Needle info on a SU H6 Carb

[Zitch]

Is a SM needle interchangeable with a W02 needle?

Rebuilt my carbs and the kit didn't come with needles, but a spare parts package I was given had 2 new W02 needles in it. Should I just use the old SM needles with the new jets or can I use the new W02 needles? They look very similar side by side but that's with my eye.

Thanks,

 

[Zitch]

I found the difference between the two but what does this mean?

SM .100	w02 .100
0.099	0.100
0.095	0.095
0.0915	0.091
0.0885	0.087
0.0855	0.0835
0.0825	0.081
0.0795	0.0785
0.0765	0.075
0.0735	0.0732
0.071	0.071
0.069	0.0683
0.067	0.0657
0.065	0.063
0.063	0.061

 

[TR3driver]

Each number is the diameter of the needle at a "station", which is just a given length from the base. This chart might make the comparison clearer:



Looking at that, my guess is that the engine will run fine, but burn more fuel. The WO2 needle is closer to the factory "rich" RH needle.

 

[Zitch]

Randall,

Thank you for the quick reply. In your opinion should I use the new richer needle or the old SM which looks to have some wear?

Ted

 

[TR3driver]

If you can see that they are worn, then I'd probably use the new needles for now, and get new SM needles on order. Also be sure to get the jets centered so the new needles don't wear.

Worn needles can lead to an insidious problem that can cause poor performance, overheating and even engine damage.

 

[MDCanaday]

If you want to be truly precise the car needs to be up on a chassis dyno and several sets of needles
run and checked for proper fuel mixtures through out the RPM range.By a process of (witchcraft??)
a good SU man with a large selection of needles can find the ideal set-up for the way your motor
wants its fuel mixed. Any performance mods will make the beast unique and finicky to feed.
The needles on my stage 3 tr4 motor took hours to dial in .......In the end there was no needle that was perfect and
the closest one got sanded lightly in the middle stages to get the right profile. This got us to 155HP@6800RPM
MD(mad dog)