276 carb flooding?

[nevhunter]

The cutout amount the Needle wear and position and needle jet wear all affect low to mid throttle Mixture. The main jet only affects near full throttle. A top feed needle works better than a bottom feed one for even metering .
Soot in the exhaust pipe, Blackening oil and low Mileage indicate rich mixture. You can also get exhaust valve pitting and rapid carbon build up in the Motor. IF you do it carefully, turning the fuel off and getting a brief improvement will demonstrate it Also having extra throttle with engine loaded will lean it slightly and temporarily help in the analysis. Having too large a carby tends to make tuning them more difficult.
Any AMAL carb with a separate Bowl I have set up exactly to the Book Just works fine. It's NOT rocket science. Nev

[MarkO]

As far as I am aware, main jet excepted, my carb is set up as per the book. Even so, I still think it is at fault due to wear in the component parts. It's only NOT rocket science if all parts are in known good order - and if they are not, it is quite a task finding out which one's are not and are the root cause of the symptoms displayed.

[cmfalco]

It's only NOT rocket science if all parts are in known good order

Engines are amazingly forgiving, continuing to run (albeit, badly) with a worn carburetor, valve timing not set correctly, incorrect valve lash, dodgy magneto, etc In most cases, a carburetor can be assembled with components from the junk drawer, with jets stamped with values printed in a 70-year-old manual, and the engine likely will run. However, that it can be done shouldn't be taken to indicate it should be done, since the engine, and the rider, will be so much happier when this precision component is treated as such. As Buddha himself often said to his followers, Nirvana can be achieved on a motorcycle only when the carburetor, magneto, and clutch are each in harmony. In the case of a carburetor, achieving bliss often can be as easy as replacing its worn needle jet. Put differently, needle jets inevitably wear, and it doesn't matter what else is done to a carburetor, bliss cannot be achieved as long as it has a worn needle jet.

The fact that Mikuni needle jet diameters are spaced every 0.0002", allowing control of richness to 2.5%, indicates that changes that small have a measurable effect on performance. As an aside, I can't think of any other aspect of a motorcycle where differences measured in millionths of an inch matter (0.0002" is 200 millionths). This is why the 6.3% increase in richness caused by wear of only ~0.0005", which can happen in as little as ~1000 miles, is enough to cause very bad behavior, indeed. As another aside, on a big single the increase in richness due to the needle jet often is a combination of actual wear, along with hammering of the jet into an oval by the needle.

[cmfalco]

Since this thread originally was about flooding, I thought it would be useful to show the tool I made when I was preparing my 1928 Ariel's pre-Monobloc carburetor.

In case it's not apparent, the function of the clear tube is the same as is shown for a GP in a Gold Star manual.

[nevhunter]

Aren't Main Jets, needles and needle jets readily available NEW? Old stuff in tins can't be relied on. The correct specified slide cutout amount should be adhered to. If it's too big you can't reduce it. You have to get another slide. Nev

[cmfalco]

FYI, I have a "collection" of ~30 used brass and steel Amal Standard-type needle jets, and the following photograph shows at least 16 different types/sizes of jets were made.

Since some of the writing will be hard to read, from top to bottom the markings and materials are:

1400 brass
1000 brass
800 brass
980 brass
960 brass
109 steel
109 steel
107 steel
107 brass
107 steel
2.67 steel (nb. 2.67 mm = 0.1051")
106 steel
105 steel
unmarked brass
unmarked steel
unmarked steel

Although some (many? most?) of these are worn, a worn 105 might be a perfect 106 (or perhaps I might decide to install a sleeve to return it to 105…) so, even though I never throw anything away, I certainly wouldn't throw any of these away.

[MarkO]

My tube goes into the plug on the float union boss. Point taken ref old stuff, but if an old jet orifice/needle can be accurately measured I don't see much of an issue. Slide cutaway I think is standard and there is no hesitation when pulling away. I'm always a bit reluctant to change slides out as you cannot mate them up well with the mixing chamber without a good deal of work - like sleeves etc.

[cmfalco]

if an old jet orifice/needle can be accurately measured I don't see much of an issue.

There's no issue at all with old needles and needle jets, since the flow depends only on the annual area between them. If measurements show they have the as-new OD and ID, they'll work just as well as ones that actually are new. That's why it makes sense for me to have a box filled with old needles and needle jets.

Main jets are a different story, because the flow depends on the precise shapes and surface roughness of the inlet and outlet as well as the diameter.[*] For this reason, even if the diameter hadn't changed, rattling around in a box for some unknown number of years could affect the flow. However, the rapid flow of air through a main jet is in the same "Reynolds" flow regime as the slow flow of fuel, so they can be characterized using a flow bench.

[*] Along with several other factors, flow rate depends linearly on the "contraction coefficient" of the orifice. The ~50% effect this can have on flow can be seen in that the coefficient is 0.62 for an orifice with sharp edges and 0.97 with rounded edges.

Some years ago I used my flow bench to test a dozen NOS Amal "500" main jets that came sealed in individual plastic bags (again, original Amal, not Burlen). Of these, 9 (75%) flowed within ±1 size of nominal, but the other 3 (25%) flowed as much as ±3½ sizes too much or too little. Despite knowing this, I have old main jets in a box and rely on the marked values along with trial and error[**] when zeroing in on a main jet, e.g. if a 200 is a bit too lean, I swap for a 210 and test again. But, if I were taking my 1928 Ariel to Bonneville to set a new land speed record, I'd calibrate all the jets I would be taking with me, even if they were new, so I'd know for sure that a jet marked, say, one size larger actually was one size larger.

[**] I developed an instrumentation package for my Gold Stars that includes a knock sensor that I attach to the head and an Air/Fuel sensor mounted in a pipe I temporarily swap, that that logs detonation events and Air/Fuel ratios as a function of throttle position and rpm on the road. If, say, I swapped for a main jet marked with a larger size and it happened to flow less, I would know.

[cmfalco]

If you've ever been curious what the inside of a pre-Monobloc needle jet looks like (and who among us hasn't spent many sleepless nights thinking about this?), feast your eyes on the two photographs below.

As the first one shows, the annual area between the jet and needle that determines the flow, and whose diameter is responsible for the marked jet size (107 in this case, although it has worn to ~0.108"-dia.), extends 1.0 cm from the tip, with a slightly larger bore (0.125"-dia.) extending another 1.0 cm to a larger chamber that's above the main jet.

A higher magnification micrograph shows the smooth surface of the metering portion of the jet, and the rough surface left by a ⅛"-dia. drill bit when it was manufactured.

Since the metering portion is ~0.001" larger than its original diameter, there's no way to know from this cross-section what the original surface looked like. Presumably, it was produced by reaming or broaching, and was smoother than was produced by that ⅛"-dia. drill bit.

[nevhunter]

The surface finish will certainly affect the flow rate. Nev