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):-