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The idle mixture screws require more turns out on the 79, finer thread. They are essentially the same if you are looking to swap in a 79 carb for a 77.
Seems a few carbs could fit into the answer to your question. This is strictly a comparison of rods/jets.
The list below is according to Lars paper on tuning Q-jet. Looks like the 77 and 79 carb comparison, if your looking just at jets and rods, are similar if not identical. Carbs with * are 800cfm.
(9) 17059211 Observed stock, sealed mixture screw setting: 6 turns on one carb, 9 turns on another, 3-1/4 on another
From: At my Bar drinking and wrenching in Lafayette Colorado
Originally Posted by signify
What is the difference between a 77 carb and a 79 , when for the same 4 speed l-82
The first visual difference between the two carbs is that the 1979 carb is set up with the provision for the float bowl EVAP canister nipple in the forward side of the bowl. The '77 carb does not have EVAP hookup provision, so the carbs are not interchangeable if you want all of your engine management systems to work ('77 cannot be used on the '79 and maintain EVAP, although the '79 could be used on a '77 by capping off the vent tube).
Here is the '79 L82 - note the 3/8" EVAP nipple sticking out of the top of the forward float bowl at a downward angle:
Here is a '77 carb. It does not have EVAP provision:
Here is a 1977 carb that I have machined and modified with an EVAP nipple and a fuel slosh separator system in the airhorn so it can be used on a 1979:
:
The 1977 carb also has slightly larger upper Idle Air Bleeds, and it has smaller Idle Air Bypass orifices in the throttle plate. This changes idle mixture screw responsiveness and settings on the two carbs.
