Quadrajet Problem Solving > Dialing in your rebuilt Quadrajet carburetor
1903 Won't Respond to Tip-In Procedure
Kenth:
I have found it´s far easier to add fuel to the equation than to relocate the UIAB´s.
If the engine lacks fuel in a certain circuit, add fuel, and the engine will be happy.
Different applications WILL have different sized idle tubes despite having the same sized airbleeds.
And, lots of EM4ME ECM Q-jets has .031"-.032" idle tube using .034" upper IAB´s in bowl with .077" lower IAB´s.
You will be hard pressed using .031" idle tubes with .070" UIAB´s and .070"-.077" LIAB´s for a performance built engine. You will need at least .036" to .038" idle tubes for result.
Then add proper amounts of idle bypass air to get the throttle blades in a proper angle at idle to reduce risk of nozzle drip and engine run-on at shut off.
FWIW
Cliff Ruggles:
Kenth is correct about the later carburetors and man use a larger Lower IAB and uppers at or close to .070". On an engine making a lot of vacuum at idle a .034" idle tube will be plenty. You can install larger idle tubes if the engine gets a cam that lowers vacuum some, but it works better to reduce the lower IAB and increase the idle tube at the same time.
A large Lower IAB vents off the signal and reduces "pull" to the DCR's and on to the idle tubes. Adding fuel with a larger idle tubes works, but you may notice a slight "skip" or "quirky" idle quality with the larger Lower IAB vs making it smaller.
The larger Lower IAB also leans out transition fuel as well. Fine for really mild engines with tiny cams, may cause issues with engines having larger cams in them and less signal or vacuum at idle and right off idle.
Early, non-emission calibrated and Marine Q-jets used really tiny Upper IAB's and would bring down PLENTY of idle fuel with tiny little idle tubes and very small DCR's. As emissions tightened up we saw those tiny little Upper IAB's become larger, then disappear and they went to indirect Upper IAB's in the main casting. Some carbs use pretty small Upper IAB's even though they were indirect, usually teamed up with a pretty large LIAB but not always.
For custom tuning the "recipes" in my work recommending .070" upper and lower IAB's work very well. I'm custom tuned many thousands of carburetors using those recipes with perfect results.
Even so I still experiment with Idle Airbleeds to this day, and may go that direction if I get a carb that just needs a tad more idle/off idle fuel and I don't want to pull the idle tubes or drill the DCR's larger.....Cliff
bry593:
I did leave the LIAB at .064 to better match the smaller idle tubes.
Drove to work this morning, seems to like the additional idle fuel. Working okay except for a slight bog after shifting at normal speeds. I suspect my APT needs to be a bit higher.
Now that I've choked back the IABP, I notice my throttle does not settle unless I blip it. Just slowly letting off the throttle will end up a few hundred rpm high and causes run-on at engine off. Might be caused by a non-worn portion of the throttle shaft (still has green plating) hanging in the new bushing. Possibly polishing next to the throttle linkage with a strip of 400 might free it up. I did try a stiffer return spring, but did not like the feel.
Kenth:
--- Quote from: bry593 on March 15, 2021, 07:08:42 AM ---I did leave the LIAB at .064 to better match the smaller idle tubes.
Drove to work this morning, seems to like the additional idle fuel. Working okay except for a slight bog after shifting at normal speeds. I suspect my APT needs to be a bit higher.
--- End quote ---
Just checking you now have .032"-.033" idle tubes and .043" upper idle airbleeds located in airhorn? Not too bad.
1. The LIAB´s switches to idle mixture discharge holes at greater than off-idle/low speed throttle blade angle.
Too small LIAB´s for the application equals to small amounts of added mixture and makes the hesitation at throttle movements like you are experiencing.
2. Also, too small IDCR`s will restrict the amounts of idle mixture needed for proper throttle respond past off-idle/lowspeed.
I have found it´s hard to go wrong with .051"-.055" IDCR´s and .070" LIAB´s for applications like yours.
After LIAB´s and IDCR´r are set you can look at main jetting corresponding to main airbleeds and APT setting using the "tip-in" with a slow hand, not quick hand method.
FWIW
bry593:
--- Quote from: Kenth on March 15, 2021, 08:11:11 AM ---The LIAB´s switches to idle mixture discharge holes at greater than off-idle/low speed throttle blade angle.
--- End quote ---
Yes Sir, I see the LIAB as an inefficient nozzle when the throttle blades are open. At that point, all idle fuel and air is metered only by the DCR. This is why you will get better gas mileage from small idle tubes than large tubes. It is true, the engine will pull only as much fuel as it needs. But, if it pulls thru the primary boosters it is better atomized and more efficient than fuel dribbling out the LIAB hole.....
--- Quote ---Too small LIAB´s for the application equals to small amounts of added mixture and makes the hesitation at throttle movements like you are experiencing.
--- End quote ---
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 issue I have right now is after it shifts and the rpm drops, while the throttle is held constant, there is a slight hesitation. I think this is power piston related. Probably need to raise it a 1/2 turn or so.
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