Quadrajet: Engine Knock, Bad Idle Mixture Solenoid...?!

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G_RIDE_81

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Oct 30, 2019
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1981 Monte Carlo, V8, TH350, Quadrajet, smog equipment intact.

I've been having some mild knock after the engine is fully warm, during acceleration as I roll into 2nd and 3rd gear then fades away as the gear progresses. It also knocks, a bit louder, if I try to pass so ive been driving like an old lady lately ha ha! Currently running 91 octane because knock is worse with 87. I have been suspecting the idle mixture solenoid might be adjusted wrong. I recently saw a video where someone had a similar problem. He disconnected the solenoid pigtail and his lean knocking went away. I decided to try the same with similar results, no knock no matter how much I mashed on the pedal. Does this test make sense? Is this a logical way to test for a wrongly adjusted or bad solenoid? Of so, does this mean my problem lies with the solenoid and not somewhere else?

I usually take my carb to a shop but they've been letting me down last couple rebuilds. I researched how to open up the carb and adjust it with that "special tool." My wrenching skills are better than they were few years ago. Feel confident I can make this repair myself if this is my problem.

Thanks in advance for any feedback!!
 

69hurstolds

Geezer
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Jan 2, 2006
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Disconnecting the solenoid will make your car run pig rich, which eventually will kill your cat converter. The dwell cycle of the jets are what meters your fuel mixture properly for the CCC system to maintain 14.7:1 air/fuel mixture (as close as possible anyway) under normal circumstances depending on the feedback it gets from the O2, TPS, and other sensors. Unplugging it will likely throw a code and end up smoking up the joint with unburnt fuel. Just not a good thing overall for your car.

Easy way to tell if there's a potential problem with your M/C solenoid. Just do an ohm check across the terminals of the M/C solenoid. Should be above 10 ohms or it could be a problem. Check this post out on how to do it.

https://gbodyforum.com/threads/1985...estoration-refurbish.84411/page-2#post-891312

monte-ss-air-horn-mcs-resistance-check-jpg.199486
 
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69hurstolds

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Jan 2, 2006
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I can't stress enough that with the CCC system carbs, you need to make SURE you get the settings of these things exact as possible otherwise it will run like crap and may cause other problems. They're emissions carbs, not performance carbs. You can't just guess and "close enough" doesn't usually work. Unfortunately, this means having the proper parts and tools and knowledge to use them. But not everyone has these, and wonder if it's worth finding them for something you may do once. Ensure you know how adjusting one thing affects the other so you know what to expect.

If you end up replacing your M/C solenoid and need to remove the carb top, if you have a rich stop screw, first MEASURE the travel of the main metering rods before removing the top. It should be 1/8" total travel. If it is, then when you do take the top off, you can leave the rich stop adjustment alone. You should always want to remove it and the access plug in the top section to readjust it properly. But it can get you close so if you use the counting turns method below, you can at least get a total 1/8" travel.

TIP: Make sure to manually press down on the M/C solenoid before moving/adjusting the height adjusting screw. You run the risk of destroying those very fine threads in the carb if you try to do it under spring load. If you don't have the M/C solenoid setting tool (1.304" spacer, essentially) you can get it very close by manually pressing the M/C solenoid down fully, and screwing the adjuster screw all the way in counting the turns as you go. Then, when you put a new solenoid in, you can press the solenoid all the way down, screw the adjusting screw all the way down, then turn it out the number of turns from before. That should get you as close as you can without tools. Not the best method, only the best guess. Then measure metering rod travel again. If you get 1/8", then you should be in pretty good shape.
 
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79 USA 1

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Sep 2, 2011
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While you are checking the mixture control solenoid also check your throttle position sensor. 3 pin is 12 power on top, sensor feed in center and ground on bottom. Disconnected the sensor and used a ohm meter between the top and second position pin position to find I had a large dead spot as the throttle blades were opened. On my 88 Monte Carlo SS the dead spot would cause the ECM to throw full advance into the distributor and it would ping no matter what fuel I ran.
Just a suggestion.
 
