Very Slight Off Idle Hesitation Question

[eguyett1985]

After reading "the book" I have been trying to get my freshly rebuilt carb dialed in and for the most part everything is working fine except for a very, very slight off-idle hesitation I've noticed; it happens when you very slowly open the primary throttle blades like backing out of the garage or slowly pulling away from a stop.  The engine idles really well and if I roll into the throttle just a little bit quicker there is no hesitation; only when I go slowly.  I probably need some clarification on what I think it might be perhaps just a slightly lean idle mixture maybe?  I'm just not sure; here's what I've got:
1972 Corvette 350 A/T 255HP stock compression, dual exhaust, and a slightly bigger camshaft (installed by previous owner and I don't have any specs).  I just took the distributor out and shimmed it, installed a Pertonix Ignitor conversion, adjustable vacuum advance, and advance curve kit.  The vacuum advance is 10 degrees and in at idle, the total advance in the distributor is 15 degrees starting it at around 1000rpm and all in by 2650rpm.  I have my initial timing at 15 degrees with the vacuum advance hooked up to manifold vacuum (total of 25 degrees at idle) with a total of 30 degrees at WOT.  I set the mixture screws with a vacuum gauge and the advance hooked up.  I have around 16"-17"hg in Park and about 10"-11"hg in gear.  The mixture screws seem to give the best results at about 5.5 turns out.
The carb is a number 7043208 with a fresh rebuild including new bushings and the following specs:
Idle Tubes - .043
Idle Down Channel - .059
Upper Idle Air Bleed - .089 (in main body)
Lower Idle Air Bleed - .059
Idle Bypass Air Installed - .096
Holes Under Mixture Screws - .096
Holes In Throttle Plates - None

When I first put the carb on with no idle circuit modifications it would barely idle in gear and had only about 7"hg, now with the above mods it's like night and day...it idles really good and goes in and out of gear nicely; just a very slight stumble right off idle now; otherwise it's great.  I'm wondering if I open the mixture screws slightly if that will help or do I need to open up the down channel restrictions a little more; I know because my carb has the upper idle bleeds in the main body not the air horn that I had to go a little bigger on my fuel restrictions initially.

[Cliff Ruggles]

You didn't mention main airbleed sizes, and the jets/metering rods being used?

I have never been overly happy with any of those carburetors without going to smaller main airbleeds and different jets/metering rods.

I also do not like the large upper main airbleeds, they require too much idle tube.

I just built one of those yesterday, and blocked the upper idle airbleeds and went with a .052 hole in the airhorn instead, .036" idle tubes and .052" idle down channel restrictions.  Plenty of idle fuel and best setting was apprx 2.5 turns out with the mixture screws.

I also installed .070" main airbleeds, differenet jets/rods and an external APT screw.  I do this to all of those models with perfect results......Cliff

[eguyett1985]

Thanks Cliff,
That may give me some direction.  The Main Air Bleeds in the main body are .043 and in the air horn are .046.  The primary jets are 71's and the primary metering rods are 45b.  Perhaps my best course is to plug the upper idle air bleeds then make the appropriate changes; what do you suggest.  Also, how do I remove the idle down channel restrictions? and what about the screws under the idle mixture screws? Have I gone too big now?

[Cliff Ruggles]

That carburetor should have .118" main airbleeds upper and lowers.  Is it or has it been commercially "remanufactured" at some point?

All of the Chevy carburetors I've seen from 1970 through 1977 (divorced choke models), used the large main airbleeds.

We sell idle down channel restrictions, press-in or screw in, in any size you would want.  They may come out if pulled with a sharp tipped drywall or wood screw.  If not, a very carefully chosen drill bit will "spin" them out.  Worst case scenario, carefully drill them out......Cliff

[eguyett1985]

Cliff,
I went back today and thought I must be missing something in regards to my main air bleed sizes and looked at them again and compared them to the photos in your book I was right the first time; they are in fact .043 in the main body and .046 in the air horn. When I put it back together I tweaked the idle mixture screws and when I went out another 1/4 turn the hesitation seemed to go away.  I see you make mention to plugging the upper idle bleeds in the main body and going with smaller ones and I noticed you recommend relocating them to the air horn in some instances and plugging and redrilling them in the main body in others; what am I better to do with my combination? Also, you mentioned installing APT? I don't see that covered in the book; how is this done in an early carb like mine? Thanks for the help and an otherwise great running Q-Jet!
Evan

[Cliff Ruggles]

The jet rod combination is fine for the main airbleed sizes.  If possible, try to get the APT screw in the baseplate free'd up so you can raise the metering rod externally.

I don't like the upper idle airbeeds to be that big, and have always had better results making them smaller or moving them to the airhorn....Cliff