Thanks! I haven't seen that guide before. I have an SU carburetor manual published by Haynes as well as the Haynes TR-3 and TR-4 manual. There's some missing information regarding TR-3Bs. leading to my SM or TW needles. This link you sent covers a lot of useful information, but didn't cover 3Bs. Thanks anyway.Ken - you might also want to check out this SU guidance:
Jet Needle Recommendations (1933–1957) | SU Carburetter Guide | SU Carburetters
Discover jet needle recommendations for SU carburetters from 1933 to 1957. Find the ideal needle profiles for various vehicles and applications to optimize engine performance.sucarb.co.uk
This is a very good illustration of the carb and says the SM is the correct needle (same as the Moss catalog) however it says its for the 1991 cc engine. I believe my engine is 2138 cc. I'm certain the carbs on my engine are original to the car and the engine is also original to the car. I've owned the car since 1975 when it had very low milage on the engine. The Haynes SU manual has all the needles listed with dimensions. I calculated the difference in flows between the two needles when the piston is raised about a quarter of an inch and get a difference of about 11 percent more flow from the SM needle than the TW. This is confusing because the manual says the TW is used on the larger cc engine. I have the SM needles in the carbs now but have to adjust the carbs to 18 to 20 flats open to get a good idle. My problem could be electrical, or mechanical but I'm making sure the carbs are in good shape before looking elsewhere. Thanks for your help. This manual is informative.Check this out:
PartsforAUC 878 for Triumph TR3A 1991 cc | SU Carburetters
Su Carburetters are The Worlds Sole Manufacturer of Genuine SU Carburettors, Fuel Pumps and Spares.sucarb.co.uk
Looks like SM would be your choice.
Thanks Charlie. I calculated the open areas for fuel flow at various piston lifts to compare the two needles. For the first quarter inch of lift, the flow areas are the same. With more lift, the SM needle produces about 8 percent more fuel flow. This makes sense when you think the 2138 cc engine is that much larger. I'll stick with the SMs while I look elsewhere to solve my misfiring issues.Ken, The difference in your 3B and a 3A engine is very little. When you find a reference to a TR4 and 4A, they are looking at a difference in some air pollution equipment that required a leaner needle. I used SM needles on my 3B and use them on my 2 TR4s.
Charley
I'm glad to learn the background of the SM needle and that eases my mind a little. I have an SU carb book published by Haynes that gives me all the dimensions of needles used in SUs. I used the dimensions of these needles to calculate area of the jet minus area of the needle at all positions of the needle as it's lifted by the piston. I plotted it on an Excel chart to make it more visual. It's interesting that the two needles are identical for the first .25 of travel after that, the SM needle produces eight to eleven percent more flow at equal piston lift.The SM needle was designated for the TR3 for the 1991 engine and carried over into the TR4 2138 engine.
I am sure they were looking more for performance than economy.
Interesting that the difference is the 8% you found. I have a book that shows the needle measurements, if you are interested.
Charley