Quadrajet Problem Solving > Dialing in your rebuilt Quadrajet carburetor
Test and tune
Pontiac Collector:
To answer your question:
Q: I'm also curious if you blocked off the exhaust crossovers in the heads, filled them with aluminum, or running them open?
A: I am pretty sure they are open.
I have loosened the air flap about as much as possible and still have the tension to close the flap. As I said before, I did try to reduce the pump shot by moving to the outer hole on the lever arm. It is running pretty good actually and I agree with you Cliff that I may be making too big of a deal out of it. It is pretty subtle and the transition feels virtually instant with no bog. It was just that little .5 second blip I could feel, but as you state it probably isn't diddly for E.T. The closest place around here to tune is Bandimere, but I am really just street driving the car, not really racing it. As I stated before, I don't really know what circuit would even tune this out (if any)?
I had considered my lower running temps might actually be hurting me, but it is the first Pontiac I've had that is afflicted with too low of temps. I think we ran a 165 thermostat (not sure why it runs @ 150-160 then, but...). Easy enough to change it out for a 180 though if you think there is some power there.
Cliff Ruggles:
That little "dip" in the A/F ratio is common, I've seen it on quite a few "playback" runs from folks who monitor them then look at the run on a laptop.
Pretty common practive for many Stock and Super Stock racers these days. I don't fully understand the graphs, bit have noticed "dips" in them on the launch and often when the transmission makes a gear change as well.
One small item to try is to install a lower hanger for the secondary metering rods.
Another thing that we often overlook is venting. Some cars, my own is among them, leave hard enough to cause some fuel to splash up and out of the center vent or slot just behind the choke housing in the airhorn. Early carburetors just have a round hole there. This also causes a rich dip in the fuel curve, and in extreme cases the engine will even falter for just a moment, then recover and run fine the rest of the run.......Cliff
Pontiac Collector:
Thanks Cliff, I did go to an F hanger from a B, but have decided to stick with the B hanger as delivered. My test re-runs showed little difference actually. The dip was there either way. I never thought about the vent hole dumping fuel, but another idea to check into. Off the topic of AFR, I was just looking at changing my thermostat and was poking around performanceyears.com forums. It reminded me that I am running a direct (not clutch) mechanical fan. It got me thinking I may be giving up a bit of power there as well and it might help explain my cars amazing ability to stay cool. Do you think it also makes sense to consider an OEM style clutch fan. Going electric won't be an option.
Thanks again for all your assistance and great information!
Pontiac Collector:
Never mind on the fan comment, I realized that this is a stock 68 Firebird 400 flex fan, so I won't be messing with it.
Pontiac Collector:
OK, its been a while and I've done a few things to the Firebird. I've added a 180 (from 160) robert shaw thermostat and moved from a Muncie M22 to an M20. This car really liked the M20 gearing. I have almost instant throttle response (only a very very slight power "dip") that I am no longer going to try and chase down. I think I've done all I can and it is running quite well actually.
Now, for my next project I am working with another Cliff's Q-Jet 7040268 on my 1970 GTO 455. Before I go further I am at 5K altitude. Since this is mostly my cruiser I haven't been too worried about throttle response (and my focus was on the Firebird). It starts, idles, part throttles, and cruises perfectly. I am trying to tune out the WOT stumble and have set the secondary air-valve spring to anywhere from no tension to about 1/2 turn where the stumble improves. What I do notice is the more tension I add, the more sluggish the WOT response feels. A dead stop punch results in minimal tire spin (street tires) and a weak launch. Without spring tension (or at least minimal), it seems quicker after the stumble event, but it could all be in my head. The choke pull-off is about 1-1.5 seconds and the secondary flaps do not have accelerator port slots in them. I moved the accelator pump linkage to the outer hole last night as well. Maybe I am asking more than my car can deliver at this altitude, but in California with a different Q-Jet (my car was dyno tuned though), a dead punch resulted in instant tire spin for as long as I desired. I have also advanced the timing curve about 5 degrees from sea level. I guess the real question is, should I tune for no stumble via air-valve spring tension and call it good, or work to richen my mixture at the WOT event somehow so I can open the air-valve quicker.
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