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Question: 81 with quadrajet bogging

P

pdelo

Guest
Hi all, new member here, longtime gearhead but 1st time vette owner. I recently purchased an '81 with a rebuilt 350 with 9.0 compression (supposedly). It has a Comp Cams 260 flat tappet cam with roller rockers, an Edelbrock Performer intake and the original electronic Quadrajet and HEI distributor. The carb was supposedly rebuilt by a Quadrajet expert(?) and the distributor was reworked also. The car starts beautifully, idles well and runs fantastic on the primaries, but if you punch the accelerator the engine bogs massively and sometimes back fires through the carb. I checked the timing, its at 12 initial and the distributor advance is working as the engine revs though I didn't determine was the full advance was. I know you have to disconnect the computer to reset the timing but the old Chilton's manual I have had me looking for a plug to disconnect that I couldn't find, so I left it alone. On the carb, the air doors seem to have the right amount of tension so I left them alone and everything else seems right. I remember something about secondary feed tubes falling out or something like that from back in the day but I'm not sure. I know most people will tell me to ditch both but I would like to see if I can get this setup running well and then go from there. I'd appreciate any ideas anyone might have, thanks
 

Antz81

Well-known member
Joined
Dec 21, 2013
Messages
936
Location
Auckland, New Zealand
Corvette
1981 4 speed
The plug to disconnect should be a four pin plug. If you look at the rear of the distributor you should see four wires coming out which go to the plug.

The tubes you are talking about can fall out, but most of the time they go unnoticed, until the air horn is removed.
 

Tom Bryant

Well-known member
Administrator
Joined
Nov 9, 2000
Messages
7,279
Location
Edgerton, Ohio, United States
Corvette
1959 black 270hp (9/2/69) 1981 Beige L81(10/20/80)
Welcome to the CAC

Usually for a Q Jet bog I go straight to the secondary air valve tension but you say it seems good. I'd unplug your distributor as Antz81 said and check your timing first. If you want to double check your air valve wrap you will need a small flat tip screwdriver and an Allen wrench. On the end of the passenger side of the carb you have access to the air valve tension spring adjustment by inserting the screwdriver into the end of the air valve shaft hole in the carb housing. Directly under the shaft hole is a small Allen set screw that goes in vertically from the bottom. Loosen the set screw and the spring tension will be released. Insert the screwdriver and turn the shaft counter clockwise until the air valve just closes. Then continue 1/4 turn more and lock it down with the set screw. This is a good basic setting that will run well without bogging, if the secondary air valve was the problem.

Tom
 
P

pdelo

Guest
Re: Quadrajet bogging

Thanks for the suggestions. I saw the four wire connector but the manual I have( which is going back on the shelf) says there is a single connector down past the four wire connector which doesn't exist on my car. So I will definitely check out the timing the correct way. As to the carb, I talked to the previous owner and he said that the float bowl was replaced as it was porous. After doing some research I read that sometimes when carb bodies are mixed and matched, the secondary fuel passages do not seal properly when the carb is reassembled, preventing secondary fuel flow, and causing a huge bog and backfires through the carb which is exactly what's happening. According to the previous owner, this condition existed as soon as he put on the carb and could not be fixed by tuning. This sounds like this could be the problem and I'll need to take off the airhorn to check. Has anyone else heard of this or had this occur in their carb and if so, how do you fix it short of junking the carb? This is something I've never heard of, I'm going to check it out but I'm up for suggestions on how to proceed from there. Everything else seems to be in good order so I'm kind of leaning toward that being the problem.
 

Peer81

Well-known member
Joined
May 21, 2003
Messages
2,497
Location
Netherlands
Corvette
'81 Black
Normally the problem starts because the second air valve opens but fuel isn't triggered to flow. You get a lean condition. Normally the easy fix is to make two half cirkel openings in the upper air valve toward the fuel openings. You can find this fix in the Cliff and Doug quadrajet books :)

Greetings Peter
 

dougelam

Well-known member
Joined
Jul 19, 2011
Messages
453
Location
Michigan
Corvette
2002 Roadster
My guess is the old float bowl is gone so there is no way to know if the replacement one is correct!
If you can't get the problem resolved are you able to tear into it? It takes a couple of special tools, just ask ant what they are

Sent from my SM-T530NU using Tapatalk
 

Peer81

Well-known member
Joined
May 21, 2003
Messages
2,497
Location
Netherlands
Corvette
'81 Black
You can take off the airhorn without special tools. To set it up correctly you need them. As with every quadrajet you can make alot of adjustments like tailer make the accelerator pumpshot. :)

Greetings Peter
 

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