 193

1100 rpm is the manufacturer specification for this engine. I know it sounds high, but this is what it is. And 700 does not sound right, believe me.

Re: Idle suddenly dropped from 1100 to 500 rpm (5/6)
 8/7/22 8:10pm
Adams' AppleLifetime Member
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Duncanville, TX - USA

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#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: 19938

Check ALL of the vacuum lines coming of the engine first...not just the one to the dist...which wouldn't have that much effect on the idle speed anyway. Holley or Quadrajet carb? Possible the throttle blade shaft(s) wore, and the thing just closed more than normal. Happens more than you think, even on fuel injected engines. 

Did anything happen right before this started? Like a backfire?



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Re: Idle suddenly dropped from 1100 to 500 rpm (6/6)
 8/8/22 10:49am
VetteCountryLifetime Member
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Belvidere, IL - USA

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1977 Black T-Top ZZ4 5Sp 16in Wheels


Joined: 10/24/2004
Posts: 423

If all checks out, check timing. Check balancer condition.

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