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MG Car Repair/MG 74 SU HS4 flooding


hi Barrie - I cannot pinpoint the reason my front carb seems to flood. I have an SU pump that seems to work ok - ticks and then stops and I inspected - no fuel excess was inside the carb. Float level seems right. I have new gross jets installed, gas in bowl  looks 3 full. Both throttle blocks ( hollow type )rise and fall ok. Oil is in pots. How do I check if the choke valve is working properly? Lately I had fuel pouring from the overflow pipe to the ground ( connected to the charcoal cannister )when motor was running ( engine ran so rough that block was shaking side to side).. Finally how can I purchase your book on the HS4? Best regards, Paul in Toronto

Hi Paul.  Are you sure you have HS4 carburettors?  I just checked my parts manual, and from 1972-74 USA and Canadian MGBs were fitted with SU carburettors type HIF4 not HS4.  If you have separate float chambers on the side of each carb then they are type HS4.  If you have integral float chambers underneath the carbs and sealed with a big rubber ring, then they are type HIF4.

Either way, it sounds as though the front carburetter float chamber is flooding.  This has got nothing to do with the choke mechanism.  The front Grose valve is not closing properly for some reason.  Possibly the float has gone porous, possibly the Grose jet is faulty, or maybe the front float is incorrectly adjusted.  It is also possible that some grit is preventing the Grose jet from closing.  

1) Remove the floats and inspect them carefully for cracks.  If they are see-through plastic, then hold a strong flashlight behind them and look for a small puddle of fuel in the bottom of the float.  Try shaking them and listen for swishing fuel.  Weigh them on an accurate scales - they should be the same weight.

2) Try swapping the front and rear Grose jets.  If the front float stops flooding and the rear one starts to flood then you have a faulty Grose jet.

3) (HS4) Remove the lid of the front carburetter and turn it upside down.

3) (HIF4) remove the front carb from the car, empty it of fuel, invert the carb and remove the 4 screws holding the lid.  Remove the lid and hold the carb body upside down.

Look carefully to see if there is any reason why the float cannot close the Grose jet.  For example, the float may be touching the lid.  

For type HS4 you can check the gap underneath the float using a small drill bit as a test gauge.  The gap should be between 1/8 of an inch (3mm) and 3/16 of an inch (5mm). If it is less than 3mm then you have found the problem.  

For type HIF4 you need a straight edge such as a metal ruler.  Place it across the base of the carb and it should just clear the depression in the middle of the float by about 1mm.  This is critical.  If the gap is more than 1.5mm then you have found the problem.  The minimum gap is 0.5mm.  

Grose jets usually come with spacer washers, so you may be able to correct the float by adding a washer and testing the gap again.  You may need 2 washers.

If you still cannot find the fault, get back to me.

Yes, I have published a book on MGBs with a chapter devoted to the SU fuel system.  It is called Barrie's Notes and it is available from the MG Car Club.  I believe that Moss Motors also sells it in the UK but not in the USA.


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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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