Quadrajet 7042220 idle air modifications

[Tway]

So I bought the SR rebuild kit from Cliff along with his book for my quadrajet 7042220 which is for my 72 chevelle 454. I wish I had bought the idle air tubes since my collars were driving down with the tubes. I had to drill the tubes down a bit to get a screw to grab the collars. Anyway just another delay since I’m in Canada and getting stuff shipped is slow.
I measured everything but have no idea if these were ever modified over the years since I can’t find any info on what the measurements were from factory. Also not sure how much to modify the air tubes and holes behind the mixture screws.

Idle tube .043
Idle down channel restriction .046
Upper idle air bleed .069
Lower idle air bleed .078
Idle bypass air .142
Mixture screw holes .081
Main air bleed .120
Fuel inlet seat .130
Secondary rods CM
Rod hangar H
Primary rod 49B
Primary jet 77C

Engine is presently at John and allyn Armstrong’s being rebuilt.
.030 Keith black pistons with -12.00 cc head.
Stock rod length 6.135 and stroke 4.00
Roller set up cam/lifters/rockers
Comp cams extreme energy roller cam 226/242 @50 and 110 lobe separation
The stock 3999241 heads had 113.06 cc chambers and 2.06/1.88 valves
The valves were increased to 2.19/1.88 with some mild porting.

[Kenth]

Your measured dimensions are about the same as I found in at least 3 untouched 7042220.
Not much that needs to be changed on it for your engine, but .095" holes for the tips of the idle mixture screws and .052" holes for the secondary POE holes as well as .052" holes for the idle channels could be suitable.

[Tway]

Thanks for the reply Kenth.
So for secondary POE is this these holes in the pic just above the pick?
And .052 for the idle tube and also for the restrictor?
I’ve also attached a pic of my idle air tubes and I assume these can’t be shortened and used (if I can get the collars down further)

[Kenth]

On my pics the idle down channels are marked red (between idle fuel tubes and accpump channels).
On the other pic marked yellow you see the secondary POE (the actual restrictions are a bit into the holes).

Your .043" idle fuel tubes are fine. FYI these are the tubes with the collars (they are NOT idle AIR tubes).

HTH

[Tway]

Had a brain fart calling those collared idle fuel tubes an air tube but thanks for clarifying and showing me where to drill.
So for me to reuse those idle fuel tubes am I correct in thinking I have to get the inner tube up flush with the top of the collar?

[Kenth]

There is always a risk of deforming the tubes by moving the collar so I would use them as is as they will still work as intended.

[77cruiser]

Or

[Tway]

Definitely like the threaded mod, especially handy if your back in tuning!

[Cliff Ruggles]

The collars will remove easily from the tubes.

Clean up the top of the tubes with a small reamer and gently chamfering the outside edge so they go back in the collars easily.  Turn the collars over and tap them back on the tubes.  It helps to clamp the tubes in a soft jaw vise and lubricate them with Loctite.  This helps them drive down flush and seals any potential leaks at the same time.  Being a tad shorter from the drilling operation hurts nothing.

Those carbs act lean at idle due to the large lower IAB's.  The cam you chose is on a 110LSA so it's going to like a little more fuel at idle to be happy.  Thankfully you didn't go any bigger with the cam and stay on a 110LSA or you'd be in trouble in that area.

I've lost count of how many of the early 70's Big Block Chevy Q-jets I've set-up for folks who was not happy with them at idle.  Reducing the lower IAB's slightly make a HUGE change to idle fuel delivery, but you would need to reduce the idle tube size at the same time as .043's are HUGE once you improve the signal to them above the LIAB's and DCR's.

I would imagine that the original calibrations were set up that way for emissions reasons as they were trying to clean up things at idle to make the EPA happy.

Coincidentally I did one just a few weeks ago where the owner had his engine rebuilt and the builder used a cam very close to what you are using, 224/230/110LSA hydraulic roller from Comp Cams.  It did not idle well and very little control with the mixture screws.  I reduced the lower IAB to .067" and went down a few thou with the idle tubes.  I also opened up the DCR's to .055". 

The owner had other issues with the carb as he just had it "rebuilt" and they installed a .110 N/S assembly (standard in many kits these days), bent the chit out of the power piston hanger arms and put an clipped the spring under the power piston.  It had other issues but too lengthy to go into great detail here.

With the correct parts installed and slight modification to the idle system he now has full control of the idle fuel with the mixture screws  I also installed one of my external APT screws so full control of the part throttle A/F as well......

[Tway]

Cliff were you able to use the check ball method your book mentions to resize the lower air bleeds or is there something else I need to do?
I’d have to order new idle tubes but the wait is worth it to have this work right at idle. If there is anything else you suggest I get from you let me know.
Do you also suggest I increase the size of the secondary POE to .052?
Should I also be grinding out a small notch on my secondary air flaps to expose those?

[Cliff Ruggles]

I use a small punch with a rounded end that I made to shrink down the Lower IAB's and resize them.

I didn't touch the POE system or notch the flaps.  I did leave a tiny bit of slack in the linkage so they "tip-in" slightly when the secondaries open so the fuel from the POE holes runs down under the flaps instead of out on top of them.......

[Tway]

So put the carb back together with a new SR kit, after talking to Cliff, I decided to try it before making any further modifications. Had the 454 out of my 72 chevelle built at John and Allyn Armstrong’s and the Qjet worked great in the test cell. Idled fine and no hesitation what so ever. Engine ended up making 492 hp@5500 and 530 Tq @3300.  Be a while before it ends up back in the car.
The bad is I noticed the Qjet inlet threads were weak when I reassembled - wish I had checked that first. So I’ve got a drip and suspect I’ll need to get a Heli coil repair done.