Correct carb app. for 1975 Blazer

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kenny78

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On national carburetors, it gives three options, light duty, federal emissions, and heavy duty. Which one would be correct and give best driveability and occasional towing etc?

I’ve never bought a quadrajet new with the factory jetting. I had a small collection and lots of scavenged pieces. No more though, a neighbors grass fire caught the barn and all my spare truck parts with it.
 

75gmck25

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It would be either federal emissions or light duty. HD is for 3/4 ton and up trucks with high GVW, which do not have catalytic converters, EGR, etc.

Rebuilt Quadrajet carb versions can get confusing because in ‘75 they went to a newer design for cars, but delayed the change on trucks until about 1980. Some sites that offer replacements don’t distinguish between truck and car carburetors, and a ‘75 truck carb is not the same design as a ‘75 car carb, so many parts don’t interchange. For example, the rods are a different design.

I used a carburetor from a 1985 3/4 ton truck on my 1975 K25 because it is the newer design but still has minimal emissions equipment (no electrical devices, etc.). It has design improvements compared to my ‘75 carb, but the only one I can think of off hand is the external adjustment for the APT. The newer Quadrajet fits without any mods to my truck. I also bought a kit and switched to an electric choke.
 

kenny78

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I appreciate the input, I did see that the trucks kept the divorced choke for longer than cars. The only reason I’m not following your route(exact thing I did on our 85 blazer) was this blazer is attainably close to original untouched, minus the interior. So a period style quad will look good under the factory air cleaner.

Plus I know quadrajets and how to tinker, not so with this Holley, which I loathe.
 

75gmck25

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One unusual “feature” of my original 1975 Quadrajet is that it has a dedicated port for the purge line from the evaporative vapor canister. The port is shown in the documentation for the carburetor, to the right of the PCV port, but there is no description of how it works (purge canister at low vacuum, etc.).
 

kenny78

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Thanks again. I’ll dial in on the one I want and report back
 

C10MixMaster

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It would be either federal emissions or light duty. HD is for 3/4 ton and up trucks with high GVW, which do not have catalytic converters, EGR, etc.

Rebuilt Quadrajet carb versions can get confusing because in ‘75 they went to a newer design for cars, but delayed the change on trucks until about 1980. Some sites that offer replacements don’t distinguish between truck and car carburetors, and a ‘75 truck carb is not the same design as a ‘75 car carb, so many parts don’t interchange. For example, the rods are a different design.

I used a carburetor from a 1985 3/4 ton truck on my 1975 K25 because it is the newer design but still has minimal emissions equipment (no electrical devices, etc.). It has design improvements compared to my ‘75 carb, but the only one I can think of off hand is the external adjustment for the APT. The newer Quadrajet fits without any mods to my truck. I also bought a kit and switched to an electric choke.



Actually there is a 1/2 ton option that was common called a heavy half ton or big 10 that are also heavy duty emissions with no cat and no egr. any truck 78 and before with a gwvr over 6000 is heavy duty. 1979 is a split year 6000 gvwr for feds is heavy duty in commifornia in 1979 it had to be 8000 gvwr. and in 1980 heavy duty was 8000 gvwr across the board.. So how do you know what you have ? The 4rth digit in the vin 1 = 1/2 ton up to 6000 , 2 = 3/4 ton or 8000 , 3 = 1 ton and 4 = heavy 1/2 ton or 6000 + gvwr...
 

kenny78

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Ok, the plot thickens. It’s a ‘75 blazer and the 4th vin is a 1. However it s gvwr at 6300. Also, I believe the air cleaner to be original, and when I cleaned off the grime both sticker and a stamped plate reference the engine meeting heavy duty standards.

I really appreciate the input from both of you all.
 

C10MixMaster

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Ok, the plot thickens. It’s a ‘75 blazer and the 4th vin is a 1. However it s gvwr at 6300. Also, I believe the air cleaner to be original, and when I cleaned off the grime both sticker and a stamped plate reference the engine meeting heavy duty standards.

I really appreciate the input from both of you all.

6300 puts it at heavy duty emissions. the 4th vin might only work on the trucks? or maybe the 4x4s already have a higher gvrw and didn't get the heavy half ton package? but at least you have figured out what you have..
 

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