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MG MGA - 'GT' Needles in 1500

My 1500 still runs rough so I checked the needles. They are marked "GT" rather than "GS." (At least they are the same!) I think these are "richer" needles. Anybody have any info on this size needle? I assume I should get the GS ones.
Steven B

Steven,

MGAs from 1955 to 1959 with a 1500cc engine were fitted with SU carbs in configuration AUC 784. The needles recommended by SU for use on that engine were:
Standard needle: GS
Week Needle: #4
Rich Needle: CC

The profile (dimensions) of the GS needle, compared to the GT needle are below:

GS GT
.089 .089
.085 .085
.0815 .082
.0785 .0795
.0755 .077
.0725 .0745
.070 .071
.0675 .0675
.0650 .0640
.0625 .0605
.060 .0570
.0575 .0535
.055 .050

The top of the needle is used during idling until about the 3rd measurement, the next 5 or so are used during pick up and cruising and the remainder is used during very hard acceleration and top engine speed.

Remember: the bigger the needle, the less fuel gets sucked up the jet and atomized in the mixture. Compared to a GS needle, this would make the GT needle slightly leaner at idle, leaner during pickup and at cruise and richer only at very top speed.

Very little damage is done by running a tad rich. However, running too lean can have disastrous results. GT needles may provide acceptable results and you probably end up "over-adjusting" in the tuning to make up for wrong profile. The good news is: the proper GS are available and pretty cheap.

Hope this helps.

Karl

K Leclerc

This thread was discussed between 16/02/2007 and 17/02/2007

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