This is a throttle needle to fit original Amal Type 5GP Racing Type carburettors. This is the 'Normal' type needle, but we can also supply the alternate 'Weak' type (identified by a '6' after the number) - for that, see Item 1143.
Price is Each
Note - these needes are UK manufactured by a specialist carburettor engineer, to the original Amal needle dimensions. As per originals, they are stamped with the GP model and type designate (as GP needles were specific to the GP size type and were available in two types - 'Normal' or 'Weak').
This model - the 5GP type carburettor were normally fitted to Shortstroke DOHC Featherbed Manx models, my own 1955 DOHC Manx has a 5GP fitted.
Background for GP Carbs:
The GP carburettor was introduced in the early 1950's and was available in a number of different body sizes (i.e. the diameter of the throttle slide) - as the size of the throttle slide bore increased, the numbering of the GP carb model number decreased - i.e. 15GP was the smallest size (comparable to the 15TT carb, and actually quite rare to find), 10GP (comparable to 10TT carbs, and fitted to the later Longstroke Featherbed Manx Nortons - I think 1952-53?), then 5GP, then 3GP. The range of inlet tract bore sizes for these GP variants is shown below:
15GP Carb: 7/8" To 1 & 1/16"
10GP Carb: 1 & 1/16" To 1 & 7/32" (Replaced the RN carb on late Longstroke Manx Nortons @1952/53)
5GP Carb: 1 & 7/32" To 1 & 3/8" (Used on most short stroke DOHC Manx Nortons)
3GP Carb: 1 & 3/8" To 1 & 1/2"
The accompanyning photograph shows a line up of 4 racing carb types - a 10TT, 10GP, 5GP and 3 GP, to show the different carb type bodies - with the model this needle is to fit ringed in RED.
The GP carburettor design was an improvement on the earlier TT and RN carb design, in that it had the needle to one side of the throttle body, and fuel enters the choke through a spray tube inclined to one side, rather than from directly underneath as on earlier designs - giving the shortest flow.
To go with this, and to give a more sensitive tuning control - each GP carburettor type had two types of throttle needle available, with a different taper - the 'Normal' type was identified by just having the model type stamped on it - i.e 'GP', or the 'Weak' type which was identified by having a '6' after the model number, i.e. 'GP6'.
Both the 15GP and the 10 GP used a needle stamped just 'GP' or 'GP6', while later 5GP and 3GP type carbs had a needle with that number on it (and a corresponding Weak needle followed by a 6 after the number).
A good starting point when setting up a carb is to look at which of the two needle types was originally fitted for your model - but as so many factors can vary the performance of an individual engine (i.e. fuel used/atmospheric conditions/condition and tuning of engine) - it is often prudent to obtain one of each type of needle and satisfy yourself which runs best (using the original Amal GP setting up instructions).
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