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 question about '72 455 HO quadrajet 
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Joined: Tue May 06, 2014 5:34 pm
Posts: 45
I almost always work on late 70s early 80s quadrajet, but today I worked on a friend's base plate installing primary bushings. The base plate in question is from a 7042270 455 HO. I noticed that with the throttle closed, all of the transfer slot is over the butterfly, nothing showing under...but, the idle screw discharge hole is .108", the biggest I've ever seen. The idle air bypass was huge as well, being .107".

What are your thoughts on this ?


Tue Mar 01, 2016 11:08 pm
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Joined: Fri Feb 08, 2013 2:37 pm
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Don't know what bushing replacement process you used, but, if the throttle shaft bores in the base were excessively worn, the reamer used may have followed the worn hole and the bushing installation lowered the center line of the shaft. Aside from that, each OE carb calibration has unique details and in this case the idle air bypass size and amount of throttle opening at idle and the location of the T-slot relative to the butterfly edge may be due to the emissions calibration of the era, which included retarded valve and ignition timing resulting in lower intake vacuum at idle than the same engine would have if it were not jacked around to suit a tail pipe sniff test. The .108" idle screw discharge hole is way large, usually .093" is as large as they get or need ot be, so someone may have been fiddling. The .107" idle air bypass is not as large as some. I think some '73 or'74 454 Chevy were .140".


Wed Mar 16, 2016 11:10 am
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Joined: Tue May 06, 2014 5:34 pm
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The tool used to bore the hole is the correct guided one from Quadrajetparts.com. My friend for who I did this for ran a 7042262 before finding the correct 7042270 and both carbs used the same casting number on the base plate, and everything is the same (t-slot and idle air bypass) but I'm not sure anymore about the idle screw discharge size. He told me he enlarged those but I'm not sure anymore if it was on that carb (getting old LOL).


Wed Mar 16, 2016 12:34 pm
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I understand the tool is a piloted reamer. They are handy and useful, but, because the constant intake vacuum continuously pulls the butterflies down, the shaft bore wear is predominantly the bottom of the shaft bore, so the pilot follows the lower axis of the worn hole. Not much can be done about it if the bore is worn significantly, like it can be in a carb that was run without an air filter for a long time or was run in dusty conditions, like a farm truck.

When the QJet first came out in 1965 the original shaft-to-bore clearance in brand new QJets was looser than old-school carb guys were used to seeing and this was a cause of concern among old-school carb rebuilders. However, there is a method to the seeming madness (of the loose fit). Rochester figured out that since shafts and bores inevitably wear, and with a tight fit originally, any wear progressed from nearly no leak to significant leak, which had negative effects on the "must meet emissions for XX,000 miles" EPA mandates. It was determined that when calibration was done with a loose initial fit and some leak to begin with, the progressive increase of leak wasn't as significant a change, because some leak to some more leak is a smaller (ratio of) change than zero leak to some leak.

Most of the bases were manufactured with an 11/32” counter bore in the entrance of the 5/16” shaft bore. To cut down on air leak you can use an appropriate length of 11/32” OD X 5/16” ID brass tubing and just press it or drive it in, no need to get out the reamer unless the 5/16” bores are really bad.


Wed Mar 16, 2016 5:48 pm
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Joined: Tue Feb 18, 2014 1:48 pm
Posts: 55
At this point couldn't you just taper the edge of the throttle blade to make the transfer slot wet so no hesitation.


Thu Mar 17, 2016 6:00 am
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Joined: Tue May 06, 2014 5:34 pm
Posts: 45
tbarb wrote:
At this point couldn't you just taper the edge of the throttle blade to make the transfer slot wet so no hesitation.


I was not saying it was causing a problem, just ascertaining the fact.


Thu Mar 17, 2016 6:35 am
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Joined: Tue Feb 18, 2014 1:48 pm
Posts: 55
It's funny you mention the transfer slot relationship with the closed throttle blade. On the AFB's I own the blade has a taper at the slot location plus where a ported feed hole would be but there is no feed hole.

On a 3810 Holley I have the relationship is exactly the same as you describe, only it seems to be just a hair above the top of the throttle blade and if you don't get approx 3/4 turn on the idle speed screw there is a hesitation.


Thu Mar 17, 2016 7:42 am
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