Quadrajet Problem Solving > Dialing in your rebuilt Quadrajet carburetor
1903 Won't Respond to Tip-In Procedure
bry593:
Forgot to mention, I did try out the needles crossed under the piston method for adjusting the piston arms. I like it. Seems accurate enough and is a whole lot easier than trying to measure things.
I also tried the tip-in procedure again at 2000rpm. No dice. Unresponsive. Never goes lean.
Current calibration: IT .032-.0325, UIAB .043, DCR .047, LIAB .064, IABP .047. The "channel mix" is rich, but the overall calibration (fuel to the engine) is lean. This engine doesn't need much to spin it at an idle. And since I'm not getting off idle hesitation, seems to be enough for the transition as well.
bry593:
The engine ran great last night on the way home from work. Ambient was about 70F and there was no hesitation as the auto-transmission shifted and the rpm dropped. This suggested my idea about raising the APT might be correct, but that the current setting was pretty close.
I raised the APT 1/4 turn. Drove to work this morning in 45F temp and the hesitation was gone. This setting is on the edge of lean and should provide highest mpg. At this point, I think the calibration is dialed-in.
Cliff Ruggles:
Couple of things.
"I don't have any off-idle hesitation. The time I did was caused by the power piston spring getting xxxxed and bound up in the vacuum channel. I dropped a brass washer in the bore to keep this from happening."
The power piston spring is NOT a player for off idle transition unless you go very quickly to heavy/full throttle and vacuum falls off.
For all light throttle or "normal" driving vacuum never gets anywhere close to allowing the power piston spring to raise the power piston.
The distributor vacuum advance is a big player in the same range. Idle to off idle at light throttle openings applies vacuum advance timing no matter which place you source it, ported or manifold. So it is also a tuning tool in your arsenal to correct issues in that area.
The Q-jet is far more sensitive to very light throttle openings than other designs so will bring fuel in early from the boosters. This simply means that you don't need to add a LOT of fuel from the idle system, but all the fuel you do add effects the overall calibration at very light throttle openings and light engine load.
So it's basically a "recipe" to find the right settings. For most carbs I use .070" Lower IAB's and conservative with everything else.
It's also a standard bit size in the 1-60 set and a LOT of factory Q-jets will already be there.
The next choice is DCR, Upper IAB and idle tube size. You will have either "direct" Upper IAB's in the airhorn or "indirect" in the main casting just above the DCR's.
Direct Upper IAB's don't need to be very big and they have a pretty direct and dramatic effect on fuel delivery to the mixture screws and transfer slots. So be conservative if you go that direction or your carb uses them.
Very early carburetors had tiny Upper IAB's when they were in the airhorn as do Marine units (many were .040" or even smaller). They do NOT need very big idle tubes or DCR's to put PLENTY of fuel to the engine when they are in play. A .040" Upper IAB, teamed up with a .070" Lower IAB, for example, will put down a LOT of idle fuel with idle tubes in the .028-.032" range and DCR's from .040-.046". So keep that in mind when you come up with a "recipe".
I'd add here as a disclaimer that there are other items in the mix that effect your specific results, like length and location of the transfer slots, bypass air, the size of the primaries (large or small), type of booster (there were several different variety used over the years), etc.
So if you want to be a custom "tuner" you'll have to do pretty much what we are seeing here, experimenting with different combinations, making small changes, more testing, etc.
To further complicate your efforts don't forget about timing. The mechanical and vacuum advance are players as well. I'd also recommend making ONE change, and if it is initially favorable drive the vehicle for several tanks of fuel to get a feel for how it's going to work under all conditions and fuel economy as well.
continued......
Cliff Ruggles:
Keep in mind the entire time that being "rich" typically gives good results as you may not initially notice any problems with throttle response, power in any range, etc, but you can end up with horrible fuel economy. This is why aftermarket carburetors initially seem like an improvement over stock parts, they are set up without any emissions in mind, almost always very generous for fuel delivery everyplace, and the companies making them know they will get them right back IF they are lean anyplace.
The last thing I'll add here and this will blow all your tuning out of the water is that folks forget that a carburetor is a load-sensing device. It will add MORE fuel simply based on throttle angle, engine vacuum, pressure differential above/below the venturi and how much air is flowing thru it. So don't get all hung up and calculating metering area and mathematical formulas based on a "static" mind-set.
I can set a q-jet up to run straight off the jets and not even use a power piston and primary metering rods and it will be FLAWLESS everyplace. You may not even notice much difference in fuel economy either, if the right "recipe" is used when doing this conversion.......Cliff
bry593:
--- Quote from: Cliff Ruggles on March 16, 2021, 06:36:12 AM ---"I don't have any off-idle hesitation. The time I did was caused by the power piston spring getting xxxxed and bound up in the vacuum channel. I dropped a brass washer in the bore to keep this from happening."
The power piston spring is NOT a player for off idle transition unless you go very quickly to heavy/full throttle and vacuum falls off.
--- End quote ---
I should have more carefully worded my description of the hesitation. As you pointed out, just sitting there idling and flicking the throttle did not result in a hesitation. Obviously, I would have noticed this type of hesitation after reassembling the carb and adjusting the idle mix screws. The hesitation occurred on a below freezing morning, as I pulled into traffic from a side street and then when leaving each stop light thereafter. The evening temps were higher and the drive home from work had much less hesitation.
Does that make more sense?
Anyway, truck is running great. Will take it on a few hundred mile trip this weekend to and from the swap meet. Hope to see 18 mpg for this 1971 3/4 ton Chevy, 383, 4L80E with 4.10 gearing.
Navigation
[0] Message Index
[#] Next page
[*] Previous page
Go to full version