When modifying the gas flow of the engine, either simply by adding a free-flowing air filter and extractor manifold or doing more comprehensive modification including camshaft and cylinder head work, it is essential that you get the right amount of fuel into the engine to match the improved air flow. If you are using SU carburettors modifying the fuel characteristics is done by changing the needle located in the piston. However, getting the right choice of needle can be a complicated process. We have detailed below a range of needles that we believe are good starting points, which may suit your purposes. We always recommend that you have the car set up properly on a rolling road to get the very best out of your classic.
The descriptions detail 'fixed' and 'biased' needles refer to how the needle is retained in the piston. Fixed needles are solidly fixed into the piston with a screw, biased needles are retained by a collar and allowed to move governed by a spring at the top of the needle in the piston. Always ensure the carburettors are in good condition before embarking on performance modifications. Check spindles, jets and dashpots for wear.
It is likely that most MGBs receiving lower modifications will be using replacement/rebuilt carburettors, most likely HS4 & HS6s as these are more suitable for power gains and have full needle recommendations. HS4s are unlikely to be adequate for serious power gains or for a 1950cc engine so a simple needle change cannot be recommended.
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