[Pmac]

“When you have eliminated the impossible, whatever remains, however improbable, must be the truth.” A.C.Doyle.
Condition: Stumble/stalls/will not restart/fuel overflowing from primary Qjet chamber and at least a cup of charcoal pellets from the canister sitting on the secondary air doors/valves of the Qjet on my ’81 C1500.
Cause: The air cleaner “THERMAC VALVE” temperature sensor did not bleed off vacuum – with the thermac air sensor stuck in constant hot Air Delivery Mode, no matter what the ambient air temp (about 75°F that day), vacuum was maintained to the snorkel motor/door(s) assembly , holding damper door up, shutting off outside air and allowing only hot air travelling up the tube attached to the heat stove (around the exhaust manifold) to enter the air cleaner through the tiny door built into the bottom of the snorkel – no fresh air source ; as the engine was up to normal operating temp, the choke fully open, and secondary throttle plates free to travel their full range of motion – the top air doors responded to the back barrels vacuum signal – as the defective THERMAC sensor’s vacuum signal held that snorkel valve shut tight (try assembling a factory air cleaner with out the top nut and watch what happens when the engine is revved to 1500 rpm, it will suck the top can down with some force – and that is just the draught on the primary side ).
When I went to W.O.T. that day (I have read posts in the past stating the snorkel valve would be pulled open by the carburetor’s fresh air demands but no so) the failed 40-year-old charcoal canister filter did not stop pellets from actuality traveling through the hose marked “ACLNR” on the canister to the “filtered air” hooded port in the air cleaner – ( that port is inboard of the air filter(s); this air cleaner still has the tall fine mesh ring that the “polywrap” once surrounded ) and charcoal was deposited into the primary chamber of the Qjet – resulting in a severe stumble, when I backed off the throttle, and the secondary air doors snapped shut, resulting cup or so of pellets dumping atop of the door plates, at least as much was ingested by the primary side.
Correction:
Removing the Air Cleaner I cycled the sensor several times with a heat gun checking each time with a infra-red thermometer – it did begin to function however but not within a practical range – (the Air Cleaner ‘can’ internal temp exceeded 140 F and the valve did not fully open) in my part of the world this system is an asset, as the outdoor temperature can be as high as 100°F and low as -40°F.
I removed the Qjet and cleaned out all the passages – and it functions! Remarkable how robust these Qjets are. I have not yet got the choke and secondary doors back to where they were pre-charcoal catastrophe – this carb used to start with one set of the choke no matter what how cold or hot it was, and the secondary air doors did open just as I wanted (it did take a week or more of set-up a few years ago to get there). The truck lives on.
So I hope this will help others – beware of the charcoal can and the THERMAC sensor.