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SU Carburettor Worn Throttle Shaft Air Leak.

SU H4 Carburettor.

Thanks and credits go to Mark for contributing this information, with symptoms, diagnosis and the solution described in this article to fix air leak on worn throttle shaft.

Well those who have been around cars using the SU H4 carbs would be familiar with this issue and have perhaps wondered why SU seemingly didn't do much to address this key trouble spot in their carburetors, namely air leaks from worn throttle shafts / carb body bushings, or maybe being relatively new to the scene this is a recent development with your own car.

Symptoms: Attempts to balance the carbs when they appear to be well balanced only to go completely out of balance at the touch of the throttle. Missing, stalling and running rough at idle and driving, or stalling and the car won't start again.

Throttle shaft.
Diagnose: With a can of carb cleaner, while the car is idling spray around the area of the suspected shaft. If an air leak exists the carb cleaner will get sucked in and the car will stall. Problem diagnosed.

The fix: Offending carb off the car, it is disassembled and rebuilt involving new oversize bushings, seals, gaskets, the locating holes in the body are reamed, it also being an ideal time to re-kit the whole carb.

We all read about those clever people with well equipped home workshops, time on their hands and either a knowledge of working with such things or an insatiable desire to get in, learn the technical details and.......etc. etc.. Well those people work out solutions and get stuff fixed.

What happens if that is not the person I am? Well, cars or carbs get delivered to the repair shop, and many dollars later they are refurbished and back on the car and working as expected.

The SU butterfly 
An alternative solution  by a contributing reader, who while working to a budget and was willing to do some work though without the time, tools and technical knowhow to rebuild his carb at home, searched his local market for a second hand carb to replace the offending SU on his car. In Mark's case, his purchase happened to be from me. It is recommended to be able to physically check the carb yourself. Paying for something delivered only to discover the same problem would not be an ideal outcome. There is some play in a used shaft so to some extent it is a calculated risk you would be entering into. Your local MG car club is also a good source for parts, which was how Mark and I connected.

Happy SU H4 trails.


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