Quadrajet Problem Solving > Dialing in your rebuilt Quadrajet carburetor

1903 Won't Respond to Tip-In Procedure

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bry593:
Edelbrock 1903 to be used on HT383, 9.1:1 static, 196/206 .431/.451 109, 10 initial + 22 mech @3k.

Calibration as purchased used, but never ran on this engine.  Note this carb was all black inside.  Was running rich!
Hot Air Choke
Needle Seat .130
Float 7/16
IT .033
UIAB .070 (in body)
DCR .055
LIAB .064
IDP .084
IABP .052
SMAB .052 (in horn)
Jet .073
Primary Rod 50M
APT Spring Silver
Acl Well .040
2ndy Rod DR
2ndy Hanger S
2ndy Bleed Tube .026
2ndy DP .052
Choke Pull Off 2.5
2ndy Spring 7/8

Second calibration per Cliff.  I could not get this to idle down until I plugged the IABP.  However, it still ran on even on 91 octane.  Pull off vac hose and idle goes up.  Zero response to APT tip-in procedure.  No hesitation off-idle or thru 2ndy.  Engine tries to die when let off throttle to complete stop.  Electric choke works fine, but loads up.
Electric Choke
Needle Seat .156
Float 7/16
IT .036
UIAB .070 (in body)
DCR .055
LIAB .064
IDP .089
IABP Plugged
SMAB .052 (in horn)
Jet .074
Primary Rod 50C (Cliff custom full taper)
APT Spring DkBlu
Acl Well .040
2ndy Rod AX
2ndy Hanger S
2ndy Bleed Tube .026
2ndy DP .052
Choke Pull Off 2.5
2ndy Spring 7/8

3rd Calibration.  Leaned it out, trying to kill the run-on and overly rich condition.  Bushed primary shaft.  Idle screws 4.0 turns out and turning in kills engine, bad off-idle stumble in cold weather (have to flutter acl pump), no engine run-on, pulling vac hose increases idle, zero response to APT tip-in procedure, choke works
New Primary Shaft Bushings
New Needle & Seat .135 <-Found tool chatter in the larger seat
New Float <- Old and new both weighed 6.5g, but new had heel farther away from fulcrum
Float 7/16
IT .031
UIAB .041 (moved to horn 6-32 setscrew, plugged old with sinker lead)
DCR .047 (8-32 setscrew)
LIAB .064
IDP .089
IABP plugged
SMAB .052 (in horn)
Jet .074
Primary Rod 50C (Cliff custom full taper)
APT Spring DkBlu
Acl Well .040
2ndy Rod AX
2ndy Hanger S
2ndy Bleed Tube .026
2ndy DP .052
Choke Pull Off 2.5
2ndy Spring 7/8

4th Calibration.  Opened IABP since idle seemed to want air when removing vac hose.  Idle screws 3.5 turns out and turning in kills engine, bad off-idle stumble in cold weather (have to flutter acl pump), pulling vac hose increases idle, zero response to APT tip-in procedure, choke runs rough
Seat .135
Float 7/16
IT .031
UIAB .041 (moved to horn 6-32 setscrew, plugged old with sinker lead)
DCR .047 (8-32 setscrew)
LIAB .064
IDP .089
IABP .052
SMAB .052 (in horn)
Jet .074
Primary Rod 50C (Cliff custom full taper)
APT Spring DkBlu
Acl Well .040
2ndy Rod AX
2ndy Hanger S
2ndy Bleed Tube .026
2ndy DP .052
Choke Pull Off 2.5
2ndy Spring 7/8

5th Calibration
Pulled the 1903 off and replaced with a junk 70 series from a 350 truck.  Runs fine, but no choke (I have vortec heads).  Surprised how much quieter the exhaust is with this carb.  No run-on, no stumble, no dying when quickly decelerating to a stop.  Pretty good for an old junk carb.

Got to say, I've spent way too much time and money on this thing.  Pretty frustrated.  Can anyone see where I've gone wrong?  Never had this much "fun" with a quad before.

I'm scared to go back to a bigger idle tube, since it already seems rich regardless of what I do.  Setting the float to 9/32" doesn't seem like it would help.  Why is this not responding to the APT Tip-in? 

Kenth:
The main issue is the single main airbleeds size. Less air equals more fuel from the same jetting.
As the tip-in is done at over 2000 rpm´s the idle circuit is out of the equation.
.052" SMAB were used in 1977-78 Truck Q-jets with small (.064") jets and rods (38B-40B) and 1979 and later Truck Q-jets with 350HD or 454 engines.
All other 1979 and later Truck Q-jets uses .087" single main airbleeds.

I would use the 3rd recipe, open the SMAB to .087", the DCR´s to .055" and the IBA to .080"-.087".

HTH

Cliff Ruggles:
Lets go back to basics here as you missed something along the way.

First post says 383CID, 9 to 1 compression and tiny 196/206/109 camshaft.

The 1903 wouldn't have needed much help to be flawless on that engine. 

Did you block the vacuum supply to choke housing when converting to electric?

How much timing is being added by the vacuum advance?

Manifold or ported vacuum to the VA?

What intake manifold is being used and does it have an exhaust crossover in it?

Lots of SMAB later carburetors will have smaller MAB's and since they are indirect they will not have a huge impact on the jets/metering rods required.

Edelbrock patterned their later SMAB units after 17080213 units so they will have small lower IAB's and smaller MAB's in the airhorn.

