Get the Needle by David De Saxe
Gosh, doesn’t time fly when you’re having fun! Before my present B (my seventh, for heaven’s sake), I last owned one in the 80’s and 90s. If those of you who keep all your back numbers can be bothered to consult the June and December 1990 issues of “Safety Fast!” you’ll find details of an experiment I conducted with a whole host of different carburettor jet needles.
The driving force (ha ha, sorry) behind my quest to find the needles to optimise the road performance of my otherwise bog standard B was my replacement of the standard Cooper, paper-element air cleaners with ones of the direct flow type. These allow a greater volume of air to flow with less restriction to the carburettors and inlet manifold, so that – in theory – if you don’t change the original jet needles fitted to the carb. pistons to match the enriched airflow, your B (or whatever) won’t be able to perform to its optimum. In other words, by allowing more air through you weaken the fuel/air mixture.
The B I had at the time was an early GHN5 car (recessed grille, Rostyle wheels). The one I own now is a slightly earlier model (GHD4, introduction of alternator/negative earth and synchromesh on first gear), but the principles of jet needle experimentation apply whatever your model of M.G. – providing it is still equipped with its original (or similar replacement) S.U. carbs.
Since I wrote the original article fourteen years ago, the composition of the petrol we now use has changed enormously. To quote two obvious changes, to-day’s fuel lacks any lead content and is more volatile – both of which features are undesirable for a BMC B-series engine as fitted to our MGBs. Most of us have taken steps to overcome some of the deleterious qualities of today’s petrol either by having hardened valve seats fitted in the cylinder head or by using a suitable additive when we fill up, so that we can burn unleaded fuel with less cause for worry, even if performance is reduced by virtue of having to retard the ignition. However, finding more suitable jet needles can help here by enabling us to recover some of the road performance lost through the need to use inferior fuel.
It should go without saying that, before you start experimenting with carburettor settings, you need to be confident that there are no fundamental nasties affecting your B’s ability to perform to its best. I mean here that your engine’s valve timing, tappet, ignition and plug gap settings are all correct, the distributor is in good condition with the contact breaker gap correctly set and the vacuum advance working properly. Piston crowns and valves should be free of excessive carbon deposit (otherwise pre-ignition is likely to occur). Finally, brakes shouldn’t be binding. If these prerequisites aren’t tickety-boo, messing with carb. settings won’t help performance much, if at all.
I mentioned above that by letting in more air with your direct-flow filters you would be weakening the fuel/air mixture. So you need to find richer jet needles, i.e. those with a taper profile slightly thinner than standard. However, it’s not quite that simple. S.U. needles are shaped with such a profile as will achieve the precise fuel/air mixture requirement for any engine load/throttle condition. For example, when you suddenly open the throttle to accelerate, the depression created in the manifold causes the carburettor piston to rise, pulling the needle upwards with it through the jet orifice. Because the needle is tapered, more fuel is able to pass through the jet, thus providing an enriched mixture necessary to provide the power for acceleration. Obviously a crucial element in this scenario is the degree and profile of taper of the needle.
All chrome-bumpered Bs fitted with engines specification codes up to 18GG were fitted with twin HS4 carburettors employing type 5 (18G, 18GA) or FX (18GB, 18GD, 18GF, 18GG) needles as standard for use with the paper-element air cleaners. If a richer setting were required, type 6 needles were suggested. An S.U. needle is described first by its length and then by (diminishing) diametric measurement at a number of equidistant points down its length in the direction of the taper. Using this description, let’s look at the comparative diametric profiles of these recommended needles (for the MGB’s 1-½ inch carburettors the length is always defined as .090):-
Experimentation could result in one or more of these slightly different taper profiles suiting your car best. Remember, no two engines are quite the same! As you make each experiment, do ensure that you install your needles correctly, i.e., with the shoulder flush with the piston face.
My former 18GG-engined B (Pale Primrose FMV 269J, come back – all is forgiven! Come to think of it, Pauline, do you know where she’s gone? – The car is still on the register but new owner unknown -Ed) performed brilliantly on CC needles, after undergoing the full range of experimentation. Strange to relate, though, my current 18GD-engined car, also of course fitted with direct flow air filters, failed to show the same enthusiasm for the experiment, since there wasn’t such a wide difference in performance between each change of needle. I tried to emulate each run as exactly as possible, accelerating and decelerating at identical points, etc., but although there were some perceived differences, these weren’t so marked as on the earlier occasion. Still, this experience doesn’t mean I wouldn’t recommend the exercise, because the BB3 needles on which I settled do give a most satisfying performance under all engine load conditions.
Having determined which needles suit your car best, you will likely find it necessary to fine-tune the exercise by slightly modifying the carburettor mixture and slow-running settings. Additionally, a slight – very slight – advance in the ignition timing might be found to be advantageous. This will certainly improve fuel consumption, but don’t advance to the point where the engine pinks on light load or, after switching off when hot, runs-on.
The observant will note that I haven’t mentioned experimenting with needles having weaker profile characteristics. The reason is that you’ll never need to do this unless, that is, you do all your motoring in the Himalayas or at the top of the Pyrenees, where the air at such altitudes is lean and meagre, and requires to be mixed with a correspondingly smaller flow of petrol – which can be achieved by obstructing the carburettor jet orifice with a thicker-profiled (= weaker) needle.
Finally, my humble apologies to those of you with later chrome- and all rubber-bumpered Bs fitted with 18V engines. The S.U. HIF-type carburettors on these (other than for certain cars destined for the U.S. market that were fitted with Stromberg carbs.) use different needles (types AAU and AAD). These are spring-loaded and are designed to bear against the inner wall of the jet. You can of course carry out the experiment as described above, but you’ll need to consult an S.U. manual (Haynes publish a very good one) to examine the multitude of needle options available. This applies equally to cars with 18GH, 18GJ and 18GK engines; if yours is a B reimported from America with its original equipment intact, you’ll find the S.U. carbs. to be fitted with AAE or AAL needles. Again, richer alternatives are available – they’re all in the book!
And on the subject of availability: Burlen’s of Salisbury, Southern Carburettors of Merton, Gower & Lee of Bushey and Mayson & Solomon of Eccleshall are among the friendly suppliers of S.U. needles who, for around £13 per pair, could set you off on a road of performance from your B more scintillating than your wildest dreams would have permitted you to contemplate. Remember, though, the importance of performing a health check on other relevant working parts of your engine first. Happy needling!