MM don't forget to use new, un-oxidized, main jets. As it is the leading edge of the jets orifice that is worried when the jets are sized with a polished burnishing tool. With very little oxidation, or Bloom as Kevin Cameron likes to call it, accumulated on that leading edge of the orifice makes a big change in what the jet will flow.
The burnishing tool worries the leading edge, bringing the jet into size.
Jets sitting in a tool box for a period of time cannot be considered reliable as for the size originally marked on the jet.
In the old days AMAL had two grades for jets, one for racing, and one for general street use. You could tell them apart where the street spec. jets have the annular groove around the hex (similar to jets supplied under 376/100), and the racing (3326) ones the was no annular groove on the hex. After many years to oxidize, with any original 3326 racing main jet it is only a guess how much fuel it will flow
If my memory serves me the racing were plus or minus 2.5%, while the street ones we plus or minus 5% (but I could be correct on my % figures).
With very little oxidation, or Bloom as Kevin Cameron likes to call it, accumulated on that leading edge of the orifice makes a big change in what the jet will flow.
I'm quite skeptical of Kevin's Bloom Theory.(*) However, despite the recess at the outlet end, the edge of the hex of another jet can make physical contact with the orifice. Although jets tend to align themselves along the long axis where no such contact can be made, years of bouncing around in a toolbox might be enough to have an effect.
(*)Not counting jets that have been exposed to moisture long enough to turn green.
Addendum: The flow through a jet depends linearly on the Contraction Coefficient, which is 0.62 for a hole with a sharp edge and 0.97 for one with a rounded edge. I put two jets under the metallurgical microscope, one that is a bright color and one that is a dark brown. Both have microscopic roughness around the edge of the orifice of ~15-20 µm and are rounded over a depth and width of ~20-40 µm.
A 1000 nm (1 µm) oxide on brass would be very thick and brown. Any oxidation of the jets would have to be ~20x thicker than that before it would begin to have a geometric effect on the profile of the outlet (like snow on the mountains, the oxide follows the profile of the peaks and valleys. Only if snow were very thick would begin to change the profile of the mountains).
Last edited by Magnetoman; 07/10/1911:20 pm. Reason: Addendum:
While a theory it was based on his experience with two strokes that were set aside before AHRMA and the like gave reason to dust them off. He was asked why would new, jets kept on a jet board, when used in exact conditions they were some years ago lead to consistent seizures. This was not a random experience with just one racer, but was a reasonable sample. When flowed the results did not match the the number marked on the jet.
When using jets you must remember that over the years there have been many who made jets, some cheeky enough to stamp them AMAL. With needle jets I can identify at least 10 people who make them from around the world including the US. Some like Keyster are brilliant, while others marked will be marked .106 are .109 and beyond.
If you watch the man flow the jets you will see how little he has to do to move the orifice into size.
Re: Correct needle for 1000-series Concentrics?
[Re: John Healy]
#778314 07/11/1912:11 am07/11/1912:11 am
No idea if this would be significant but the surface of an oxadized jet should be substantially rougher than the surface of a fresh clean jet. Remember that oxides grow at different rates according to the orientation of the grain they are growing on So the oxidised surface should have fingers of oxide poking into the tunnel for different lengths and at different angles. They only way to prevent this happening would be polish the jet hole to the point where tha oxides fuse to become a semi metallic glass as in the case with polishing for electroplating.
So he could have been right in his observations of a reduced flow in oxadized jets but not for reasons of reduced diameter.
However we might be getting into SEM territory and how one would measure it with out slitting a jet in 1/2 lengthwise is a bit beyond my pay level.
So the oxidised surface should have fingers of oxide ...
Originally Posted by Magnetoman
I put two jets under the metallurgical microscope, ...
I couldn't use the highest magnification/resolution objective because the working distance is less than the recess at the outlet of the jet, but I still could resolve ~1 µm in the eyepiece with the next-highest magnification/resolution objective. If there were any fingers of oxide they were smaller than 1 µm and hence negligible.