Needle choice for XK 3.4

My XK120 has a B-Type head (1957-59 XK150, Mk VIII, Mk 1 3.4) #12500, with matching cams, and the block is 3.4 bored +.040". Everything else is standard, H6 carbs, single exhaust. What would be a good starting point for needles in the SU carbs?

Chris
Needles for the SU HD6 carbs [1.75in ] on a 3.4 ,with B head and 8-1 comp are as follows
Weak SJ
Standard TL
Rich WO3

You can also go to the SU needle chart web page called 'mintylamb.co.uk and pick your SU main jet size which is .090 and then chose different needles and compare them.

Many thanks for this, Morris - most useful information!
Chris

Another quite useful software tool is WinSU:

It’s helpful if you’re dealing with an engine which has been modified (or fitting an SU to an engine which didn’t originally have them).

I’m thinkin’ the jet size is .1"…

Jet size .100" is correct for original H6 carbs.
Your original needles were RF (standard) or RG (weak) or 53 (rich) with the pancake air filters.
Your B head has better flow characteristics, and if you mean the cams came with the head, then they are 3/8" lift where your originals were 5/16" lift.
A lot of other things affect performance, such as float levels, correct oil in dampers, distributor advance, cam timing, coolant temperature. Go over all these before beginning experiments in needle choice.

My B head has the 3/8" lift cams, normal B Type valve sizes, and AC pancake filters. I will check what needles are in there now, but reckon I will go with Morris’s suggestion of standard needles for B head (TL) and .100" jets, as a starting point.

Sorry jet size is .10 my error.
Thanks Lee

Lee

You are correct, .100main jet for HD6 1.75 carbs, and .125 for HD8 2inch carbs for 150S