They don't need a lot of idle tube for what you are doing and stock .055" DCR's would have been adequate with the stock upper and lower IAB sizes.

I suspect the issues early on were due to leaking at the N/S assembly or some fundamental issue which started you on the path of adding fuel to the idle system and moving the upper IAB's to the airhorn.  That move would NOT have been needed and sent you into a rich condition.

Keep in mind here that the main system is fed thru the transfer slots so it does impact transition from the idle to the main system and will add some fuel at cruise at very light throttle openings.

The Edelbrock 1910 is basically the same carb you are using with a rich idle calibration and the secondary airflap stop ground off some to provide a little more CFM.  They used several different calibrations in the 1910's the most common is nearly identical to the 1903 with only larger idle tubes and a little more bypass air.  They are known for idling very well on engines with pretty "hefty" cams in them and lower vacuum at idle speed.

I've build hundreds of these carburetors and they are very nice units, but I have found numerous fundamental issues with them over the years which we can discuss later after you answer the basic questions above.

In any case it should have been pretty close right off the bench with just a few minor enhancements which would have included good rebuild parts and slightly larger idle tubes and possibly just a tad more bypass air......Cliff

bry593:
Hey thanks guys.  Appreciate you guys listening to my woes.  This thing is driving me crazy and it's a short trip...

Q. Did you block the vacuum supply to choke housing when converting to electric?
A. Yes, per your recommendation I blocked everything except the timed, aux and PCV ports with 3/16, 1/4 and 3/8 Al bar driven in and red Loc-Tite.  Zero vac leaks from those ports (did a smoke check).

Q. How much timing is being added by the vacuum advance?
A. I had the factory ZZ4 dizzy advance, but the curve started at too low of Hg.  After discussing with you, I bought your higher Hg advance unit.  Don't have my notes at the moment, but IIRC is 14 degrees.  Cured my transition ping from cruise to light throttle. 

Q. Manifold or ported vacuum to the VA?
A.  At first I was running ported to lower the rpms and help prevent the ignition off run-on.  After the seat replacment and throttle shaft bush, I switched over to manifold

Q. What intake manifold is being used and does it have an exhaust crossover in it?
A. GMPP Vortec, no crossover in Vortec

Lots of SMAB later carburetors will have smaller MAB's and since they are indirect they will not have a huge impact on the jets/metering rods required.
Yes, seemed small but worked well with Calibration 1.  Only issues were engine run-on and could not get APT to go lean.

They don't need a lot of idle tube for what you are doing and stock .055" DCR's would have been adequate with the stock upper and lower IAB sizes.
Are you sure stock DCR was .055?  I thought someone had drilled these out? 

I suspect the issues early on were due to leaking at the N/S which started you on the path of adding idle fuel and moving upper IAB's to the airhorn.
I reduced idle circuit from IT .036 to .031, but still unable to get APT to go lean.  At 2000 rpm tip-in engine slowed even with APT screw turned all the way down.  Why?

Keep in mind here that the main system is fed thru the transfer slots so it does impact transition from the idle to the main system and will add some fuel at cruise at very light throttle openings.
Yes, IT .036 to .031 killed my transition fuel.  Bad hesitation especially in cold weather.

I've found numerous issues with them over the years we can discuss later.
I've checked piston arms for level, blown air thru channels, checked well plugs, etc.  So far everything checks out.

It should have been pretty close right off the bench with good rebuild parts and larger idle tubes and a tad more bypass.
It's all good stuff from you except the second needle and seat (Hygrade).  I soldered up your .036 idle tubes to .031, but can easily drill them .033.  Are you sure IDCR .055" is original?  I thought it was supposed to be .047?  What size should I make the UIAB now that I've moved it to the horn?  Should I also reinstall the 50M rods and 73 jets?  I was thinking .086" IABP.

Cliff Ruggles:
Moving the upper idle bleeds to the airhorn made it go rich at idle and didn't help part throttle either.

I save that move for heavily modified engines with hefty cams in them and very little vacuum at idle speed.

If you do move them to the airhorn in a later SMAB carb a .052" UIAB, .070" lower, and .034 idle tube with .046" DCR's would be enough idle fuel.

Adding timing at idle speed typically raises vacuum some and lowers the throttle angle.  I would doubt if your engine combo would need much beyond 10-12 degrees initial timing to be happy, although I'm not familiar with that particular camshaft on a 109 LSA so don't know exactly how it would behave in a 383 build with some compression in it.

If it helps some the toughest engines to tune I've ran into over the years were larger CID, moderate compression and tight LSA camshafts with early intake closing.  I've had a few pulling my hair out to get them to work well everyplace, and it's a good thing I'm nearly bald because I would have been by the time I got finished with them!

As far as "fundamental" issues with Edelbrock q-jets I've ran into a few that had one or both jet holes machined at different depths in the casting or too deep in the casting making tuning difficult if not near impossible.  It's not uncommon for them to miss drilling passages by either not connecting them/missing the intended target.  Most of the time that problem was with secondary POE which wouldn't really impact anything you are doing.

In any case the cold intake isn't helping things and all tuning needs to be done when it's fully heat soaked, so give it plenty of warm up time before making idle adjustments and changes to the idle system.

The 74 jets and 50C rods are fine and not making it rich.  I've used that combo in quite a few of those carbs and had to get the APT up to 3.5-4.5 turns for most of them before they had adequate fuel at light load/part throttle. 

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