Quadrajet Problem Solving > Dialing in your rebuilt Quadrajet carburetor
Test and tune
Cliff Ruggles:
Have you tried loosening up the secondary airflaps slightly using the adjustment spring? The flaps may need to come open just a tad quicker.....Cliff
Pontiac Collector:
It had already backed it off about 1/8+ turn from what it was set at delivery, and that made a pretty big difference in what seemed to be mildly sluggish response. I will have another look at backing it off a bit more. The reason I hadn't considered more in that area was that I felt it wouldn't really overcome the rich dip event which occurs immediately upon WOT for a duration that is well before the secondaries are really opening up (a choke pull off rate of 1-1.5 seconds). The dip is quite obvious in the graph. It is a rapid decline from and AFR of 14.7 ish to around 9.8 in a few tenths of a second. At that point it starts moving from 9.8 toward 12 for a about .4 more seconds. This is probably too much detail so I will take your advise and loosen it some more.
I'll let you know how it goes, thanks for your support.
Cliff Ruggles:
I'm also curious if you blocked off the exhaust crossovers in the heads, filled them with aluminum, or running them open?
I would also run a 180 degree thermostat if you have full control of the engine temps. With my own engine, it runs quicker at the track when allowed to come completely up to 180 degrees. If I try to leave at 150-160 degrees is "lays down" ever so slightly and runs slower at ever point on the run.
The higher engine temperatures allow for the A/F mix to atomize more effectively on it's way to the combustion chambers.......Cliff
FORREST MILLER:
Any chance that rich dip comes from an over-agressive accelerator pump shot???
Cliff Ruggles:
Yes, but it is not always a negative. A LOT of fuel is required with any large CFM 4 barrel carburetor the instant you go to WOT to avoid a lean condition, which will result in a "bog", hesitation, stumble, and in extreme cases backfiring up thru the carb and catching it on fire!
FWIW, I've seen folks armed with LM1's and other A/F monitoring devices chase this condition to death, particularily with Holley double pumper and other Holley DP clones, just to find out that they didn't pick up any ET or MPH, or improved 60' times by continuing to lean it out to avoid a quick "dip" in the A/F curve.
Not that there's anything wrong with having some fun with these things, but any and all testing should be backed up with some dragstrip runs to monitor the results to see if it leads to improved performance, and exactly how lean one can go before the vehicle starts to slow up........Cliff
Navigation
[0] Message Index
[#] Next page
[*] Previous page
Go to full version