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> drilling carb jets, inspired by a weber tuning thread
larryM
post Oct 8 2018, 09:39 PM
Post #1


emoze
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inspired by recent thread on tuning webers for 2.7
http://www.914world.com/bbs2/index.php?showtopic=333667

many carburetor books, including Holley, advise against self drilling jets

- pretty much all agree on the same issues

"As to drilling jets, .... Whenever a jet is drilled, PLEASE file off the number so the next guy isn't driven to drink trying to figure out what happened!"

here is a relevant forum that summarizes it
http://forums.sohc4.net/index.php?topic=3713.0

"speaking as a machinist, if you were to open up the diameter of a jet, you should get yourself a lathe, chuck it up and remove unwanted material with a strait-flute reamer. the mental image of some yokle with a makita screw gun drilling out something as precision as a carb jet is disturbing to me. for one: twist drills are not as accurate as you think, sure they work good for drilling out the cavity in your mom's last tooth, but they dont cut perfectly on center and can only be trusted within a few thousandths of an inch to a sixty-fourth depending on the diameter of the drill. B: twist drills dont leave a nice surface finish, and a drill that small going thru brass, to obtain a decent finish you would have to turn it insanely fast, upwards of 4000 rpm.

. If you MUST practice home-dentistry on your carb jets, use kerosene as a cutting fluid, brass does not like to get hot, and thicker oils do not evaporate off and dont posess the cooling properties that kerosene does. ... as i've said to so many people, so many times, "someone who went to school a lot longer than you said to do it this way, and the owner of the company agreed, why dont you?" here's to putting our faith in engineers and leaving the duct-tape and safety-pins to MacGuyver!"

AND

"Twist drills do not make round holes. And, they often push metal rather than cutting it, if not maintained sharp. Have you never deburred a hole? Where did the burrs come from? Also, twist drills make holes bigger than their shank size. Round holes are made with reamers or are cast, possibly stamped in a die, as such.

The jet orifice is not simply a hole size. If you use magnification, you will see that the jet is ramped both into and out of the restriction orifice, and the orifice wall is rounded, not flat. The reason is that these forms help to keep the fluid flow in a laminar, non-turbulent manner over a wide range of fluid speeds. Stepped orifice walls will introduce turbulence, often in a modal fashion related to fluid speed or the pressure difference between the orifice inlet and outlet.
You've probably seen this effect with a garden hose nozzle, where at low pressure the outgoing stream is solid, but when the pressure is increased, an air pocket begins to appear at it's output. When the pressure is increased more, the solid stream begins to flail about wildly.

Drilled jets often flow LESS at holes sizes larger than smaller sized properly formed orifices, when subjected to high flow rates. Precision, well formed, holes are seldom achieved with $3 tooling, unless you are not measuring, or not looking very closely."

fwiw - ebay has 180 Weber mains for $10-$15 ea - (which i just ordered) - what's our time worth????
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