276 carb flooding?

[MarkO]

The surface finish will certainly affect the flow rate. Nev

Not sure about that. The controlling annulus will be the tip of the jet for 10mm or so where the needle locates and the needle itself, then the main jet when throttle is wide open. The rough surface finish is on an intermediate larger bore which I don't believe has any effect. Indeed I wonder why there are 3 different bores in the jet at all?

[cmfalco]

The surface finish will certainly affect the flow rate.

The annular area between the 2.50 mm (0.0984") needle and the rough walls of the 0.125" hole is nearly 4× larger than between the needle and smooth walls of the 0.106" metering-portion of the jet, so the effect of those rough walls on the flow through the jet will be negligible.

I wonder why there are 3 different bores in the jet at all?

Two of those bores are for the 0.106" metering and for the tap needed for the main jet, reducing the mystery to the reason for the ⅛" bore, and for the length of the tap-drill size. There are explanations for both.

The overall design of the carburetor dictates the length of the 0.106" region. If it were made longer, the slide would have to lift further before the taper of the needle entered that region to start enrichening the mixture. If it were made shorter by cutting off the top of the jet, it would leave the top farther below the main bore of the carburetor where the "vacuum signal" would be too small, and if made shorter by reducing the length of the lower portion of the 0.106" bore, it would flow too much fuel unless the diameter of the needle were increased to compensate. Although it might seem the tap drill could have been run to the start of the 0.016" region, that would have left the walls too thin. Instead, a ⅛" bore was used to be large enough not to affect the flow, but small enough not to leave the walls too thin.

So, why did the depth of the tap drill extend so far beyond that needed for the main jet threads? The same needle jets were used in carburetors for small motorcycles, which had tiny main jets, and large motorcycles with huge main jets for alcohol. Having a one-size-fits-all large chamber in all of these provides a "ballast volume" of fuel to avoid fluctuations in fuel/air ratio when, say, a throttle is suddenly snapped open.

[MarkO]

cmfalco - I get why the 0.106 bore is the length that it is and thanks for the explanation for the larger bore. I still don't get the need for the middle bore. It does nothing for fuel metering...does it? So why does it exist? Tooling purposes perhaps?

[cmfalco]

I still don't get the need for the middle bore.

As a possibly-relevant aside, I already noted in a previous post that Mikuni's needle jets are spaced every 0.0002", indicating that changes at a level of ~0.0001" have a measurable effect. Motivated by that, plus having assembled an instrumentation package for measuring air/fuel ratios, some years ago I decided to see if I could control a carburetor even better by making my own needle jets. Better, at least, for the short time before wear reared its ugly head. As the next photograph shows, I have a drawer filled with drill bits and reamers from that time.

Sadly, neither I nor the very skilled research technician I had at that time, succeeded in making holes of usable quality for use in a needle jet. Of course, one possibility is both of us are incompetent. However, another possibility is that, unlike the rest of the fabrication steps, this step in the operation of making a needle jet requires specialized equipment and/or someone who has developed specialized skills.

Anyway, the ⅛" bore allows one "crude" machinist to "mass produce" all the partially-finished jets that then would be supplied to skilled, precision machinists to make the final 0.104", 0.105", 0.106", etc. bores.

[nevhunter]

Mr falco, when I mentioned finish it was in response to your comment just before it. ie where the needle runs and where the Metering is done. Nev

[nevhunter]

Needle jet wear will have most effect before 1/2 throttle as the annulus has much more area/flow beyond that due to the taper of the needle. At full throttle, the Main jet is in full play. When I say it's set up as per the book I MEAN with properly produced and good condition parts. These carbys can run quite well and it's worth persisting to get them right. Economy, nicer running, oil cleanliness ,no Plug fouling, putting stinky foul things onto those following you all desireable and PART of the challenge with this hobby. Comments too. "That Thing runs well, doesn't it?" Better than"Haven't you got that thing "SORTED" yet?"Nev

[cmfalco]

when I mentioned finish it was ... where the needle runs and where the Metering is done

Ah, sorry I misunderstood. Oh well, at least it prompted a calculation that someone might find useful or interesting.

[nevhunter]

All good grist for the Mill. Nev