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MG Car Repair/HS4 Carb Idle Problem



Thanks for your help in the past.  I'm stumped again.  I am working on my 1971 US Spec MGBGT.  Specifically, I am having carb trouble.  The car is fitted with HS4 carbs.  They were gummed up pretty well from years of sitting so I rebuilt them recently.  I have rebuilt HS4, HIF, and ZS carbs before with no problem.

I did a thorough disassembly, dip, and cleaning and reassmbly per the Bentley manual.  The car starts and runs reasonably well but it will not idle down below about 1800 rpm. Even with the idle screw backed out all the way.

I did notice that unlike previous carbs I have worked on, the throttle shaft does not rotate completely around so that both tabs on the stop come together completely.  It lacks about 1/16" on both carbs.  Are these supposed to rotate all the way until they meet or will the butterfly stop it just short by contacting the bore?

I have confirmed that the choke (cold idle) mechanism is not holding the throttle open.  I inspected the butterfly valves, and just to be sure they were seating corrctly I loosened the screws and snapped the throttle closed and re-tightened them.  I cannot see light around the butterfly when closed.  I have sprayed starting fluid around the carbs at various places and I am able to get a small increase in idle speed when I spray around the throttle shafts but not a huge difference.

These butterfly valves do still have the spring loaded button on them.  They have not been soldered down.  Is that the most likely cause of the high idle? Maybe a weak spring on these? I know it is common to replace these with a solid disk or solder them down.  Is this the symptom that causes people to defeat them?

I don't have any other ideas? Do you have any suggestions?

Hi Dave,

That was a major problem we had in the dealership even while the cars were still not very old. We were not allowed to modify them as it was a Federal offence to us to do it, but an owner could do it, so most just replaced the throttle plates with the plain ones and some had the skill to solder them shut. They were only there to lean out the mixture on a quick throttle closing for Federal emissions. They caused us a lot of grief in the dealerships.


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Howard M. Fitzcharles III


MG from 1956 (USA versions only) up and Engine theory.


Dealership line mechanic on MG, Triumph, Jaguar for 15 years, Instructor in commercial mechanics school 2 yr. Product information manager for piston and valve manufacture, Instructor & hotline answer man for import car parts importer 15 yrs.

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