So I've got a 72 350 all stock that I converted to a quadrajet from the 2bbl and also changed the distributor from points to a HEI from a 79 Olds. I am currently experiencing a hesitation or flat bottom out under wide open throttle. Here is what I've checked, replaced, and tested. The distributor has a new pickup coil, module and coil. New cap, rotor and wires. New plugs gapped at .040. Quadrajet was rebuilt. All new vacuum lines. The engine has been resealed along with new timing chain and gears as well. Multiple adjustments on the carb getting the choke and fuel adjustments in the right spot which I believe is there. One pump to set the choke and it'll start right up. Initial timing set at 12 BTDC and 34 all in with vacuum advance disconnected and plugged off. I do have the vacuum running through the Distributor vacuum control switch. Vacuum gauge is sitting at 15 steady. I do feel as if I had to set the curb idle a bit high but otherwise it runs good however not really what I was expecting going from 2bbl to 4bbl. When I go to stomp on it though is hesitates and at times will get a quick backfire. Also at times when I shut off the vehicle it will run on for a couple seconds. Also note that when we first installed the motor I had lost air through the vents, was only coming through the floor (not sure this is relevant). Through multiple adjustments I have since gained back proper air ventilation through the vents. Some of this may not be meaningful but I thought I'd throw it all out there for suggestions.

 

Blowing through the floor vents is a classic signature of loss of vacuum, most likely from the vacuum line disconnected from the vacuum reservoir ball on the firewall.

You stated a lot of adjustments but none about the carburetor secondary air door tension. That is critical for proper WOT transition.

Also need to verify the accelerator pump operation, which is also critical for WOT transition.

Did you replace the throttle cable and bracket with the correct one for a QuadraJet?

15” vacuum for a stock ‘72 is low - should be closer to 18”. Maybe more tuning is needed.

 

Buy a wideband O2 and take the guesswork out of diagnosing it.

 

The wideband is a great idea, it could be way out to lunch. As said, check the secondary air door adjustment. Every Qjet I have touched, the door basically had no tension. Another issue may be the choke pull off releases too quickly. Most replacement release way too fast. Cliff Ruggles sells ones he actually adds a restriction to. Also, try another 4 to 6 degrees of base timing. I bet that 8 to 1 350 will like it.

 

Thank you all for the ideas. The secondaries do seem to have proper tension and I can get them to open. When the engine was warm I did check for fuel and I did see squirts of fuel in the primaries when pressing the throttle so I have to think the accelerator pump is working. I did replace the throttle cable with the correct one for the 4 bbl. It sounds like I will double check all vacuum, inspect for leaks and try to advance the timing more. I honestly didn't go any further with the advancement just because it seemed like the idle smoothed out so I figured it was good. I will take it further and see how she likes it. I'll keep you posted.

 

Just wondering how you came to that conclusion. Mine was fine for years, then after an intake manifold change I had to do a LOT of WOT runs in order to get it adjusted correctly afterwards.

 

Maybe I should have phrased it differently. Upon inspection I can get the secondaries to open. They are not "flopping" around like I see in many other posts. Visually they appear to check out. So I can understand you correctly, did you have to physically continue to make adjustments or do you mean they continued to adjust during your WOT situations?

 

I performed multiple standing start WOT launches and made incremental adjustments until there was no hesitation or bogging. Just a small adjustment of the tension screw makes a noticeable change in operation. I made changes of around 1/16 of a turn of the screw at a time in between the WOT launches.

 

You can't visually "check out" the secondary air valve spring tension.

X 2 Fun71 is giving you proper guidance.

 

I am suffering the same problem. Fun71, did you start with the secondary air flap "tightened up," and worked it looser with your testing? Or the other way around?

 

Thats from not enough tension on the spring for the secondary air valve. The stumble is from the "door" with not enough tension.

 

Is the hesitation from a dead stop and/or rolling? Will it maintain a steady pull to full throttle if you don't slam the throttle? Can it maintain high rpm for an extended period without running out of fuel?

Is the float level in the rebuilt carb definitely correct?

 

I think it's easier to start tight and loosen as you test.

 

So I had a little time this morning to get into it. I was able to get initial timing set at 22. Vacuum increased to 16 and hesitation is gone along with backfiring. It runs good however I agree with Fun71 that I have to adjust the secondary flaps as I am just not too sure I have maximized its potential. Also thinking that I need to put a limiter on the vacuum advance as I have read others have done. Will get back at it and keep you all posted. Thanks for the help!

 

Thats good news.

 

Went back at it and made some final adjustments. The secondary flap tension seems to be where it needs to be as I am not getting anymore hesitation and when I open it up it goes. We'll keep an eye on it moving forward as now I know what to adjust if I need to. I fabricated a vacuum advance limiter and brought the initial timing back to 20. I was able to adjust the mixture screws and got the vacuum up to 18. I was also able to bring down the curb idle to about 800 and it seems to like that. No more run on/dieseling after I turn it off. 1 small issue is that I need to fine tune the choke and fast idle. I haven't been able to get that high idle to adjust and on a cold start it will chug along for about 30-45 seconds until it comes up enough in temperature so this is something that I need to work on next. Glad that I got the driving condition handled. I may also consider switching the vacuum to manifold just to see if I can pull even a little bit more than the ported through the distributor vacuum switch. Thank you everyone for the help. It's been a long time since I've worked this deep on my own car and it's great to know that with time and patience you can figure things out on your own.

 

If the cold idle speed is set to spec and it chugs the choke pull off probably isn't opening enough, check its adjustment.
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