Hello all,
Question: Does any one know of an adapter to allow the use of fixed-type
needles in an SU biased-type piston?
Background:
I recently got some new SU carburetors for my TR4A. New, not rebuilt. They
are very nice and appear to be made to at least the same quality as the
originals, and maybe better.
Anyway, I took the suction domes off to check the needles and make sure they
were the correct ones. As soon as I had the pistons out, I realized that
these were NOT exact duplicates of the originals. These were fitted with
BIASED needles. The original TW needles were fixed needles. As far as I
can tell, there was no exact equivalent needle to the TW profile in the
biased types. So, what gives????
I removed the needle, and..... it IS a TW needle, but it had the biased
collar.
This got me back to thinking about how much I dislike biased needles because
they wear. There is no great advantage to the biased design. I think that
it lessens the possibility that the pistons will stick due to a mis-aligned
needle. The biased design causes the needle to slightly drag on the jet
causing wear to both. There is more wear at the idle region, which will
cause it to get richer at idle. When corrected at idle, then the running
mixture is set leaner, which is not good for the engine.
Long story short, I had been thinking about fabricating an adapter to allow
a biased needle to be set rigidly in place so it acts like a fixed-type
needle.
Then I realized that the TW needle's biased collar looked added-on not
machined in place the way my BDM needles for my TR7 look. So, I took an old
worn-out (due to the biased design!) BDM needle and I was able to easily
drive out the needle (1/8" diameter shaft) from the collar! So, they HAD
retrofitted the biased collar to a standard TW needle!
So, all I need to do is make a little collar about 0.340" outer diameter,
with a co-axially drilled 0.125 center hole. 0.408" length. I would assume
someone has already done this. Does any one know of something like this?
If not, I may commission a shop to make a run of these adapters. Does
anyone want any of these? A larger run will reduce the unit cost
considerably.
Tony Rhodes
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