Cleaning and Assembly
Old fashioned carb cleaner soup was fortified with a heavy dose of methylene chloride, and would clean the filthiest carb spotlessly given a few hours of soaking. Unfortunately, newer formulas are not as effective, and the soup sold here in California is expensive and the results are usually disappointing. These days most effective alternative is ultrasonic cleaning, though these machines are out of reach of the casual hobbyist. For our ThermoQuad project we soaked the carb in a bucket of Simple Green cleaner, brushed and rinsed, and finished with aerosol solvent-type carb cleaner. Using the straw on cleaner spray can, we blew out all of the internal passages with solvent, making sure all of the passages were clear. Finally, the parts were blown clean and dry with compressed air.

Before beginning assembly, it pays to fully organize the carb components. We like to lay the parts out, grouping related components together. This will not only make the assembly process quicker, it will help avoid having to open the carb back up to install left over parts. ThermoQuad kits come with several extra gaskets, owing to the different primary size variations and a configuration change. Match the replacement gaskets to the originals, or carefully check the fit if the old gaskets are unavailable or destroyed. To avoid sticking gaskets and make future disassembly easier for tuning changes, saturate the gaskets in penetrating oil such as WD-40.

Assembly is pretty much the reverse order of disassembly, loading the three major castings with their sub-components, and then stacking these together and bolting it up. Most of the work here is in the upper airhorn, starting with the accelerator pump components, then laying the gasket in place, replacing the pump transfer tube, installing the secondary jets, the needle and seats, then hanging and adjusting the floats. The main body gets the primary jets, secondary air baffle, and main well o-rings, while the base plate gets the adjustment screws and the metering rod step-up lever and pin. Stack the subassemblies together, and run the bolts in lightly seated at first, and then progressively tighten in an alternating pattern.

Finish the external assembly with the accelerator pump discharge nozzles and check needle, metering rod assembly, dashpot, linkages, fuel inlet and any auxiliary devices. The carb kit provides detailed instructions on bench setting the various adjustments.

Rebuilding Pitfalls
When cleaning the carb, be aware that some solutions will damage the carb. Acids are out, since they will almost instantly strip the delicate zinc plating from the steel parts, and many will oxidize and blacken the aluminum castings. Strong industrial "purple" parts cleaners will have the same negative effects as acids, so chose your potion carefully.

Failed rebuilds usually come down to common mistakes. After installing the needle and seats, test by inverting the airhorn and blowing into the fuel inlet; the needle and seats should be sealed and closed, while air should flow freely with the airhorn upright. Make sure the square-cut rubber o-rings in the main wells are in position when the carb is assembled, as a compromised seal here will result in a severe rich condition. Always test the dashpot (choke pull off) diaphragm. This dashpot also controls the secondary opening via a controlled bleed-down orifice in the vacuum fitting, and is critical to the operation of the secondary. Install the metering rod yoke with the rods hanging in place, remembering to install the return spring first. Make sure the yoke is installed with the raised dimples to the choke tower side, or the metering rods will bind.

If there is one aspect of the ThermoQuad that can be termed complex, it is in the linkages. There are over a dozen linkage adjustments, and though it may be tempting to just bang the linkages back on, these adjustments make all the difference between success and failure. Accelerator pump volume and timing, primary enrichment, Air Valve position and tension, and even the secondary opening itself relies on the linkages and the associated pawls and dogs are adjusted correctly. Go through and set everything to the specified specification before trying any "trick" settings. If your ThermoQuad stumbles, bogs, leans out, or runs pig rich, more often than not some aspect of the linkage is maladjusted.