It's a '77 M4MC (the one I'm looking at, mine is a '78, potential carb is a 17057241, mine is a 17058241). Only difference is air horn is slighty different (has the triangle shape instead of the little nipple up front on the air horn). The '77 has the boss still for the vent tube, but just didn't get it I guess for the A bodies. Maybe they vented a different way with check valves, thermal switches, and other BS that doesn't make sense...How old is that carb? They used to have a vent on the top of the air horn with a spring loaded rubber disc. At rest it closed the hole and at any throttle opening it opened. The hole was closed up approaching emissions carbs but you still saw the castings for the spring and plug mechanism. I forget what year it happened but the vent tube was added to the carb, the tank had a non-vented gas cap, and the charcoal canister appeared.
I guess it was '78 with the downsized A body that they added the vent on there. At least for Buicks. If the boss is there, I'm sure the passage is there.
Edit: don't know if i mentioned, but the potential carb is from everyday performance. he has added the tube, so i assume the passage is there. lol.
Last edited: