MINTY LAMB AND CARB NEEDLES

I've just had a pleasurable read through the November issue and noted in your report on the Minor's carburettor woes that you mention the needle is a number 6. That is quite different to the H6 needle, which is the rich needle 1098cc option. For many years I have found the website at www.mintylamb.co.uk/suneedle to be a brilliant way of viewing the profiles of different SU needles, helping to work out alternative needles when looking at non-standard changes on an SU equipped engine. This site really helps focus on detail changes in areas of the needle profile where you have a need to alter mixtures. For example, attached is a screen grab from the Minty Lamb website where it shows the profile of the standard AN needle on the lowest green line, the 'rich' option H6 as the next line up and the 6 needle you currently have fitted at the top. As you can see, just off the idle area setting on the lower left of the line, the 6 needle moves up very quickly and is significantly richer than the H6 'rich' option needle is to the standard AN needle. (The 6 needle is actually the 'rich' option for early MGB twin carb engines.)

[For the benefit of readers who did not see my initial report, the problem was that whenever I used full throttle on the Minor, the engine got bogged down from the mid-rev range. However, if I lifted my foot off to about 3/4 travel on the accelerator, I could feel the engine get a new burst of life and rev freely on up to the higher reaches of its puff – Ed]

I suggest the next move should be to fit a new AN needle, and fit that with the shoulder of the needle level with the base of the piston as you now have with the current needle. Fit this without any resetting of the mixture nut initially because the idle settings of almost every needle are effectively the same, and since setting the mixture of an SU is done at Idle, this means that once set with one needle it should also be applicable to any other. Then it is the difference in the needle profile that dictates changing fuelling rates during driving. Try the new needle with a test drive and if any adjustments are needed, then they should only be quite minor (pun intended!).

The other thing that comes to mind with the obvious very good performance of the fuel pump is that it shouldn't be so strong that the pressure overcomes the float valve. Clearly if that was at a significant level, then there would be the traditional dripping (or pouring) of fuel from the float chamber overflow and the characteristic petrol smell from 50 paces, but a small degree of overpressurisation every pump pulse could, over a longer period of idle, see a deterioration of engine idle smoothness and lowering idle speed, probably with an over-rich hunting developing as the slowly increasing internal fuel height in the float chamber translates to a higher fuel level in the main jet and associated richening of the mixture. When the engine is under load, the fuel demands of the engine relieves the float valve.

With your new carb there shouldn't be any wear issue in the float valve, but being a small float on the HS carb means it tends to be a little more susceptible to being overcome by higher fuel pressure. I would usually like to see no more than about 2.5-3psi fuel pressure to avoid this. If you do suffer from any idle degradation when the engine is idling for more than a minute, it may well be worth checking the fuel pressure and if pulses are exceeding that level, then an in-line fuel pressure regulator would smooth out those pressure pulses, and is usually somewhat cheaper than replacing a good working pump.

Interestingly I have seen an increase in the instances of pumps over-pressurising in recent years and whilst this can often be laid at the door of non-original fuel pumps, it can still apply to original SU pumps too. And of further interest, my first experience of an SU pump over-pressurising was in 1978 on a standard 1971 MGB where a Filter King type of regulator/filter provided a 10% improvement in fuel consumption and a smoother, more stable idle.

Roger Parker

Hello Roger, great to hear from you, and thanks for that feedback – very useful indeed and a likely candidate for the source of my problem. I have just put the Minor into storage for the winter, not because I want to take it off the road until spring but mainly to create a little more room in the garage to work on the Midget and Mini Clubman. I have ordered an AN needle though, and will fit it as soon as I get the Minor back. There is no problem at idle incidentally, just the full-throttle bogging down so I have high hopes of this needle swap doing the trick – Ed.