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My 1979 (Early, October 1978) 4-speed pretty much stock L48 and quadrajet has been bogging down on acceleration from idle. It's especially annoying when trying to do an uphill start in traffic with a bus inches behind my rear bumper, since my parking brake is... well, 45 years old. I had the full pump replaced recently as it was leaking, but that did not help. I also just changed the distributor and adjusted it to 36deg at 3k RPM with vacuum line disconnected, and the engine runs quite smoothly now except for the initial bogging. My cold starts were a disaster but I think I fixed that by replacing a cracked choke coil cap and adjusting it properly, and also adjusting the new distributor helped. Also changed the fuel filter in the carb and installed a pre-formed steel line from pump to carb to replace the old hose that had "not for fuel" written on it...
Reading this amazing forum, I learned a lot about my engine, but it looks like the range of remaining possible causes for bogging on initial acceleration is still very wide (from leaking head gasket to bad fuel filter to loose pedal or link, etc). As I was adjusting the new distributor, I noticed a hissing sound at or near the carb when I accelerate. I had not noticed that in various quadrajet videos I have watched before. When it hisses, the RPMs drops for a split second (or at least fails to pick up) and then picks up (and the vacuum drops too, measured at the brake booster port before the check valve). If I accelerate very slowly, the RPMs go up smoothly.
My 77 came with a quadrajet. Always stumbled off idle. Had never heard of Lars back in those years.I tryed, my friends tryed, a couple so called mechanics tryed. So, after years of it stumbling off idle I replaced the intake manifold and the carburetor with a Edlebrock manifold and a Holley carb.
Problem solved!
Today, we have Lars. I would suggest you contact him. It does sound like an accelerator pump issue. But could be a number of things.
So, you can be like me. Just give up on that awful Quadrapuke. And throw it in the woods.
Or, you could contact an expert and get your carburetor properly repaired.
Your accelerator pump is not functioning. There should be a stream of fuel in each primary bore when you actuate the throttle.
sucking sound is normal.
From: At my Bar drinking and wrenching in Lafayette Colorado
Your accelerator pump is dead. It is probably one of the cheap non-ethanol-compatible pumps, and the diaphragm has either collapsed, or swollen up and sheared off the accel pump shaft. Replace the pump with a good pump. You get a good pump from Cliff.
Thanks a lot for all the comments, the accelerator pump is likely the original, it looks like something I could do. Once that's done, any benefit in fiddling with the two mixture screws at the front of the carb? I would have done it except I have a smog / A.I.R. pump that seems totally in the way of the passenger side screw...
From: At my Bar drinking and wrenching in Lafayette Colorado
"Fiddling" with any adjustment only has benefit if you know how to "fiddle" correctly, and if there is a reason to change the current adjustment and do any fiddling in the first place. Most people who engage in unnecessary fiddling tend to make things much worse than they were. If your idle mixture is not correct, you can make improvements to idle quality by correcting the out-of-spec condition. Your Service Manual has steps and procedure for adjusting the idle mixture without the use of a gas analyzer using the "lean drop" method. However, if your car and carb are all original, your idle mixture screws will have steel caps covering them, preventing any "fiddling" with their setting. If the caps have been broken out and removed, it means that someone has already done a bit of fiddling, and you may have an issue with the work having been done incorrectly.
oh, yes, I would fiddle by the book for sure. But my question was whether I should even bother: my understanding so far is that it would only affect idle quality but no effect on top end, right? New accelerator pump is on its way. Thanks!
From: At my Bar drinking and wrenching in Lafayette Colorado
Idle mixture screws have no effect whatsoever on engine operation at "top end". The idle circuit does contribute some fuel at light throttle cruise, but does not contribute to any engine operation under power, and has no effect at all to wide open throttle performance.
Great, thanks. Sorry I was going to edit my last post but you are too quick! Top end is not the issue in this thread, I should have said "adjusting the idle mixture screws will have no significant effect on initial acceleration when coming off idle"; still correct? I ordered the pump from Quadrajet Power, I also wanted an air horn gasket just in case and they had the one matching my carb's number.
From: At my Bar drinking and wrenching in Lafayette Colorado
Correct - idle mixture screw adjustment has no effect on performance when accelerating coming off idle.
