I ditched the double pumper on my c10 for a q-jet. It's a 10:1 350 with vortec heads, isky 280 mega cam, and aluminum dual plane manifold, pretty basic combo. The carb was originally on an early 80s 305, but had richer secondary rods and a power piston spring for a "mild cam" put in when rebuilt. The off-idle and primary circuit drivability is second-to-none. Blows the holley out of the water, to the point that the cam feels smaller. Just how a 'jet should be. Additionally, it doesn't have the typical bog I'm used to sorting out. You can abruptly put your foot through the floor at virtually any rpm, and you're off without a hitch. Runs great clear to 6,000 rpm. The ONLY time I can create anything abnormal, is when applying a load more gradually. Specifically when riding the clutch to launch it on street tires, where instant wide open just results in tire smoke. Here, and only here, I get a pretty aggressive hesitation. I'll try to attach a comparison clip of the two separate conditions. Simply a secondary opening speed issue like the normal stomp bog? We are some of the most q-jet faithful of the gm family, so hopefully somebody has an idea.
https://youtube.com/shorts/v3C6l3_SE90?si=oviepi3ZWPUB3Aj2 Here is an example. 1st bit is just simply stabbing it, with the obvious ensuing spin. 2nd bit is rolling into it, and met with the bog.
Saw the vid, no prob. The q-jet has two big players for secondary function. One is the spring tension on the air valves, the "flaps", but I think you need to look at the secondary vacuum break. Once upon a time the orifice on the vacuum break was plastic, and the diameter of the vac orifice could be changed with a drill measured in thousands of an inch. Enlarging it makes for a quicker hit. BUT over-doing it is bad. Larry's probably the go to guy on this for street-strip, and we have some Quadrajet folks who know their stuff here too. Devon
I am just guessing, but it sounds to me like the carb is working super well overall? I would be careful not to make too many changes. loosening/tightening the secondary spring tension a tad as Devon describes might be a good idea as a test? I also wonder if this might be a situation where your vacuum advance is going a way too soon? Sound like low RPM but loaded is the issue? Maybe a hair more initial advance would be a good test too?
Yes, overall it's fantastic. The usual bog under sudden hard acceleration being absent is the only reason I'm puzzled. Even leaving it in 4th gear going down the highway at 2500 rpm, you can just flat foot it and it goes instantly. Never a stumble to be found. I suppose the vacuum advance is something I could play with. Truck has 14 initial and 32 total in it. Vacuum pot on the carb is intact and working.
Vac pot is intact and seems to work normally. No matter how hard you rev it under no load, the air doors stay closed, as I understand they should. I assume this contributes to why there's no bog when you just stand on it. I'm wondering if maybe the air door tension could need to be tightened some, whereas under a gradual increasing load the vacuum signal goes away, and allows the air doors to open, at which point spring tension isn't sufficient on its own, and they open too quickly. I guess ultimately it's down to trial and error, having a wideband would be a nice tool here.
It sounds like rolling into the throttle prevents the pump shot from covering the lean spot as you go to WOT. Here’s a couple of things to look at that might help adding more fuel earlier that hasn’t been mentioned yet. Cliff, Larry or another expert might know the best way. - changing the primary metering rod springs to come in earlier, might affect out of secondary acceleration though. - Modifying the pull over enrichment circuit on the secondary air doors. It will help get fuel going to the secondaries. Edelbrock and the OEMs on certain models notched the front edge of the doors to help fuel flow start just as the doors opened. Cliff’s book explains how to do it depending on what style POE you have.
I had a few moments before work today, and added tension to the air door spring with some effect. I could go further, and might try that, but it tells me you're probably over the target about the lean spot. I imagine helping the fuel come in sooner would be better and more responsive than just restricting the air door opening until way late.
Restricting the secondary air door causes more fuel to flow through the primary side. So it seems like a decent way to adjust for this one case.
Put a vacuum pump on the choke pull off, ensure choke pull off is holding vacuum. Then release the vacuum, time how long it takes to fully release. This component is a key player in controlling how long it takes for the air door to open. Too quick - motor will bog. On my set up, you could have a quick release when spinning tires on the street. Bolt on drag radials or slicks, engine would bog if choke pull off opened too quickly.
If adding spring tensions helps then it's most likely the pull-off. Make sure the link is tight up to the secondaries when the pull-off is fully applied, and that it releases smooth and about 1.5-2 seconds. If it opens nearly instantly or really slow it will cause a hesitation going quickly to full throttle no matter how tight you wind up the spring....
