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On my 1973 stock L48 350ci I have a Quadrajet with a TH400 trans. It runs great until I punch it and instead of accelerating it just sits there making this crazy hollow bogging sound. I know 90% of all fuel problems are electrical so I changed out the plugs, points, cap, and rotor. Then I dialed in the timing and took it for a test drive. No difference. It’s got an electric choke and I read if this is out of adjustment it can prevent the air valve for the secondaries to open causing bogging. Changing that made no difference. The choke damper is working. I disconnected and plugged all of the vacuum lines except the vacuum advance. No difference. There’s so many linkage arms on the choke side of the carb I have no clue if they need adjusting or bending but I’m thinking the air valve is not opening. Thanks friends!!!
You mentioned you dialed in the timing, to what? Initial? Total? At what RPM? What does your vacuum advance look like?
When you wing the throttle looking down the primary bores do you see a shot of fuel? If not then you have a bad accelerator pump. This can cause a lean bog.
Your choke not opening doesn’t prevent the air valve from moving, but the secondary throttle plates themselves. When the car is warmed up with the choke off is this lever moved out of the way of the throttle shaft?
Finally your secondary air valve pulloff. Is it holding vacuum? And when you let off the vacuum does it take a few seconds to fully extend? A lot of new pulloffs being sold today let off too quickly, which causes a lean stumble as you go into the secondaries.
If none of that works, check your fuel pressure.
Last edited by Piersonpie; Apr 22, 2025 at 02:08 PM.
Finally your secondary air valve pulloff. Is it holding vacuum? And when you let off the vacuum does it take a few seconds to fully extend? A lot of new pulloffs being sold today let off too quickly, which causes a lean stumble as you go into the secondaries.
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Even if your carb is completely rebuilt, the only place you can get this correct part is from Lars or Cliff Ruggles.
ALL the aftermarket "pulloffs" open too fast, and cause a bad bog.
Blue arrow is called the choke pull-off, but it also slows the secondary air valve opening.
You can test it easily. Apply full vacuum where the short rubber hose is. and let it go. It should take ~1-2 seconds to fully extend. Not instant. Instant = bog.
From: At my Bar drinking and wrenching in Lafayette Colorado
The 1973 did not come with an integral choke carb that can be converted to electric, so the first queastion is, what type/year carb do you have? Is this a commercially rebuilt late '70s carb converted to electric? Post the carb number and a couple of photos so I can see what you're doing...
Lars
You mentioned you dialed in the timing, to what? Initial? Total? At what RPM? What does your vacuum advance look like?
When you wing the throttle looking down the primary bores do you see a shot of fuel? If not then you have a bad accelerator pump. This can cause a lean bog.
Your choke not opening doesn’t prevent the air valve from moving, but the secondary throttle plates themselves. When the car is warmed up with the choke off is this lever moved out of the way of the throttle shaft?
Finally your secondary air valve pulloff. Is it holding vacuum? And when you let off the vacuum does it take a few seconds to fully extend? A lot of new pulloffs being sold today let off too quickly, which causes a lean stumble as you go into the secondaries.
If none of that works, check your fuel pressure.
Thanks Piersonpie. I looked closely at that area you highlighted and there is no lever, pin, etc that touches the secondary shaft on the outside of the carb. That may because upon closer inspection this carb is made by Jet. I don't know if they take Rochester carbs and rebuild them or if this is some kind of cheap clone. But from the photos I've seen it has the same adjustments as the Rochesters. I adjusted the air valve spring by loosening the allen screw and putting a tiny amount of spring tension on the valve then gently tightened the allen screw. It seems to have made the bogging less but it still bogs. So next is the dashpot adjustment. My dashpot takes about 2 seconds to go back to its "home" position after pushing the rod in. Don't know if thats okay or not. There's a screw that's part of the dashpot/air valve rod linkage. If I screw it CW it shortens the linkage and if I screw it CCW it lengthens it. I have no idea what the initial setting is supposed to be. I'll try to put it midway and see what happens. Again, thanks for the information!!
Thanks Lars. It turns out it has a Jet carb that's either a rebuilt Rochester or a clone made to look like a Quadrajet. I'll try to post a photo. But it does have the same types of adjustments as the Quadrajet: choke, air valve, mixture, dashpot, etc.
From: At my Bar drinking and wrenching in Lafayette Colorado
So you have a hacked, commercially rebuilt carb... that opens a can of worms for all the things that will be wrong with it. E-mail me for my paper that outlines a list of all the issues your carb will have that will cause the problem you are having. If your carb does not have the pin and lever shown by Piersonpie it is a good indication that it has been pieced together with a non-Chevy throttle plate (Pontiac carbs do not have the pin). The fact that you have an adjustment screw on your choke pulloff lever arm indicates that you have an M4M carb (integral choke) instead of the correct 4MV carb for your car. That adjustment is the choke pulloff adjustment that cracks the choke open upon engine start. I suggest you don't continue to make random adjustments and turning screws without knowing what they are or what they do... not likely to make the issue any better... choke pulloff adjustment has nothing to do with a bog going into the secondaries. If you have it set at the "halfway point" it is grossly out of adjustment, which will cause you cold-start & cold-run problems, but not affect WOT performance.
Bogging