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G_RIDE_81

Apprentice
Oct 30, 2019
88
19
8
Disconnecting the solenoid will make your car run pig rich, which eventually will kill your cat converter. The dwell cycle of the jets are what meters your fuel mixture properly for the CCC system to maintain 14.7:1 air/fuel mixture (as close as possible anyway) under normal circumstances depending on the feedback it gets from the O2, TPS, and other sensors. Unplugging it will likely throw a code and end up smoking up the joint with unburnt fuel. Just not a good thing overall for your car.

Easy way to tell if there's a potential problem with your M/C solenoid. Just do an ohm check across the terminals of the M/C solenoid. Should be above 10 ohms or it could be a problem. Check this post out on how to do it.

https://gbodyforum.com/threads/1985...estoration-refurbish.84411/page-2#post-891312

monte-ss-air-horn-mcs-resistance-check-jpg.199486
I only ran it like this about 2 miles for test run. Ok, I'll check the ohms, thanks!
 

G_RIDE_81

Apprentice
Oct 30, 2019
88
19
8
I can't stress enough that with the CCC system carbs, you need to make SURE you get the settings of these things exact as possible otherwise it will run like crap and may cause other problems. They're emissions carbs, not performance carbs. You can't just guess and "close enough" doesn't usually work. Unfortunately, this means having the proper parts and tools and knowledge to use them. But not everyone has these, and wonder if it's worth finding them for something you may do once. Ensure you know how adjusting one thing affects the other so you know what to expect.

If you end up replacing your M/C solenoid and need to remove the carb top, if you have a rich stop screw, first MEASURE the travel of the main metering rods before removing the top. It should be 1/8" total travel. If it is, then when you do take the top off, you can leave the rich stop adjustment alone. You should always want to remove it and the access plug in the top section to readjust it properly. But it can get you close so if you use the counting turns method below, you can at least get a total 1/8" travel.

TIP: Make sure to manually press down on the M/C solenoid before moving/adjusting the height adjusting screw. You run the risk of destroying those very fine threads in the carb if you try to do it under spring load. If you don't have the M/C solenoid setting tool (1.304" spacer, essentially) you can get it very close by manually pressing the M/C solenoid down fully, and screwing the adjuster screw all the way in counting the turns as you go. Then, when you put a new solenoid in, you can press the solenoid all the way down, screw the adjusting screw all the way down, then turn it out the number of turns from before. That should get you as close as you can without tools. Not the best method, only the best guess. Then measure metering rod travel again. If you get 1/8", then you should be in pretty good shape.
Yeah, my recent research says they're very sensitive to adjustments. It'll be a few days before I get to work on this again. And thanks for the tip!!
 

G_RIDE_81

Apprentice
Oct 30, 2019
88
19
8
While you are checking the mixture control solenoid also check your throttle position sensor. 3 pin is 12 power on top, sensor feed in center and ground on bottom. Disconnected the sensor and used a ohm meter between the top and second position pin position to find I had a large dead spot as the throttle blades were opened. On my 88 Monte Carlo SS the dead spot would cause the ECM to throw full advance into the distributor and it would ping no matter what fuel I ran.
Just a suggestion.
Oh, I haven't read this anywhere. So, the reading should climb or drop as I open the throttle? Your problem was the reading zero'd out somewhere in the middle?
 
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Clone TIE Pilot

Comic Book Super Hero
Aug 14, 2011
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Galaxy far far away
Its a lot easier to work on CCC systems with either a analog dwell meter or a old OBD1 scanner. If its just spark knock you likely have a vacuum leak leaning out the fuel mixture. Disconnecting the M/C solenoid causes the fuel mixture to go full rich which can offset a vacuum leak. The off position for the M/C solenoid is rich, the on position is lean. Finding the vacuum leak is the hard part.
 
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