The reason for leakage is because the seat is worn too wide and deep. When that happens the needle can only seal in one specific position. The seating area is also much larger and that makes it sensitive to dirt. The needle seat should only be a very thin cone and that cone also has to be rounded for the needle to successfully seal in different positions when the engine is not running. The force from the float is very light, so a thin seat is better. When running, the needle bounces constantly up and down both from gas consumption but mostly from the violent shakings and vibrations on the road. The seat wears quite a lot and needs to be refurbished regularly. The float height must also be regularly reset.

A part of the reason for wear is the needle weight, so avoid adding any weight to it. A peculiar fact is that the ball on top an original needle is hollow! in order to keep the weight down. So it is not removable or replaceable. It is a mystery how Schebler manufactured that. An all steel needle is much heavier.

One picture shows a NewOldStock bowl, observe the pristine very thin original seat. The other pictures is a reconstruction of a previous restauration to show the steps, and the picture with the grinding cone shows a worn seat that had 4 riding seasons behind it. 3 seasons overdue! Of course that carb did leak when I put the bike on a sidestand. The needle in the pictures is not the needle used in that carb.

To dress a worn seat, the seat pedestal needs to be ground down to remove the old seat. Any remains of the old seat is bad as a freshly dressed needle tip will not seal in the remains of an old seat. I carefully ground the seat pedestal down with a flat bottom reverse cone grinding stone in the Dremel, ground until there is next to nothing of the old seat left. In this case the seat was worn so much, that it was a risk that the needle ended up too short.

Bending down the needle lever a lot because of a too short needle, is not good. The risk is that the ball will bind in some positions in its socket. Best is when the fork is straight and level, so the needle needs to be lengthened or replaced.

Because I didn't want to grind the pedestal more, next step was to drill the hole in the bowl larger with the next size drill. Observe to drill from under!! any nick or damage on the upper hole edge will ruin everything. Drilling the hole oversize has a limit, the hole can't be a lot larger than original as the tip cone is not very wide (or a larger tip has to be made). A larger hole also reduce the pressure per square seat area that the float exert on the needle, so the hole should be kept as small as possible. But when that was done I had a nice square hole edge, no sign of the old seat cone.

Then it was time to check and adjust the length of the needle with the float lever, replace or resolder the tip further down the shaft. It is difficult to center the tip again on the needle in most cases but it has to be done. Length is not critical to the fraction.

(WTF is that box over the text? Can't remove it)

The lever can be adjusted a bit, just not too much. 

Next is to shape the old needle tip. To do that I use a grey rubbery very effective polishing wheel on the Dremel. When it is new it wobbles and undulates, very unbalanced. To shape that I grind it round, centered, square and nice against a running grinder or regular grinding wheel.

Then I put the needle in a drill, support the tip against end wood and set the drill on low speed. Then carefully just touch the tip with the Dremel wheel until all wear is removed. It is so easy to accidentally grind off the small protruding needle! It's not needed for function but the possibility to move the tip down further is ruined.

The tip angle is not important to duplicate exactly to a T, close is good enough, the seat will be shaped to the new angle. Finally polish the tip shining to a mirror finish with a lint wheel in the Dremel. The tip has to have a very fine surface. The ball and its socket in the lever, is also very important they are clean. Ideally the ball should have free movement in all directions with just a small play up and down. 

Then the big time has come. To form the seat. To form the seat at this stage when the hole is sharp and pristine, grinding powder or paste can, should or must not be used. Any grinding substance will inevitably make the seat leak! I tried all common and uncommon stuff, toothpaste can be used but it is worthless.... A paste made of cigarette ash and spit can be used as a very good grinding paste to refurbish a slightly worn seat. Funny enough ash from a cigar doesn't work! Maybe it has something to do with the cigarette paper....

When the engine stops the needle has to seal in any position it can assume, and keep tight even if it would be disturbed slightly. Worn needle ball, worn ball socket, worn lever pin, a float that is tilting, all that is disturbing the needle and makes the needle unable to seal in a seat that is tight only in one position. That is the reason the seat has to be rounded. To achieve that I took a hard plastic ink reservoir tube from a pen, put on top of the needle, pushed hard and rolled the needle in a circle for some 4-5 agonising minutes to form a rounded seat less than a millimeter wide.

After that it is just refit and check float level. Put a cork in the bottom hole and fit the float bowl plum and level to the gas line and open the petcock. Instruction book says the float should ride horisontal on the gas surface and in the center of the bowl. The float must not in any position, low down or high, rub against the sides or carb center stem, and the needle should be centered in the bowl cap opening. When/if bending the lever it is safest to remove it first from the bowl.

Some modern foam floats are possible to carefully file down at the sides and in the center without losing much buoyancy, so you can get away with a bit sloppy lever pin bearing... Restore the lever can be awarding but is a bit precise, takes some good eyesight and steady hands, some precision tools and material.  

Now, the book mention a float level at 7/16" (11mm) from the bowl edge to the top of the float. That is specifications on a NEW schebler cork float, unworn everything in perfect shape presumably a sunny warm day in the spring with 1928 year's petrol in the tank.

That is a very impractical measure these days since we have repro floats with different shape and buoyancy and different petrol. Best is to measure from the bowl edge down to the petrol surface. There is no factory measure on that, that I know of. The measure I took on my carb was 18mm (0.7in).

Instruction book also tell for the first start, a preliminary rich adjustment is 2 turns open on the high speed needle, and 3 turns open on low speed needle .

If everything goes tits up, the seat is shot to kingdom come, then the hole can be drilled larger, and a fine brass tube can be inserted. Fine size brass tubes can be found in scale model building shops. Then the tube end is dressed for the needle tip and work fine as a seat. The tube is safest soldered in place because temperature or vibration can loosen it up.

On the NOS bowl the distance from the lid top, down to the seat is 21mm.