1975 carburetor question.
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Pro
Joined: Aug 2013
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From: Cincinnati Ohio
1975 carburetor question.
I have a 1975 L-82 4 speed. If I set the idle with the A/C off, when I turn the A/C on it is too slow and wants to stall. If I set the idle with the A/C on, when I turn the A/C off it is too fast. I think I may be missing a "dashpot" or some kind of idle solenoid or something that allows setting the idle with the A/C off and steps the idle up when A/C is on. Does anyone know anything about this??? Thanks
Melting Slicks
Joined: Apr 2009
Posts: 2,632
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From: Great Plains Iowa
I have a 1975 L-82 4 speed. If I set the idle with the A/C off, when I turn the A/C on it is too slow and wants to stall. If I set the idle with the A/C on, when I turn the A/C off it is too fast. I think I may be missing a "dashpot" or some kind of idle solenoid or something that allows setting the idle with the A/C off and steps the idle up when A/C is on. Does anyone know anything about this??? Thanks
Yep, it requires a fast idle solenoid like this one on my 454" with A/C. This solenoid came off an old '68 Firebird and I made a bracket to mount it. After you get one installed you set the idle speed without the A/C on then turn on the A/C and adjust the solenoid to give the same idle rpm.
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Joined: Aug 2013
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From: Cincinnati Ohio
It is electric .
Very common for the solenoid and the bracket to be missing off running cars. People also often clipped the tab/ or brownish wire too.
Your factory bracket was same color as carb , it clipped on the outer Top edge edge of your Q jet and was secured with ONE kinda special looking little screw. I think an original solenoid bracket screw is kinda tuff to find. But you can make something work.
I pretty sure the solenoid is reproduced after market now. About 30.00 ......a good original NOS might be 150.00 on eBay . I think DR Rebuild corvette supplier has them.
The bracket is harder to locate. I see them still attached to old QJet carbs on eBay from time to tine. Depending on who you buy the part from, it could be cheaper to buy a ruff old QJet that still has the bracket on the carb. Corvette specialist shops that deal in original parts would want 50.00 I bet for the bracket . You might buy a dead QJet carb for 15.00 off eBay and Robb the bracket.
You need a factory wiring diagram ....to see if the wires are all still there.in the circuit . DR Rebuild has nice a BIG easy to see ones ...probably a factory assembly Manuel might be of help.
Circuit pulls the solenoid in when the compressor is engaged. Bumps up the rpm a bit. The brass screw or plunger of the solenoid turns in or out to adjust your rpm assist.
That usually us a dark tan wire with a clip running to the solenoid . Clipped to the top of the valve cover. If you still have the black conduit look in there and see if the wire has been clipped . If so, just add a piece of same gauge wire on with a wire crimp connector .
Very common for the solenoid and the bracket to be missing off running cars. People also often clipped the tab/ or brownish wire too.
Your factory bracket was same color as carb , it clipped on the outer Top edge edge of your Q jet and was secured with ONE kinda special looking little screw. I think an original solenoid bracket screw is kinda tuff to find. But you can make something work.
I pretty sure the solenoid is reproduced after market now. About 30.00 ......a good original NOS might be 150.00 on eBay . I think DR Rebuild corvette supplier has them.
The bracket is harder to locate. I see them still attached to old QJet carbs on eBay from time to tine. Depending on who you buy the part from, it could be cheaper to buy a ruff old QJet that still has the bracket on the carb. Corvette specialist shops that deal in original parts would want 50.00 I bet for the bracket . You might buy a dead QJet carb for 15.00 off eBay and Robb the bracket.
You need a factory wiring diagram ....to see if the wires are all still there.in the circuit . DR Rebuild has nice a BIG easy to see ones ...probably a factory assembly Manuel might be of help.
Circuit pulls the solenoid in when the compressor is engaged. Bumps up the rpm a bit. The brass screw or plunger of the solenoid turns in or out to adjust your rpm assist.
That usually us a dark tan wire with a clip running to the solenoid . Clipped to the top of the valve cover. If you still have the black conduit look in there and see if the wire has been clipped . If so, just add a piece of same gauge wire on with a wire crimp connector .
Where does the dark tan wire to the idle solenoid go to? I seem to have no evidence of an idle solenoid ever being there. The mounting holes for the solenoid bracket on the carburetor seem to not be tapped. I'm 99% sure it is the original carburetor. Are the screws to mount the bracket self tapping? If so, could mine have never had one in the first place?
Melting Slicks
Joined: Apr 2009
Posts: 2,632
Likes: 112
From: Great Plains Iowa
Yes its electric and it's wire is teed into the A/C clutch coil wire. As my '71 is far from being factory original I didn't care that the fast idle parts aren't original. I'm just interested in making my '71 a nice running car.
