The SU carb is frequently criticised as being hard to tune and always needs fiddling with. If you have found that they need constant tuning then they weren't set up properly in the first place.

The SU fitted up to approx. 1972 was of the H or HS type. H type for MGs up to and including the MGA. HS2 for Midget, HS4 for MGB and HS6 for MGC. (The number denotes the size in 1/8" steps from V. i.e.: H4 = 1 ½.) The HIF type was fitted from about 1973. These have the float bowl underneath. The choke is a barrel that turns and opens up a tapered groove to allow more fuel to feed into the motor after the piston and before the butterfly. The O ring on this barrel rots with age and our fantastic new petrol. If this happens fuel can leak past causing excessive fuel consumption.
H4 and HS4 types had an adjustable jet block to centralise the jet. The workshop manual gives a description of how to do this but doesn't say that the jet adjusting nut and spring need to removed and the jet refitted for this operation to work properly. Hold the jet up as far as possible and with the jet block, just loose lift the piston up and down to centralise the jet and then tighten the jet block. Check again that the needle is not sticking in the jet. Refit the spring and adjusting nut and check centralising again. Incorrectly centred jets wear the jet and needle very quickly. This results in an incorrect mixture - usually over-rich at idle.

The HIF type is more difficult to tune as the jet adjuster has a bi-metallic strip in it and is therefore affected by the under-bonnet heat. That means that setting the mixture with the bonnet open, the engine hot and the carbs cold results in a mixture that is far too lean when the bonnet is closed and everything gets superheated in a traffic jam. One indication that the mixture is too lean is a LOT of choke and hard starting when completely cold. So to get the best results close the bonnet and go for a drive for about 10 mins and then set the mixture. On the late models, the running changes dramatically when the electric fan comes on if the mixture is way out.

Of course, it is a waste of time doing any tuning if you don't have an air flow meter - listening to the suck is not good enough nor should you go by the height of the piston. DO NOT under any circumstances stretch the damper spring if they are lifting to different heights. A piece of 8mm plastic tubing and a small container of water works with a bit of practise. Put the tube against the piston at the bottom and the other end in the water and the water will rise in the tube. Check the airflow with the throttle open a bit to 2000 rpm as well as at idle. The first ½" of choke travel is for the fast idle and the mechanism should be adjusted to achieve this. Check the airflow when doing this too. When the mixture is correct the jet is usually about 1 - 2 mm down the tube. If it is much above the jet block then either the jet or needle or both are worn. The HIF and the very last of the HS type had a floating needle to overcome bad mechanics not being able to set them correctly. This was/is a good idea except that the constant rubbing of the needle in the jet wears the jet badly, especially with around town motoring. To check for wear use a magnifying glass. There is a small chamfer around the top of the jet and if worn, this will not be there on the manifold side of the jet hole. The jet usually lasts about 70,000 miles of around town use. The HS4 type suffer almost no wear if centralised correctly as my MGB GT has done 260,000 miles on the original jets and needles.

I don't have any explanation for it and two carb service shops I have queried haven't had any problems, but I have found that the 'Fuelmiser' after market jets will not tune properly in the HS4 carbs. A very bad flat spot just above idle could only be cured by the replacement of these jets with GENUINE SU parts.

Garth Bagnall

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