Quadrajet Problem Solving > Dialing in your rebuilt Quadrajet carburetor
Fine tuning Q-jet
Cliff Ruggles:
Another thing few if any folks realize with these things is that a carburetor is a metering device an it is NOT, and I will repeat NOT a "fixed" delivery system. By design, and I absolutely LOVE this part, it will vary fuel delivery based on throttle position, engine load, air flow across the venture/boosters and pressure differential above and below the throttle plates. Very clever these engineers were with designing them and they are very effective at not only fuel delivery in the correct proportions but also very fine atomization of the fuel molecules BEFORE they hit the intake tract an make it to the combustion spaces.
I'll slow this down a bit but anyone reading this needs to understand the importance of finding the best "recipe" for your particular combination. This includes a lot more than just setting up the carb and distributor. There are a LOT of other players in the game and things effecting the final outcome. To mention a few is the compression ratio, combustion chamber shapes, camshaft events, intake chosen, spacers used, scavenging efficiency of the manifolds, headers and the rest of the exhaust system. I'd also mention engine temperature (thermostat setting and actual temperature of the engine parts) is a player here, plus the air induction system (heated or bringing in cool/fresh outside air). I'll go on to mention engine load from a "tight" converter, lower numerical gearing, vehicle weight, rolling resistance......etc, etc.
I certainly hope this doesn't "muddy" the water. Nothing at all wrong with using monitoring devices to help you in your efforts for perfection, but folks need to realize that they are simply tools to assist us and most to see the results of our tuning efforts. Used or not we ALWAYS tune for best results in all areas. IF you are using a Quadrajet in your efforts I have the knowledge and experience to help you cut very quickly to the chase. At this point in my learning curve I know all the part numbers, what they came with, how the factory set them up, and what is needed to dial them in for what you are doing. I can get you so close in most cases it's basically plug and play.
This is why I retired from carb building a few months ago and have moved on to selling parts, rebuild kits and custom rebuild kits with tuning parts. I've spend the time and funds to make sure you get the best parts out there, the correct parts, and tuning parts needed to dial you in, or basically "plug and play" right off the bench.
So as these discussions go on folks should realize that this is NOT something you can nail down going out and buying an A/F metering and simply monitoring. You are going to have to spend some time with your set-up to nail down the best overall results. This Forum is there to help you that effort........Cliff
bob69:
Hey all, I appreciate the discussions. Never too old to learn something new :).
old cars, the car does not "hesitate" as such. But it just does not accelerate like it should. Slow uptake in that first second or two. The acceleration after that is ok but again kinda weak for a 350 when compared to my other cars.
I had a 1969 nova with a nicely built 454. It would smoke the tires easily from a standing start with just 1/3 throttle. I got real good at rolling starts before hammering it. Very strong motor, plant you in your seat real good. I had it at a 1/8 track once. Ran 93 mph with a rolling start. It had a 7029207 with mabs at .070. Ran great. Loved that car right up to the day it was stollen.
I had a 350 hp 327 with with a 7028213 q-jet. That motor would easily rev to 6500 quickly. Loved the way it would raise the front end when I jumped on it. The 350's acceleration is not even close to the 327. The same goes for the other chevys I've had.
At this piont I'll probably chuck the carb and try another.
Before I bought Cliffs book I followed the q-jet tuning steps outlined by Lars. His steps are basic the same as Cliffs. No surprise. So I've done things like tip in and removing a vacume line as tests.
As Cliff mentioned, you have to tune the ENTIRE set up not just the carb and distributor. For example, I scored a low mile GM remanufactured 700r4 from a 1990 nine passenger Chevy van. Took the gm rebuilt tq too. After the 700r4 / tq was installed the engine would not idle. To get a 650 idle in drive, the park idle had to be around 1300. After a lot of tuning and installing an idle stop solenoid the engine was still not running great at idle.
After much research I found that the 1990 nine passenger chevy van 350/700r4 tq had a stall around 1100-1200 rpm. The tq was dragging down the motor resulting in a too high idle rpm. Changed the tq to a 2200 stall. I dropped the park idle to 900 and drive idle was 600. The idle stop solenoid was removed. After some tuning to the carb the idle is silky smooth and the engine turns off when the ignition is turned off.
You may have heard the phrase - no replacement for displacement. Well experience is much the same. That's why I enjoy reading these posts. Never know what you can learn.
old cars:
It is hard to reflect sometimes what someone means as problematic online without seeing/touching/hearing in person because we all describe things differently.
AFR gauges (readings) can be controversial to say the least. I will agree there not for everyone. Even the installation location and leakage can affect readings. Not sure if I would call carburetors stone age. Maybe my posts have not been helpful. Good dicussions with relative information are always good though , even when it slides off the topic.
Cliff Ruggles:
I'll add that the WORST calls I get here are from folks who have installed A/F gauges and are trying to tune their carburetors.
They will call up here all upset and say things like "I saw 15.9 clear up to 17 to 1 AFR a few times cruising and changing speeds in traffic".
The FIRST question I asked was IF you noticed any hesitation, stumble, bog, "flat" or lack of power, surging, or any other negative symptoms. "No" they reply, "it seemed to run just fine". Then WHY are you worried about?
Well, I was over on the Chevelle Forum and a "guru" who posts there said that my A/F should be reading 13.61427427 to 1 at cruise or my carb was all messed up.
See what I mean? We tune for results, and certainly NOT what any resident "expert" posts on the subject.
The very same "guru" also told him that he can't use ported vacuum to the advance. I asked him why? "Because he said the engine will run hot, overheat, plus it will continue to add timing at heavy/WOT throttle, detonate my engine, crack the piston ring lands and knock all the rod bearings out of it".
LOVELY, another well educated "guru" that doesn't even understand how vacuum advance works and should as quickly as they can get back to their "day job"....IMHO.......see what I mean about using the Internet and an A/F meter to tune with.......FWIW.....
bob69:
Cliff, one last thing to check. Could the carb be draining the fuel from the accelerator pump well while running? What would the possible causes be.
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