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MG Car Repair/MGB 74 SU HS4 flooding causes?


MGB plugs
MGB plugs  

MGB 1974.5 SU HS4
MGB 1974.5 SU HS4  
hi Barrie -thanks for your answer and tips to 4\1\2013. I have more questions..I checked floats for free movement, air tight, checked spacing and swapped gross jets - all works same as before when I run engine. Now I am not so sure which carb floods. See plugs picture #1-4 left to right. They seem to be burning ok. I must keep the choke on full to keep engine running ( eventually leads to gas pouring from overflow cannister ) If choke is left off, and unless I do not really open up the throttle, engine stumbles and falls. I have also attached other pic of my carb set up with open bowls if this reveals anything. Any other troubleshooting ideas? Finally - can you verify firing order is 1-3-4-2 for 18GA ( I seem to have 1-2-4-3 - is that even possible and engine still runs? ) Many thanks Paul in Toronto

View of HIF6 Jet
View of HIF6 Jet  
Hi Paul.  From your pictures I can see that you do have HS4 carbs.  Perhaps the North American cars continued to use them after they changed over to HIF4 carbs in the UK.  Anyway, it makes things easier for you to check out the float chambers.  

I can confirm that the firing sequence is 1-3-4-2.  These numbers should be cast into the front branch of the exhaust manifold.  Remember, the rotor arm is rotating ANTI-CLOCKWISE looking at it from above:

1   2
3   4

Yes, you could get the engine to run with the leads plugged in 1-2-4-3 but the engine will only be running (very roughly) on cylinders 1 and 4, with the plugs of cylinders 2 and 3 getting very wet.

I suggest you proceed as follows:

1) Fit the lids to the float chambers but do not connect the overflow pipes to the lids.  Turn the ignition on but do not start the engine.  The float chambers should both fill with fuel but the pump should then STOP and there should be no fuel overflowing from the top.

2) Remove the dashpots from both carburetters (your photo shows one on and one off).  Inspect the tops of the jets.  They should both be down BELOW the level of the jet guide by about 1mm or so.  My guess is that you have adjusted them up above the guides.  If so, drop them down using the adjusting nuts.  I will try to attach a photo of what they should look like.  The photo is from an HIF6 carb, but the principle is the same.

3)  I notice that you have a gulp valve on top of the inlet manifold.  They often cause carburation trouble.  Remove the spring clip, take off the circular lid and inspect the rubber diaphragm.  If it doesn't look to be in perfect condition, replace it.

Let me know how you get on.  

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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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