Instructor
Joined: Mar 2013
Posts: 180
Likes: 21
From: Chesterfield Mo
I have a 1975 L-82 4 speed. If I set the idle with the A/C off, when I turn the A/C on it is too slow and wants to stall. If I set the idle with the A/C on, when I turn the A/C off it is too fast. I think I may be missing a "dashpot" or some kind of idle solenoid or something that allows setting the idle with the A/C off and steps the idle up when A/C is on. Does anyone know anything about this??? Thanks
Melting Slicks
Joined: Jul 2009
Posts: 2,091
Likes: 40
Where does the dark tan wire to the idle solenoid go to? I seem to have no evidence of an idle solenoid ever being there. The mounting holes for the solenoid bracket on the carburetor seem to not be tapped. I'm 99% sure it is the original carburetor. Are the screws to mount the bracket self tapping? If so, could mine have never had one in the first place?
See next post
Last edited by LS4 PILOT; Jun 1, 2015 at 11:28 PM.
Melting Slicks
Joined: Jul 2009
Posts: 2,091
Likes: 40
Ok.....I checked my Corvette fact book.
I was mistaken .
The pics of the mint 75 engine , show no carb solenoid , it appears the top of the carb housing changed from 73-74 QJet configuration . I don't see the boss attachment points like on mine.
So......it appears this item was dropped when they redesigned the ...new electric choke version carb for 75 and up
I guess you'd have to make up your own bracket ,etc, and wire it in to the compressor circuit is all I can tell you......I suppose people just lived with the issue back then.
I was mistaken .
The pics of the mint 75 engine , show no carb solenoid , it appears the top of the carb housing changed from 73-74 QJet configuration . I don't see the boss attachment points like on mine.
So......it appears this item was dropped when they redesigned the ...new electric choke version carb for 75 and up
I guess you'd have to make up your own bracket ,etc, and wire it in to the compressor circuit is all I can tell you......I suppose people just lived with the issue back then.
Last edited by LS4 PILOT; Jun 1, 2015 at 11:24 PM.
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Thread Starter
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Joined: Aug 2013
Posts: 633
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From: Cincinnati Ohio
The 1975 supplement shop manual shows the solenoid in the Corvette engine wiring diagram. I think I may put one on even if it shouldn't have one. I don't like the difference in idle speeds without it. I don't want to run a wire directly from the solenoid to the compressor. Is there a way to tap in on the drivers side or do I have to go under the console directly to the control switch? Also what color wire to tap in to?
Thread Starter
Pro
Joined: Aug 2013
Posts: 633
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From: Cincinnati Ohio
The 1975 shop supplement shows a solenoid in the Corvette wiring diagram. I may put one on even it is not supposed to have one because I don't like the difference in idle speeds. I don't want to run a wire directly to the compressor. Is there a wire I can tap in to on the drivers side or do I have to go inside the cabin to where??? The control switch??? What color wire do I want to tap in to??? Thanks
Team Owner
Joined: May 2005
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From: Huntsville AL
How much of an RPM drop are you seeing?
I had a 73 base motor. A/C car that did the same thing. Turn the A/C on and it actually stalled the engine. It was about a 500 RPM drop. I had the idle bumped up to keep the car running when the A/C was on.
It was something like 1200 without A/C, 700 with A/C.
I played around with it for a while, then figured out that the A/C compressor had way too much drag on it.
I got a rebuild A/C compressor, new dryer, etc.
That fixed the problem. I was now seeing a 200 RPM drop with the A/C on.
I had a 73 base motor. A/C car that did the same thing. Turn the A/C on and it actually stalled the engine. It was about a 500 RPM drop. I had the idle bumped up to keep the car running when the A/C was on.
It was something like 1200 without A/C, 700 with A/C.
I played around with it for a while, then figured out that the A/C compressor had way too much drag on it.
I got a rebuild A/C compressor, new dryer, etc.
That fixed the problem. I was now seeing a 200 RPM drop with the A/C on.
Thread Starter
Pro
Joined: Aug 2013
Posts: 633
Likes: 36
From: Cincinnati Ohio
How much of an RPM drop are you seeing?
I had a 73 base motor. A/C car that did the same thing. Turn the A/C on and it actually stalled the engine. It was about a 500 RPM drop. I had the idle bumped up to keep the car running when the A/C was on.
It was something like 1200 without A/C, 700 with A/C.
I played around with it for a while, then figured out that the A/C compressor had way too much drag on it.
I got a rebuild A/C compressor, new dryer, etc.
That fixed the problem. I was now seeing a 200 RPM drop with the A/C on.
I had a 73 base motor. A/C car that did the same thing. Turn the A/C on and it actually stalled the engine. It was about a 500 RPM drop. I had the idle bumped up to keep the car running when the A/C was on.
It was something like 1200 without A/C, 700 with A/C.
I played around with it for a while, then figured out that the A/C compressor had way too much drag on it.
I got a rebuild A/C compressor, new dryer, etc.
That fixed the problem. I was now seeing a 200 RPM drop with the A/C on.
Last edited by 409/409; Jun 4, 2015 at 08:31 AM.
Thread Starter
Pro
Joined: Aug 2013
Posts: 633
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From: Cincinnati Ohio
Thanks Jeff. It now idles at a rock solid 750 rpm with A/C on or off.
