Quadrajet Problem Solving > Dialing in your rebuilt Quadrajet carburetor

1903 Won't Respond to Tip-In Procedure

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bry593:
Alright, that's two for tips under the piston method.  I'll try it next time.

Do you ever see jets that are obround due to the needle rubbing? 
I would assume this a common issue due to spring tension of the primary needle retaining clip.

Cliff Ruggles:
I see jets every single day that are worn or have been drilled so I replace them with new ones.

I get carburetors in here from every "builder" out there and see at least a half dozen fundamental problems with them and why they weren't working for the customer.

Not too long ago I got in a fully restored early Buick 430 carburetor that was done by a shop in NJ.

The owner had it fully restored and it was running really rich.  A couple of trips back to the "builder" made no improvement so it end up here.

I took it apart, and aside from using a small N/S assembly, clipped POS power piston spring and crappy accl pump (soft blue seal already swelled up some) I couldn't see why it was so rich.  It had the correct 71 main jets and 45B metering rods in it.  Hanger was bent slightly down not up so not contributing to the rich condition.

I took a closer look at the main jets and they looked pretty big for 71's so I removed them.  They ended up being .078"!  Most likely at some point the owners friend or one of his beer drinking buddy's thought he help him out while the carb was apart and give it some more fuel. 

Moral of the story is to be meticulous and leave no stones unturned with this sort of thing, plus use new parts and high quality parts if you want the carb to be reliable in long term service......FWIW.....

bry593:
Cliff and Kenth,

Thanks guys for stickin' with me through this.

I couldn't leave well enough alone.  Engine seemed like it wanted a bit more fuel, so I tried to figure out why it was running well at:

IT .031
UIAB .042
DCR .047
IABP .052
Jets .074
Rods 50C

First off, remember that this engine has a roller cam and short piston skirts.  It also has a crazy tight 109 center cam (19.5 in Hg), vortec heads and moderate compression (9.1:1).  Put this all together, and it is more efficient than most.

If Cliff will recall from our phone convo a year ago, I originally used the .036 jets, but couldn't get it to idle down until I blocked the IABP.  This is when I was using the bore UIAB at .070.  I also had high sensitivity to engine run-on after the key was turned off.  Engine run-on typically means too much fuel in the motor after shut-off.  That's why I killed it back with a .031" idle tube.

I ran some calculations on fuel through an orifice and found vacuum of 19.5 in Hg through a .031 idle tube, pulls the same as 16 in Hg through a .033 tube.

I then looked at the ratio of air to fuel between the UIAB and IT with my current IT .031, UIAB .042.  Keeping the ratio the same, I extrapolated the following table:
                                                                                            1.75-1.85
IT (in)   UIAB Rec   UIAB Rec (mm)   Input Actual UIAB   Rec UIAB/IT   Actual UIAB/IT
0.031   0.042           1.07                   0.042                   1.84                   1.84
0.032   0.043           1.10                   0.043                   1.84                   1.81
0.033   0.045           1.14                   0.043                   1.84                   1.70
0.034   0.046           1.17                   0.047                   1.84                   1.91
0.035   0.047           1.20                   0.047                   1.84                   1.80
0.036   0.049           1.24                   0.047                   1.84                   1.70
0.037   0.050           1.27                   0.052                   1.84                   1.98
0.038   0.051           1.31                   0.052                   1.84                   1.87

Thing is, you only have a few sizes in a numbered drill set to achieve the orifices.  So, you gotta choose what is closest and hope for the best.  That is what "actual" indicates.  The actual drill bit you have in your index.

Being that the idle screw was just barely off the stop at IABP .052, I decided to choke out some bypass air to reduce my idle and allow better use of the idle screw.  I choked it back to the next drill bit, .047".

I also thought it wouldn't hurt to run the float a bit lower than my tested and true 3/8.  So I put it at 7/16.  With a .135 needle seat orifice, I should never have a float issue.  Obviously, I don't plan to go racing.  4500 rpm is my shift point.

