Q-Jet vacuum ports
06-19-2014, 11:19 PM
#2
Le Mans Master
All the ones on the bottom plate should be. The aluminum colored plate in the pic.
06-20-2014, 06:50 AM
#3
Burning Brakes
The ones closest to the manifold - the bottom ones. Flip the carb over and notice where these port "holes" are located - directly into the manifold.
Good luck.
Good luck.
06-20-2014, 08:26 AM
#4
Team Owner
The largest one (front/center at the baseplate) is intended for the PCV valve 'dump' port (inlet for pcv vapors). IMO, the best manifold vacuum port for the distributor advance can is the one protruding from the right side of the carb, thru the choke mecanism (should have a 2" long brass tube connected to it).
06-20-2014, 10:32 AM
#5
Tech Contributor
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All the ports in the bottom plate are not manifold vacuum. Specifically, the port on the forward, passenger side corner, coming out at a 45-degree angle, is ported vacuum intended for EGR operation. All other ports in the bottom plate are manifold vacuum, including the one on the rear, passenger side, coming out at an angle near the power brake fitting.
Further, the forward center port, just above the fuel inlet fitting, is manifold vacuum. The port on the forward driver's side can be either ported or manifold, depending on the carb and the throttle plate being used - you can test it by spraying some WD40 into the nipple and seeing if the fluid comes out of the small vacuum hole above the throttle plate in the driver's side primary throttle bore, or if it comes out of the throttle plate at any location below the throttles - Q-Jets were built both ways, so you can't say with certainty if that port is ported or manifold without testing it.
Charley -
Only the 4MV (divorced choke) carbs used the long tube on the passenger side. The stock tubes were steel. Rebuilt carbs are often fitted with brass replacements. I agree: On the 4MV carbs, this is a very good manifold vacuum take-off. The M4M (integral choke carbs, as pictured above) did not have this port installed, and there is no provision for it due to the divorced choke placement. The vacuum location for the long tube is used as the vacuum bleed hole for the hot air choke housing, drawing hot air into the choke.
Lars
Further, the forward center port, just above the fuel inlet fitting, is manifold vacuum. The port on the forward driver's side can be either ported or manifold, depending on the carb and the throttle plate being used - you can test it by spraying some WD40 into the nipple and seeing if the fluid comes out of the small vacuum hole above the throttle plate in the driver's side primary throttle bore, or if it comes out of the throttle plate at any location below the throttles - Q-Jets were built both ways, so you can't say with certainty if that port is ported or manifold without testing it.
Only the 4MV (divorced choke) carbs used the long tube on the passenger side. The stock tubes were steel. Rebuilt carbs are often fitted with brass replacements. I agree: On the 4MV carbs, this is a very good manifold vacuum take-off. The M4M (integral choke carbs, as pictured above) did not have this port installed, and there is no provision for it due to the divorced choke placement. The vacuum location for the long tube is used as the vacuum bleed hole for the hot air choke housing, drawing hot air into the choke.
Lars
Last edited by lars; 06-20-2014 at 10:43 AM.