SU needle sizing

For all cars made before 1945
Posts: 5
Joined: Wed Sep 19, 2012 9:28 am
Name: john Walker

SU needle sizing

Postby MARTINI » Sun Sep 23, 2012 12:24 pm

My 12/50 trials special has a 12/70 engine and gearbox. the present carburettor is a inch and a half bronze bodied SU ,bronze piston also.It is running vastly rich and cannot be leaned out with the present needle, I will replace the 90 thou jet , but what needle should I try, to get roughly in the right area for correct running. The carb is bodged onto the original cut down inlet manifold with jubilee clips and rad hose. I will, as soon as I can, fabricate new inlet stubs and balance pipe and convert to twin SU. In the meantime the Trials season approaches and I need to get car running as well as possible.Any advice welcomed. I know of the Minty site for needle comparison , but it is the correct ball park start point I am after. Many thanks

Silver Eagle
Posts: 10
Joined: Thu Aug 16, 2012 5:18 pm
Name: Philip N W Robertson

Re: SU needle sizing

Postby Silver Eagle » Fri Jan 04, 2013 4:26 pm

If the carburettor is has had significant use, the jet and needle are likely to be worn.

The jet would be oval and the needle will have a shiny line down the side, so a quick start would be to replace both as like for like before experimenting with different needles.

Another check to make is to test how fast the suction chamber falls off the piston. put the piston in the suction chamber, put a bolt with a large washer in one of the suction chamber screws, hold the piston upside down blanking off the piston transfer holes with a finger or two. With the piston in the top of the suction chamber, let the suction chamber drop down, this should take about 5-7 seconds. If it is significantly quicker, it will give a rich mixture.

I do not know the 12/70 that well, but 1.5" sounds a bit large for the 12/70? I am reasonably familiar with the SU carburettor, I have 3 on my Silver Eagle. I have found that Burlen Fuel Systems are very helpful on S.Us.

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