Quadrajet Primary Metering Jet discrepancy

[Shane F.]

Hi all,
I am in the process of restoring a Rochester Quadrajet 4MV p/n# 7040204 for my 1970 LS5 Auto.
It is the original carburetor and has what I believe to be the original Primary Metering Jets stamped #78. The jet orifice measures as I would expect at 0.78"'
My question is around the purchase of #78 jets ordered from Quadrajetparts.com. They do not measure 0.78'', I get 0.73'' at most with a digital Vernier. I raised this issue with the supplier and they sent me another set, unfortunately the same issue.
Anyone experienced this before, do I drill out the new jets or install the originals? I read somewhere that the leading and trailing orifice chamfer effects overall flow.
Thanks in advance,

Regards Shane fever.

[Joe R.]

Re: Quadrajet Primary Metering Jet discrepancy

I would not recommend drilling out the jets as they will never flow to .78 after drilling. Use the originals unless damaged as they do not wear out. They can be damaged by the metering rods so I would carefully inspect them.

JR

[Shane F.]

Re: Quadrajet Primary Metering Jet discrepancy

Ok, thanks Joe.

[Domenic T.]

Re: Quadrajet Primary Metering Jet discrepancy

Shane,
Sometimes the digital vernier lies a bit when measuring small orfices. I back up measurements using the shafts of number drills. They are not always the exact size but work well. Example being that a .077 shank should go in a .078 hole. you need measure the shafts after selecting a drill as most drills drill a bit larger and a good set will be a bit smaller to compensate. I have drilled jets in the past with good results but there are doo's and dont's.

Dom

Dom

[Joe L.]

Re: Quadrajet Primary Metering Jet discrepancy

Shane,
Sometimes the digital vernier lies a bit when measuring small orfices. I back up measurements using the shafts of number drills. They are not always the exact size but work well.


Dom

Dom------


Yup. I agree. That's the way to do it and is what I do.

[Shane F.]

Re: Quadrajet Primary Metering Jet discrepancy

Thanks Dom,
You are correct, the measurements with with the digital vernier were not accurate. I did the go/no go drill trick as you suggested, after careful drill selection and measuring I got a completely different result. The new Vendor supplied jet is sized correctly, the original was larger than stamped, hence my confusion.
Regards
Shane

[Joe L.]

Re: Quadrajet Primary Metering Jet discrepancy

Thanks Dom,
You are correct, the measurements with with the digital vernier were not accurate. I did the go/no go drill trick as you suggested, after careful drill selection and measuring I got a completely different result. The new Vendor supplied jet is sized correctly, the original was larger than stamped, hence my confusion.
Regards
Shane

Shane------


Perhaps someone had previously drilled out the original jets. If so, they should also have changed the primary metering rods. In that case, you may need to replace with the correct rods.

The correct primary metering rods for your carburetor were GM #7034849. These are double taper type rods and should be stamped "49B".

[Shane F.]

Re: Quadrajet Primary Metering Jet discrepancy

Hi Joe,

I think Bubba may have had a look around at some stage, the primary metering rods were mismatched, stamped #41B and #44B. Probably an attempt to compensate for the worn or drilled jets.
I have a pair of primary rods stamped "49C". Are they a later design, the taper measures the same but with a more defined step. Do you know if they are interchangeable?

[Joe L.]

Re: Quadrajet Primary Metering Jet discrepancy

Hi Joe,

I think Bubba may have had a look around at some stage, the primary metering rods were mismatched, stamped #41B and #44B. Probably an attempt to compensate for the worn or drilled jets.
I have a pair of primary rods stamped "49C". Are they a later design, the taper measures the same but with a more defined step. Do you know if they are interchangeable?

Shane------


I believe the "C" suffix primary metering rods were a special taper rod used only for some non-Chevrolet applications (maybe Oldsmobile or Buick??). I do not know if they would work for your application but, if it were me, I would not chance it unless I could confirm with someone very experienced with Q-Jets.