Ok, filing flats in needles is how people used to do it when using fixed needle carbs. The needle would never touch the jet and secondly it was clamped into position and could not rotate, result, filing needles works.
Unfortunately the same methods did not get changed when the su was converted to bias sprung needles. Filing needles on those carbs results in the flat section rubbing in the jet and effectively sawing into the jet and making the jet bigger. Also the needle in bias carbs rotates this has the effect of presenting a different part of the needle to the air stream as it rotates and hence produces the effect of ‘some days it runs really well’.
The point is filing flats on bias sprung needles is not the best way to do things.
Next, the idea that all needles of the same needle code being the same is a falicy. The tolerance on production is 0.0006” wide. When I modify needles about half the time I am removing less than that from the needle to make a fueling change. So I usually carry 4 or 5 versions of the same
Needle. I have on occasions tried 2 or 3 of the same needle to get a better fueling curve simply by working within the tolerance band for the same needle. Indeed I did have one engine where changing the needle for another of the same
Type when from just too rich at part throttle to being ideal.
It is not possible in most cases to take on off of the shelf needle for it to fuel perfectly in applications where you need a replacement simply because the tolerances they are manufactured to are much too wide. 0.0001” on a typical hif44 application at 2 to 3000rpm is measurable in the change in fueling it will deliver. The smaller the carbs become the more sensitive to needle variations they become.
Ac
Edited by ACDodd, 25 March 2024 - 09:47 AM.