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MG Car Repair/SU HIF carbs


Hi Barrie car is a 1977 british market RHD.Was interested to read the correspondance from Ron  re petrol height in the jets with dashpot etc removed.Was there a satis factory ending to the story.I am asking as have come across same problem-customer says after warming up car after a couple of miles car would appear to run out of fuel.He tried changing the pump/filter/pipe lines/timimg/changed to electronic distributor(ugh)/new coil/new needle valves/think he stripped carbs as all appears new inside.I tested car and it was as he said after a short run it gave all the symptoms of lack of fuel but it held tickover and with choke pulled out could struggle home.I have tried with an independent petrol pump and supply-replaced distributor with points type again wired independent of the cars wiring-new lucas 12v coil-replaced modern type plug leads with copper cored type wire -set new champion N9Y plugs.No matter what I tried problem was still there.Removed carbs and found leaking float in rear-replaced checked float level  adjusted to the 1mm clearance-checked to see if jet was free to go up and down with the mixture control screw and all the other checks for SU`s replaced on car with out dashpots and turned pump on,petrol arriving ok BUT no sign of it in the jet tube-blew gently down the tube and could hear straight away the the bottom of the tune was not completely covered let alone up the jet tube! seeing where the choke picks up its feed at least showed how the engine could run with choke pulled out!Now I have stripped down quite a few SU`s over the last 50 years but always the earlier models and this is the first time that I have opened up the bottom end so if the owner had made a mistake in his re-assembly I did not see it-I did not go any further than changing the leaking float and carefully measuring and adjusting. the float height.Checking action of needle valves and ensuring overflow pipes open to atmosphere checking piston free fall etc.Needless to say that valve gaps checked,ignition timing checked,and for just ease of adjusting carbs and distributor, vaccum advance blocked off.Any light that you could throw on this problem would be much appreciated-by the way I am based in France.

Hi Paul.  The design of the HIF carb choke mechanism is unlike any previous SU carb.  When you pull the choke out a bar rotates which opens a secondary "cold start enrichment device".  The fuel for this device is pressurised, and it bypasses the needle valve.  

I therefore have 3 possibilities for you to check out:  

1) Your primary jet is blocked,  

2) The float chamber does not have enough fuel in it.  This can happen if the bi-metal adjusting device is not correctly located on the dimple on the end of the adjusting screw,  

3) I have also come across a situation where the jet is adjusted so high that it touches the bottom of the piston, completely closing the jet.  This usually happens with badly worn jets.  

Let me know what you find.

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Barrie Jones


MG Sports Cars from 1949 to 1980 - Including MGB, MGBGT V8, TF1500, TF, TD and modern Midgets. I also specialise in SU carburettors, Lucas wiring, suspension, steering & brakes.


I have owned my 1955 TF1500 since 1966. Technical specialist for the TD and TF with the MG Car Club T Register. Also owned 20 MGBs and currently own an MGBGT V8. Written 3 books on MGs and produced a DVD on how to strip and rebuild the TD/TF gearbox.

MG Car Club.

Barrie's Notes on the 1953-55 MG TF (author). /=====/ Barrie's Notes on the MGB (author). /=====/ Barrie's Notes on the MGB GT V8 (author). /=====/ The Essential Buyer's Guide to the MG TD, TF and TF1500. (author)

M Phil, C Eng, BSc (Eng).

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