NEEDLE DETERMINATION

To find the dimensions of a needle that will match up to the optimum results obtained by varying the jet drop during tests, it is first of all necessary to plot out the test needle.

The needle diameters in thousandths of an inch are plotted on the horizontal axis and the needle length in 1/8 in. increments on the vertical axis, conforming with the method used to dimension needles in the needle charts. The piston heights corresponding to various speeds can then be super- imposed on the vertical axis.

All needles with the same profile over the first 1/8 in. of their length will require the same jet drop for idling, thus allowing an immediate assessment to be made of the effect on mixture of the remaining dimensions. If these idling dimensions -are not maintained accurate assessment of the effects of different needles cannot be made until the alteration of the jet drop necessary to give the same fuel discharge at idling has been established.



In order to maintain a constant idling setting when using different needles every effort is made to maintain the shoulder and 1/8 in. dimension at standard figures, i.e. 0•089 in. and 0•085 in. for 0•090 in. jets, 0•099 in. and 0•095 in. for 0•100 in. jets and 0•124 in. and 0•1205 in. for 0•125 in. jets.

The position of jet drop below the bridge, and the piston lift above, are determined at idling conditions. Assuming that the idling dimensions will be retained this jet drop (shown in Fig. 26) must be added to piston lift at all readings to establish the actual distance of the metering diameter from the shoulder of the needle.

If this occurs the engine won't idle properly and will hunt around all over the place. You need a minimal clearance betwen the piston and the jet bridge to get stable idling.

I'd like to know how you go about making up a new one, the material you used etcetera. I was lucky and had a few twin SU pistons lying around to choose a good pair, but would like to fix the bad ones up.

machined up a little bit of delrin .010" oversize and bopped it into place, then trimmed the excess with a razor blade - works like a champion

I have 3 in my wallet for you Mick, make sure you get them off me on Sunday

You can have my babies Simon. Thank you very much

:)
From what I've read over the years, if the pin/striker etc is missing it allows the piston to close completely. If this occurs the engine won't idle properly and will hunt around all over the place. You need a minimal clearance betwen the piston and the jet bridge to get stable idling.
RonR

Me.....No mine is fine. Additionally we've never used a Choke on eather car, not even hooked up..and no slop in shaft