1903 Won't Respond to Tip-In Procedure

[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.

[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]

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.

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]

The LIAB´s switches to idle mixture discharge holes at greater than off-idle/low speed throttle blade angle.

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.....

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.

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.

[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]

"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. 

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.