You're not going to get a good accel pump from that outfit - I have tested them, and they swell up and seize in pump gas. Suggest you follow my recommendation above.
Lars
From: At my Bar drinking and wrenching in Lafayette Colorado
...and don't buy Cliff's book from Amazon. Jeff Bezos doesn't need the money. Buy it directly from Cliff for the same price so he gets the extra few bucks from the sale. He's an old retired guy like me: https://cliffshighperformance.com/pr...et-carburetors
I bought Cliff's book years ago. I don't even own a quadrajet anymore. Great book. Highly detailed.
fabulous read if your a motorhead.
No I won't sell mine.
Ok I got the parts and installed them. The old pump was not that bad, the cup was not shredded but there was a crack in the plastic disk at the wire end of the spring.
old accelerator pump. Notice the crack in the plastic disk just next to the rubber cup
But I noticed three things: 1) my air horn was pretty clean overall, except in the area near the accelerator pump:
looks like we have some dried up gas near the shaft of the accelerator pump
2) the screws retaining the air horn were not tight at all. Especially the 2 big 1/2" ones were barely more than finger tight. 3) the arbor holding the 2 primary needles popped right out even before I started working the old gasket out. I re-inserted it and it stayed in place but maybe at some point I will need to change the friction washer that holds it.
I used the technique of 76vette on YouTube to insert the new gasket (lift the front part to about 30 degrees and slide the gasket under the arbor of the primaries, then work it around the primary needles. For posterity and since I saw people having a hard time on YouTube putting the air horn back while managing the choke lever and the accelerator pump pushing the gasket out, I decided to use a 2" L-bracket I had lying around and taped it with masking tape to avoid scratching the gasket, then taped it to the carb using some strong Nashua duct tape (white tape in the photo). That was just enough to hold the accelerator pump down while I put the air horn back.
a 2" right angle bracket is holding the accelerator pump down so it won't pop the gasket up. My carb was still pretty full of gas so I got some squirting on the gasket at that time but I guess it should be ok
it was then easy to place the air horn (after good cleaning of the mating surfaces), start a few screws, and just slide the L bracket out.
once the air horn is back, just slide the L bracket out
Finally tightened everything from the center out. I had to do at least 6 rounds going very easy with the tightness each round, until all screws were at the same amount of tightness.
So far it looks promising. Because it was getting late and I did not want to disturb the neighbors I just idled a bit and gave it a few accelerations to check for leaks and it looks great, no leaks and seems that the bogging is gone. I will drive the vet to work tomorrow and will be back if I still have bogging issues.
Thanks again very much everyone for your most helpful comments!
hehe, maybe, or maybe it's a California thing. My carb definitely has a lot of ports and lots of emission-related hoses. That new gasket matches the old one exactly, every hole and every little feature are there and no extra holes. Carb number 17059213. I bought the car in 1993 knowing nothing about corvettes and the seller at the time told me it's a rebuilt LT1 engine (which I also did not know anything about, but I just liked that the car is very strong), and he indeed changed the horsepower plate near the parking brake handle and applied some LT1 decals to the hood. Now that engine looks like L48 to me (can't find a number on it) and my understanding is that an LT1 would not have a quadrajet anyway, and where would it come from, 1971 or what? on a 1979 car? I think what he did instead was convert from automatic to 4-speed manual and put a really high differential ratio. So it is a very strong puller but it does not go very fast, that combo works great for me.
driving to work today the bogging is gone, no problem doing hill starts without using the parking brake! Warm idle is a bit fast after I replaced and adjusted the distributor, will adjust that tonight.
From: At my Bar drinking and wrenching in Lafayette Colorado
Okay, so you do have a truck carb - that's not a Vette carb or passenger car carb - it's a 1979 350 Heavy Duty Truck carb. In that case, you have the right gasket for that truck carb. However, you do have the wrong float installed... that brass float is horrible, and will not "float" at the correct level. Put the correct float in the carb and set it to .300" if you want it to run right...
...and that's not an LT1 engine - your initial video with sound is clearly a hydraulic-cammed engine. The main feature of the LT1 is that it's a solid lifter engine. Tranny type and rear end ratio has nothing to do with it being an LT1.
Lars
1979 bogging and hissing on sharp acceleration
