Carb needles

[fogdot]

Still trying to nail the idle on my TR3A. I looked at the needles, and they are stamped "369". They are supposed to be "SM" needles. Do I have the wrong needles?

 

[piman]

Hello Fogdot,

it doesn't matter what needles you have as far as the idle is concerned as they are all (as long as they are the right size for the jet, i.e 0.090" for a H4) close to the same idle dimension, typically 0.089".
I'm sorry that I don't have my reference book available to confirm the correct needle.

Alec

 

[fogdot]

I think I have found the reason I couldn't nail the idle speed. I've gone through the timing and tuning routine about three times, thought I had it, but after a road test, the idle wouldn't go below 1000. I put an extra spring to hold the throttle closed, and tightened up the return springs on the throttle shafts, still no luck.
Always in the back of my mind, I had that other idle thread where there was a twig interfering.
I found interference between the linkage and the manifold. There is a horizontal throttle link that comes forward from the firewall, then a vertical link that connects to the throttle shaft.
The vertical link was too short, and ran up against the manifold at the bottom end, giving the inconsitsant idle speed.
I lengthened the vertical link, and shortened the horizontal link, and now there is plenty of clearance through the whole range of movement.
It was late in the day, and the idle is smooth and steady, but I guess I'll have to go through the mixture tuning again before I can put these tools away.

 

[piman]

Hello Fogdot,

one of those quirky little things that can happen, glad you found it.
For what it's worth, with S.U.'s of the correct specification and a healthy engine, you won't be far away from the 12 flats adjustment of the jet nut from flush with the bridge. If you haven't heard of that, just screw up the jet, (with the damper\piston assembly removed) until the jet tube is flush with the bridge then screw down 12 flats.

Alec