maybe there is a stock weber needle and seat that is close to the nikki needle and seat, then you'd know what to order the gross jet for?

 

I did some checking on the needle seat that I had laying around in my garage. From my measuring I am inclined to say that it is M8 x 1.0 thread. Can anyone else confirm?

Jamie

 

OK, measurements are:

SEAT:
Threaded portion, not including the narrow top 'nipple' that the final screens sit on, is 7.52mm long.
Outside diameter of threads is 7.81mm, so M8.
Thread pitch is 1.00mm peak-peak; M8x1.0
Main body overall length including threaded portion and 'nipple' is 22.90mm
Main body length not counting threaded portion & nipple is 14.00mm from the land that touches the carb body to the outside face.
Main body OD is 10.28mm.
The aluminum crush washer that goes between the seat land and the carb is 0.46mm thick, uncrushed.
Seat main bore (where the needle body slides) is too small for me to measure directly via caliper; looks to be about 5.92mm by visual inference.
Seat main bore depth is 11.32mm from outside to the face that is drilled for the needle
Needle seat bore (where the rubber tip sits) appears to be 2.00mm


NEEDLE:
The needle itself is a bit harder to quantify, because 1) it has a compression spring inside it, and 2) the tapered rubber tip protrudes into the seat bore, but I can't directly measure how much (it's deep in the bore at that point) so the working length has to be inferred.

The three-piece needle, with spring uncompressed, from tip to tip, measures 17.25mm in length, and 5.37mm across the high-points.
Fully compressed, the needle is 15.70mm cap to tip. So, total spring travel is 1.55mm.

When inserted in the bore and seated but with spring uncompressed, the overall length of the assembly is 27.25mm, and the needle protrudes 4.54mm.
Adding the bore depth to the needle protrusion, that means the "working" length of the needle, cap to seating circle, is 15.86mm uncompressed, and 14.31mm fully compressed.

That total spring travel (1.55mm) is the finagle factor, because we don't know the amount of that travel that must be compressed in order to close the valve against fuel pressure. Does the spring have to be fully compressed to close the valve, or only partly compressed... or not compressed at all?

I don't know a good way to tell.

 

Thanks DD and Jamie for your hard work! Hopefully, we can find something that will match up. If I find anything, I'll post back.

That's a good question. I would think that whatever state the spring is in during the initial float adjustment would indicate what's needed to completely shut off the fuel flow. The workshop manual doesn't clearly say either way.

 

Measuring the compression of the spring while the float rests on it in the inverted 'spec' will be tricky; it's not easily reached for measurement in that position.

Might be able to do it optically. I'll experiment if I get a chance.

I suspect the spring is only minimally compressed to seal the valve; consider:

Stock fuel pressure: 3.7-4.7PSI
Seat needle bore diameter: 2mm
Area of a circle 2mm in diameter is Pi(r^2).
Radius(r) is 1mm or 0.03937 inches. Squared, that is 0.00155; multiplied by Pi, that's 0.00487 square inches.
Total fuel pressure exerted on the needle is only 0.018 to 0.022 lbs.

I don't have a scale accurate enough to determine the spring rate, but it's moderately stiff for it's size. It's not going to compress a great deal under that loading.