As recently mentioned on another thread...........

I stated, and Ian replied the following:-


['Some bikes also specify a larger main jet fitted when running a correctly-assembled Vokes....']

That's a bit odd...A smaller main jet is fitted usually when an air filter is added to a non filtered bike...

The air flow is restricted by the filter so petrol supply is reduced to compensate...Ian.......

This makes sense...........

But the Matchless G3L Tecnhical Inspection Standards Handbook, Electrical and Mechanical Engineering regulations, Wheeled Vehicles B418, issued 21 October 1953 (same format as the BSA M20 one) specifies the following:-

Main Jet:

(A) With Air Cleaner, Size 160

(B) Without Air Cleaner, Size 120

This is for the G3L running on a 275 carburetter and 120 is the normal size jet. Seems that when a Vokes was fitted, AMC specified a larger jet, not smaller......

But a check of other manuals and parts lists for BSA and Norton show that the standard size main jet was retained regardless of whether a filter was fitted or not......even Ariel, the W/NG using the same 275 carburetter as the G3L, specified the same standard (120 size) main jet.............I'm not sure about Triumph or Royal Enfield though (3HW and WD/CO) as I don't have the info here to check........

Strange that a Vokes was still specified as a fitting during the early 1950s, or maybe they are referring to a different type of air cleaner ? Perhaps the change of main jet was a post-war thing ? And for AMC only if an air cleaner was fitted.......

The Ministry seem to have based carb settings on their own ideas, particularly during the period when they became obsessed with fuel economy and the results were not necessarily correct or practical...

Both the Triumph 5SW and the Velocette, for example, won't actually run correctly on the jet settings specified by the WD...

BSA service sheets and manuals show an erratic situation vis a vis jet sizes from model to model..

A randomly selected service sheet shows the C10,C11, B31 and M20 for example having no change in jetting with an air filter fitted, yet for the B33, M33, C12 and some A series twins reductions in jet sizes are listed...

Also, manuals, service sheets and parts lists have different recommendations for different years.
The B31 for example is shown with no change in jet size for some years with a filter fitted but a reduction of three sizes for other years...

My recommendation would be to follow Amals advice and to use the laid down settings only as a guide or base setting. Then, adjust the carburation to achieve the correct mixture if required after testing and not to assume what is stated is definitively correct.....Ian

email (option): ian@wright52.plus.com

Whats a rivet counter to do?? Fit a 160 main jet as per the Technical Inspection Standards Handbook, Electrical and Mechanical Engineering Regulations, Wheeled Vehicles B148 and put up with the black smoke, eight stroking etc, or risk fire, brimstone and heavenly damnation by fitting a 110 jet?

..It's a tough call....and you'd better fit all second hand parts as well to preserve the patina..Ian

email (option): ian@wright52.plus.com