MG MGA SU Carbs - new needles etc. BBS discussion at MG-Cars.net

MG MGA - SU Carbs - new needles etc.

Have just had the carbs done over by an expert! I was not happy with only 20mpg. and could not lean them off at all. We found the needles were #6 which I am told are suitable for a supercharged engine! I now have new needles matched to the "MGB" 1950cc motor and test run with the fuel analyser on board. Also we soldered over the vent holes in the dashpot caps as they are the internal vented type and don't need the extra vent to atmosphere. I am told that internal venting is achieved through the ribbed section at top of the dashpot casing, as shown in the photo below. I have filled up with fuel and will report on consumption after a few miles. The new setup, with those fabulous vent stacks (thanks again to all concerned) and new K&N filters, has given the old girl a new lease on life! We also discovered the throttle was not opening fully so an adjustment has resulted in it going like 's--t' off a shovel. Boy racer reinvented, Barry.

Barry Gannon

For those not familiar with the B engine PCV see attached photo (3 brg 1800 ) Mike

Mike Ellsmore

Barry, The comments from Chris are worthy of note. You should consider changing to the simple MGA setup. The MGB system is not an engineering improvement but due to Government regulations. The only advantage of the MGB system is that it does not drip oil on your driveway. If you do not wish to use the low maintenance MGA system then you should be prepared to correctly fit and service the MGB system. The connection to the manifold should be at the centre of the balance tube so as to affect each SU mixture equally. You should disassemble the PCV valve and clean it every couple of years. The hole in the restrictor plug at the side vent should also be cleaned and blow compressed air through the hoses. Image attached. Mick

M F Anderson

The H4, HS4 and HIF4 are all basically 1 1/2 inch carbs with 90 thou jets and require very similar needles. The one thing that does affect the needle more than anything else is the filter arrangement - standard paper filters produce more of a pressure drop across them than K&N filters and no filter at all produces no drop! Engine size has little effect because the SU carb is a constant vacuum device - the piston rises to admit more air and fuel dependant on load so it is not affected by air density or engine rpm in the same way as a fixed jet carb like the Weber. There is one minor difference in the needles - early ones are fixed in the piston - later ones are sprung loaded towards the engine and are longer - this helps atomisation of the fuel. Comparing the early needles - GS & 6 fitted to MGA engines and MB, 5 & FX fitted to MGB engines. The no 6 is the richest of those but there is not a huge difference between them all - see graph below.

Chris at Octarine Services

Comparing the FX with later biased needles shows again they are all very similar with the later ACD being weaker at the top end and the common AAU being slightly richer at the top end. I suspect that emissions drove the needles weaker - up till 1974 all the UK MOT required was no smoke!

Chris at Octarine Services

Finally when you start fitting free flowing filters the needle requirement changes to - compare the AAU to AAA, AAM & AAB needles all very similar to each other in the low to mid range but richer than the AAU then above that they get progressively richer. The needle required for any one engine installation depends on the whole mix - the only way to determine which is the best fit is by rolling road and a skilled operator - often taking a slightly lean needle and using fine abrasive paper to progressively richen the needle until the optimum air fuel ratio is obtained throughout the rev and load range.

Chris at Octarine Services

This thread was discussed between 31/10/2013 and 25/01/2014

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