OK, so I dug out my 800-cfm Q-jet to put on my new 464, and after much poking around and looking at my Doug Roe book, I realized that it doesn't have the idle bypass holes drilled like it should (see the pic below). Got to comparing it to my John Osborn - prepped 750 cfm Q-jet, and of course the smaller carb doesn't have the holes drilled in the throttle plate either, but every other part of the idle air bypass is there (so you can drill your 750 Q-jet for the bypass?) Any ways, the new engine NEEDS the bypass air feature, so how big are the holes in the throttle plate supposed to be?
James, I have several at home and will try and check tonight. I have a precision drill set so can give you an exact size. I have never seen one not drilled.
Thanks, I'd appreciate that! Looks like my 800-cfm carb had its throttle plate switched out at some point, makes me wonder if all 750's could have the air bypass added by simply drilling two holes. o No: I looked through my Doug Roe book and a couple of Chassis Manuals trying to figure out exactly what/where the bypass is, it's not a straight path like the books indicate. :error: Took me awhile to figure out where it does run, I'll have to post some pics one of these days. Thanks again! :beer
James, just checked an original untouched 71 Satge-1 carb that is 800 cfm and it is .0890 or a #43 drill bit. The other Stage-1 carb I am running I drilled it out a little more to try and get my cam to behave better at idle. I do not remember how much over I drilled it but just a little at a time. At least this is the stock starting point. You may also want to find the correct base plate as who know what else is different.
Thanks for the starting point! :TU: If it hadn't been for one little pic in the Roe book, I'd have never figured out where the bypass passage ran (chassis manual is less than descriptive). Are there part numbers or id marks on the throttle plates?
Yes there is a part number arranged in a circle on the back area of the throttle plate. I have a 71 and 72 at home will check those #s as well.
1971 big block carb base plate is #7029024 and 72 is 7024284. The part #s are very small arranged in a circle and are tough to read. The 71 has one extra vacuum port in the front but otherwise looks the same. I am not sure if the 350 carbs used the same base plates. These are the only two years I have access to. Hope this helps.
I found them - that's wild, look at my carb and base plate numbers: 750-cfm carb: I got it from John Osborn, sent him another one as a core. Carb #7044244 - '74 model year w/ automatic trans application, right? Baseplate #7024284 - same as your '72 (Stage 1?) it is NOT drilled for the bypass. 800-cfm carb: An AC Delco rebuilt unit I picked up years ago from a dealer that was closing down Carb #7042240 - '72 model year B,C,E-body w/ auto trans, right? Baseplate #7030784 - who knows what year, also NOT drilled for the bypass. I think I'll drill out the #7024284 baseplate and use it under the 800-cfm body, since it's got bronze throttle shaft bushings, and apparently 'goes' under a big Q-jet body anyways.
I posted pics of the complete air bypass passages in this thread here , in case anybody's interested.
