Carb. Valve Lapping

[Plankhill]

What are folks using these days for lapping compound to lap in metal bowl valve needles?

[speedytinc]

NONE.
The needle is polished.(original stainless)
Per Service manual. A light tap on the needle in seat. Suck test to confirm seating.

Hint. After passing the suck test, turn the needle 180 degrees. Test again. If you have a seal, you are good to go.
If not, your needle point is probably off center. This was an issue with repop SS needles.
The answer was to recut the needle tip or go to a viaton unit.

[Scott_Conger]

Actually, I'm all but certain that the original "needles" were Monel, but that isn't the point of the question

So here's the deal: you do NOT want to lap the valve

What you almost certainly have right now: is a slightly concave polished ring on the cone of the needle and an equally wide and polished seat in the brass barrel

What you do most certainly WANT right now: a needle which is perfectly STRAIGHT cone with no groove and not necessarily "polished" per se - this wants to seat against a perfectly SHARP corner down inside the brass body. Then, with a small hammer tap ONCE to impress the needle's taper/cone into the brass body to form an imperceptable mating seat to the cone

Folks sometimes successfully run the needle in a drill and by luck, get round enough to make a new, wide seat in the bore, but the cone will invariably have a conoidal, slightly bullet shape and just as soon as things begin to wear again, the shape of the "cone" no longer matches the "seat" that it created. That cone must be PERFECT for long term continued operation.

Why do I want a TINY seat?

Because your seal is dependent upon the pressure per square inch that the float can achieve through leverage, floating on the gasoline. The SMALLER the seat surface, the HIGHER the sealing pressure per sq/in. It is counter intuitive to the average person's sense of logic. Not only does this allow for a much tighter seal, it also presents a much smaller surface for junk to hang onto or impress into and create a permanent weep.

The unfortunate part of this is that the body must have the bottom (inside) cut DOWN to remove all vestiges of wear, and present a good sharp "ring" of metal for the needle to impress into. This usually creates trouble because the needle must necessarily sit just that much further down into the barrel and it simply may no longer stick proud enough of the body, to be lifted to full seat by the float. If it isn't one thing, it's another.

These are the actual hows and whys involved in fixing it. For the most part it will not dissuade anyone from "lapping" things if they're convinced already that it's the right thing to do, 'cause that's how Pap did it and if it was good enough fer him, it's good enough fer me. If that's the case, so be it...