Hi, got some questions about permanently installed manifold vacuum gauges.
Years ago I had one in my Chevy van on the dash - worked great! It was above the carb. Installed one in my old Camaro (below the carb - in the console) and I keep finding an oily residue in the vacuum lines and the sintered bronze snubber in the barbed inlet of the gauge that tends to plug it up and slow the response - especially on the colder spring & fall days.
Questions:
what is this residue? gas?
where is it coming from? (yes, the manifold, but why)
The vacuum hose connected to the engine is a steady downhill run to the gauge. Make a loop in the hose that is higher than its connection to the carb at the firewall THEN drop down to the gauge. That should solve the problem.
Quadrajet carburetor? If so, maybe the jet well plugs are leaking and gasoline vapors are making their way to the gauge after leakage into the intake manifold.
If your vacuum port is near the EGR port, then oil fumes from the EGR are getting into the vacuum line. Same if the vacuum port is near the PCV valve port.
Thanks for your replies guys. The carb is a Holly 4165 (spreadbore Rochester replacement) and the vacuum for the gauge and distributor advance is being taken off the port right next to the PCV valve connection point (see detail in photos here):
The odd thing is that, now that I’m looking the vacuum connections that Holley shows - they connect the choke diaphragm to the port nearest the PCV valve inlet. Mine is connected to the other port and the distributor / gauge signal is coming from the choke port. It’s been a long time, but my mechanic may have installed it that way after the rebuild years ago - true manifold vacuum.
What Holley shows as the Timed Spark Advance port up on the side of the carb - is capped!?
Houston, it looks like we’ve found the problem … !!! Keith, looks like you nailed it!
What do you guys think? Connect the choke diaphragm to the port next to the PCV valve to match the Holly documentation, cap the one beside it and move the vacuum advance / gauge connection to up on the side of the carb?
Good point - I’ll have to see if there are any spare ports available so I can get the connection away from the carb.
That aside, in the video they mention the timed advance vacuum signal on the carb is opposite what we see in the manifold- not the desired connection for the gauge!
The best place to take the vacuum is directly from the plenum just below the carburetor. Never take it from a port on the carburetor. You may need to drill and tap a hole to make a port. If you drill a hole, best place is in front of and centered between the primaries. You. may have to remove the manifold and carb to do this and make sure you don’t get any shavings into the manifold.