This Jaguar MK IX comes equipped with a pair of SU HD6 carburetors with an integrated SU starting carb (a form of automatic choke that adds fuel when the car is cold). The car is running very rich and the bottoms of the carbs are fuel-stained, so we need to figure out why.
Testing our starting carb showed that it was functioning properly. The first clue comes from the fact that this car has a leaky water pump. The starting carb runs off a thermo switch (much like a fan switch) mounted in the cylinder head. If this switch is bad, or if it is not actually sitting in the coolant, it won’t properly shut off the starting carb. We removed the switch and hooked it up to a multimeter. The switch is normally-closed, so we had connectivity when cold. We dunked the switch into a cup of water heated to 180 degrees and within 2 seconds the switch shut off, proving it was working.
While not easily visible, the jet in the front carb is set about 60 thousandths lower than the rear carb, causing additional enrichment in the front carb.
Disassembling the carbs completely, we put the bodies in the ultrasonic bath to clean out the passages and get the gunk off the exteriors.
The throttle shafts were removed and inspected for wear as we felt play in them on the car. The wear is over 5 thousandths and we will need to re-bush and install new throttle shafts in the carbs before reassembly.