Replacement genuine SU waxstat jet assembly for single HS4 carburetors. This is only for single carbs using 10 degree float bowls and is identified with a blue band. 
 
The waxstat jet was a very smart design at the time that encorporated a thermostat that leaned the mixture slightly when warm to allow for a better warm up cycle and more efficient driving. Trouble is they can fail and cause a very lean mixture, or erratic tune. These are great for daily driven cars, but If you don't want to spend this much or you want to avoid failure down the road we also offer a conversion kit to switch your carburetor to a standard jet. See