So my new calibration is:
 
IT .032 to .0325
UIAB .043
DCR .047
IABP .047
Jets .074
Rods 50C
APT 2.5 (still does not go lean with a quick hand over the airhorn)
Float 7/16

Note that idle mix screws are 3.0 turns out.  It actually works at 2.5, but I can eliminate the pip, pip, pip form the exhaust with another 1/2 turn.

I think your single taper 50C rods will optimize gas mileage.  But, I do like the double taper 50M because once it hits the second taper, it comes alive well before full throttle.  However, if you are pulling a trailer and the piston rises to this level, you will surely consume more gas than with the single taper. 

Cliff Ruggles:
Keep in mind when doing all of this that changing the float level also changes the fuel level in the bowl.  This has a DIRECT impact on all of your results.

Same thing with the size of the fuel inlet seat and fuel pressure.

I can set the fast idle on my engine at 2000rpm's with the engine fully warmed up.  Starting at 4psi, for example I can use the fuel pressure regulator and start increasing the fuel pressure (raises the fuel level in the bowl).  You can hear the engine speed increase as you increase pressure as it's raising the fuel level. 

So every "recipe" you come up with is impacted by the fuel level, which is controlled by fuel pressure, fuel inlet seat diameter, float level (and size of the float).

Thru all of this I'm still  not sure why you chose to move the upper IAB's to the airhorn instead of leaving them in the main casting.  The indirect bleeds in the 1903 are fine at the stock size and location.  If you move them to the airhorn they become a direct bleed and have a much greater impact on idle fuel delivery typically requiring much smaller idle tubes and DCR's to keep the carb from being way too rich at idle speed.

I may have mentioned it before but will throw it out there again.  The tiny 109LSA cam is NOT helping the scenario or making the engine easy to tune.  I've never once in all the years of doing this been the first bit impressed with cams like that and they have thrown me some pretty big curves with tuning.  Pinging was among them, and "quirky" idle quality.

Not saying here that you can't make one work OK.  Just keep in mind that small camshafts on tight LSA's do several things.  They close the intake valve EARLY, pull power DOWN in the RPM range, and NARROW up the power curve.  This very quickly takes a lower compression engine and makes it behave like it has higher compression.  Peak VE occurs earlier, so with the good cylinder filling going on at lower RPM's we get a pretty high "spike" in cylinder pressure and more times than not the engine will ping at both light throttle and heavy/full throttle if we try to put very much timing in it.

I think the "quirky" idle simply comes from closing the intake early but still having a decent amount of overlap from the tight LSA.  That's just a guess on my part but all of the engines I've had in here to custom tune showed me those characteristics when outfitted with similar camshafts.....FWIW.......Cliff

bry593:
I also prefer a bore type UIAB.  It's just a better place to put it.  The large passage, typically .065-.070, does not protrude into the bore and the upward angle all work together to virtually eliminate clogging.

Here is my assumption on why the airhorn bleed must be smaller than the equivalent bore bleed.  The airhorn bleed enters the fuel channel at 90 degrees, while the bore bleed enters at a reverse angle.  The bore bleeds cause the air to crash nearly head-on into the oncoming fuel.  This has the effect of reducing air flow.  Hence, the horn bleeds introduce more air and are necessarily smaller.  At least that's the way I see it...

The reason I moved the UIAB to the airhorn was to allow an IT .031.  Being that the bore UIAB was .070", I couldn't really take it any smaller.  How would you make this smaller?  I don't think I can fit an 8-32 without taking out part of the bowl top surface.  And there is not much sealing surface around the upper idle channel.  So, it seemed logical to move it to the airhorn.  There I can easily change air bleed size. 

I agree, fuel pressure is definitely something to consider when setting float levels.  I observed that typical, over the counter pumps are pushing 9psi.  So work best with a lower float level, especially for a driver.   

As for the cam, I do have a ZZ4 roller squirreled away somewhere.  It is 112 208/221 .474/.510.  Would definitely wake this engine up.  Also easy to tune with a .036 IT, .050 UIAB, float 3/8, static timing 12.  But, I'd have to change out the springs, retainer, pull the condenser, radiator, accessories, intake, etc.  Probably should have considering how much time I have in this quirky tune.

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