Santa (technically, my sister’s boyfriend) brought me a lovely gift this Christmas: a carb rebuild kit for my spare Thermoquad including all of the gasketry, springs, and other small parts needed to overhaul the mechanical bits. Introduced by Chrysler in 1969, the Thermoquad is a four-barrel carburetor designed with two small primary and two large secondary bores. At idle or cruise, the primary bores supply fuel to the engine in small amounts for efficiency, and under load or acceleration the secondary bores open up to dump fuel into the engine, adding power. It’s built around a phenolic resin chamber sandwiched with two aluminum plates, which was supposed to keep the temperature of the upper bowl 20˚ lower than standard metal carburetors of its day–because vapor lock was standard equipment on a 440 big block. So I’m going to have to study up on this Malaise-era marvel of engineering.
My engine, near as I can figure, is a 1979. As the 70’s wore on and emissions standards got stricter, engine displacement decreased and smog equipment strangled engines. IH trucks weren’t as strictly governed as passenger vehicles of the era (trucks over a certain weight and load limit were exempt from CAFE regulations, hence IHC advertising the Scout as the XLC in 1975 onwards) but smog equipment was still added to pass emissions tests, and the carbs got more complicated as the decade drew to a close. The Thermoquad sprouted all sorts of ports and valves and complicated linkages to govern choke, temperature, airflow and throttle, and the result is a lump of metal that looks like C-3PO screwed a pinball machine.
I’ve got three Thermoquads in my collection. The first is the one that came on Peer Pressure, which I had overhauled by my friend Rodney a few months after I got the truck. Tracing the serial number (9128S), it was originally made for a ’79 IHC 345ci engine with an automatic transmission. Rodney is a gearhead of the first order (he has a slingshot dragster parked in his driveway) and no stranger to carburetors. He made it sing, but that was eight years ago.
The second is a spare I picked up from a fellow north of Baltimore. This too is a 9128S. It’s the spitting image of the one on Peer Pressure, minus the funky extra linkage on the throttle arm to the left. It’s also the rebuild candidate and the one I’ll be working with here.
The third is a spare I picked up from my friend Jason, who converted his Traveler over to fuel injection two years ago. Tracing the serial number (9203S), it was originally made for a ’79-80 IHC 345ci California smogged engine with an automatic transmission. The top of the air horn at the fuel inlet is different, and there is one vent tube where my other carbs have two. It’s also super crispy and in need of a major wash. It will be my bench reference and possibly a source of spare parts.
The first order of business is to arm myself with knowledge. I downloaded a series of getting-to-know-you videos from YouTube where a gentleman steps through the features of the TQ, and another where he goes through the steps of rebuilding one. I’ve got the original ’72 Chrysler TQ service manual, an updated ’82 Federal Mogul manual, and the Dave Emanuel bible.
The first order of business is to clean them both, which will require a bucket of Simple Green and a couple cans of carb cleaner, which I’ll pick up this weekend. Then it’s on to the teardown.