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Necessity of needle replacement?

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100DashSix

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I was wondering if it’s recommended to replace the needles when rebuilding carbs; the old ones I have look like they’re in fine condition and aren’t bent at all, but my rebuilt kits each came with brand new, shiny, ones. The new needles are a slight bit shorter and seem to be tapered differently, but I am sure they are the correct replacement needles for the type of carburetors I have. Should I put in the new ones or keep what’s worked before? Do the new needles lend some sort of advantage?
 
Hi 100-6, If you received new needles in the kits and you are sure they are correct inspite of the differences between the old ones by all means use them, follow the manual's instructions for positioning them correctly in the piston.---Fwiw---Keoke
 
Another two cents worth.
Carb needles are calibrated in thousandths & ten thousandths dimensions. It is impossible to tell by inspection if they are worn & pretty difficult to even measure. The older SU carbs with floating needles are not likely to wear much if the jets are properly centered. The newer SU's that have the needles spring biased to purposely rub on the jets are almost certain to wear both the needles & jets enough to disturb calibration.
D
 
Thanks for the very knowledgeable advice. The old needles are original as the carburetors have never been taken off the car. I'll put the new ones in since I don't know for certain if the old ones are worn. The carburetors are from an early BN4, when they were still using cork seals. (I had been told the carburetors after the early BN4s no longer used cork above the jet locking nut and on the jet bearings--correct me if I'm wrong.) So I'll try using the new needles along with the replacement jets from the kits.

Ah and another quick question that doesn't warrent a whole new thread: should the jet be lubricated with a spot of oil, or kept as clean as possible?
 
If you have the early H4 carbs, the bore should measure 1.5 inches. They will have a two vertical bolt mounting flange. If this is true, the jets should be exactly .090" in diameter. The needles should be marked "AJ".

If you have different carbs, the procedure will be a bit different.

(The later carbs would be the HD6. 1.75" bore, four bolt flange, & a diaphram seal on the bottom of the jet instead of the corks. The Jets would have a .100" bore & use a "CV" needle.)

The outer surface of the sliding jets should be polished as smooth as possible with crocus cloth to prevent undue wear on the corks. The corks should be soaked in light oil for a day or so to make them more pliable. The flat side of the cups should be toward the spring & the cupped side away from the spring. The corks will form into the cups as the jet is assembled.

Make sure that the jet is centered with the needle by screwing the jet adjuster all of the way up, loosening the large jet fixing nut, pushing the slide with needle (top chamber attached) all of the way down, & tightening the large nut. Check that the slide will easily drop all of the way down. If not, recenter the jet again. After the jet is centered to the needle, back the jet adjuster down about two turns as a starting point for mixture adjustment.
D
 
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