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Topic: Carburator butterflys not moving correctly high engine RPM on start up

in Forum: C3 Engines


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Carburator butterflys not moving correctly high engine RPM on start up (1/3)
 8/2/23 6:00pm
Babyblue 78
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Vette(s):
1978 Corvette Light Blue Pulled from the swamps of Miami! Its a driver and back on the road!


Joined: 12/3/2007
Posts: 280

Ok I know I will not have the technical terms for this problem but I have enclosed a picture of what my mechanic (not a corvette guy) thinks is the problem.When I start the engine it wants to cough a couple of times and then it wants to rev to over 4500rpm's  and I then turn off the ignition. (Scares the crap out of me like it will blow up!)  The picture of the round spring facing  directly out with several circles of wire and what looks like a flat screw head in the center.  This is what he thinks is causing it. The spring seems to be worn and it was just laying on the intake when I took it to him. 

I do not know the name but I can't have it racing to high rpm's and it never did this before. I thought I was pumping it too much on starting but the throttle linkage all looks good and correct. Any help in this matter or at least a name for this part and what or how it affects the carburetor would be greatly appreciated.



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Re: Carburator butterflys not moving correctly high engine RPM on star (2/3)
 8/2/23 9:48pm
Adams' AppleLifetime 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: 20122

That spring is to keep the secondary blades closed until the primary linkage "kicks" the secondaries open. If the spring is not on properly, or broken, the secondary throttle shaft will be free to stay open a small amount and cause the high idle. I would suggest the carb be removed and a new spring installed. The spring needs to hold tension on the linkage to keep the throttle blades closed. If the spring was just laying there un-attached, I would suspect it is broken. The pics are small, and my eyeholes are old, so I can't really see clearly what has happened. You should be able to manually push the secondary throttle shaft/blades closed, and they will normally stay closed...until you hit the throttle hard enough to open the secondaries again. 

This should give you(and/or your mechanic) and idea of how the spring is supposed to be installed. If yours is broken, it will need to be replaced. 

Quadrajet secondary return spring

...and a link to A new replacement spring if needed.




|UPDATED|8/2/2023 7:48:43 PM (AZT)|/UPDATED|


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Joel Adams
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Re: Carburator butterflys not moving correctly high engine RPM on star (3/3)
 8/9/23 12:59pm
aceintheholeLifetime Member
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Manteca, CA - USA

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1978,two tone,Metalic Rootbeer & gold 1975 L48 4 speed


Joined: 11/20/2005
Posts: 3623

The throttle return springs would certainly create this issue if not connected. Looks good in the pic though. If that were the issue it would immediately correct itself when they were reconnected. 



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Jimmy B.
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