TR2/3/3A TR-3B carb question

[Ken H]

I'm rebuilding my TR-3B carbs and need to know if the needles should be SM or TW. I've seen both used in different sources and don't know which is correct for the engine.

 

[NutmegCT]

Ken - what carbs do you have on the engine? What type of driving do you do (road, rally, high speed, car shows only, etc.)
Tom M.

 

[Ken H]

AUC878. Car shows and neighborhood driving in Florida. Hot and sea level.

 

[NutmegCT]

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.

 

[NutmegCT]

Ken - you might also want to check out this SU guidance:

Jet Needle Recommendations 1933 - 1957 | SU Carburetters

The Jet Needle Recommendations 1933 - 1957 page for the SU Carburetters website.

sucarb.co.uk

 

[sp53]

i would get the SM

 

[Ken H]

Ken - you might also want to check out this SU guidance:

Jet Needle Recommendations 1933 - 1957 | SU Carburetters

The Jet Needle Recommendations 1933 - 1957 page for the SU Carburetters website.

sucarb.co.uk

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 H]

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.

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.

 

[charleyf]

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

 

[poolboy]

Get them both...compare how they work with your driving style.

 

[Ken H]

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

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.

 

[charleyf]

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

 

[Ken H]

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

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.

A month ago, the engine was missing some, but I would drive it for a few miles and the plugs would indicate rich with the SM needles. That caused me to tear down the carbs thinking one of the cork gaskets on the jets was leaking causing too much fuel to the engine. The gaskets were in poor shape, but I can't confirm if they were leaking or not. I think the carbs are in good shape now, but I needed to know if the SM needles were correct for the engine. Your input gives me more confidence that they are correct. How did you learn the history of the SM needle?

 

[charleyf]

How did you learn the history of the SM needle?
[/QUOTE]
I guess the answer would be experience and books. I had a TR3 in the 60's. Drove it for four plus years and never touched the carbs. Bought a TR3B in the late 90's and restored it. (My son now has that car.) That involved a lot of reading, at least for me with no computer help. I rebuilt those carbs with the help of books/ manuals. I now have restored 2 TR4's and another TR3A. I now use the HS6 SU carbs in all 3 of my cars. When converting the HS6 carbs I needed to do more research especially regarding which needle to use. Ultimately I stayed with the SM needle even in the different model SUs.
Charley