[Mgs] MGB Carb Qs

PaulHunt73 paulhunt73 at virginmedia.com
Wed Dec 27 01:50:59 MST 2017


Opinions below.
  ----- Original Message ----- 

  I plan to replace the weber (45DCOE) with HS4s on my 1968 GT.  The needles are bent and a float is missing.  Is it reasonable to expect success by replacing these parts (and maybe jets and float valves), after a good cleaning?  What needles and jets?

  Yes.  But if the pistons, covers and dampers have beome mismatched you can still get problems.  Always 0.090" jets on 4-cylinder MGBs from the factory.  Needles on North American Mk2 18GF engines with AUD265 carbs were FX, and in 1969 on the 18GH engine AUD326 carbs used AAE.  18GK engines in 1971 used AUD465 carbs with AAL needles.


  I have shiny carb air cleaners to use on my 1972 Roadster with HIFs, but when I replace the stock air cleaners, the car bogs and won’t accept the throttle.  Should I change jets?  What jets?

  The throughput at idle is so small no air-cleaner should be having any effect at all.  Part of the HS and HIF systems involves two small ports either side and above the main orifice.  These ventilate the underside of the piston and allow it to rise when the throttle is opened, and must have a clear passage through any gaskets and filter base-plate into the clean side of the air filter, see here http://www.mgb-stuff.org.uk/sut.htm#3  Still 0.090" jets on HIF carbs.

  Should a car start without choking?  Is that a indication of a rich mixture if it does?  I have a car with HS6s that fouls the plugs when I start it and move it in and out of the garage, but don’t drive it.  The car needs choked to start but once started the choke can be relieved immediately.  Do I have a mixture issue?  Do I need a meter?

  Unless the ambient temperature is high I would say that all SU-equipped MGBs should need some choke to start.  How much, and for how long, varies from car to car and has to be learned.  For example my roadster HS choke can be pushed back half-way immediately it starts, even in cold weather or it floods, whereas the V8 HIFs need full choke for a few seconds after starting and only gradually eased back from there, even in summer.  The plugs are the best indication of rich mixture, but no choke, or some to start then pushed all the way back straight away and it will pull away, does indicate rich mixture to me.

  PaulH.
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