Bike runs well. Idles noisily at 1000rpm when warm. It's always (in recent years) run rich. Plugs are sooty as all get-out. I tried an experiment after reading about needles and the things they stick into wear out and richen the mixture at lower throttle openings (which is where I and old ladies usually reside). I lowered the needles one notch. Bike ran badly. closed idle adjustment 1/8 turn. Bike runs pretty well. Pulled the plugs and #3 is still sooty. #2 is perfect. #1 & #4 are sooty but not as much as they were. Carbs are synched. Rubber isolators were new about 2 years ago. Timing is nuts-on. Valves are correctly adjusted. Floats were adjusted on the bench with see-through tubing on the drains. Plug caps are fairly new. Grrrrr! Sure would like to get this thing right. Are the needle sets available anywhere? Should I lower needles for 1,3 & 4 to the last notch?
TL:
You mentioned that you turned the air screws inward a little: are they at 7/8 turn, now? With our modern 'gas' (to use the term loosely), that would be about the same as 1 turn with 'real' fuel. The ethanol carries along some extra oxygen of its own that breaks out during high temp combustion to help consume "leftover hydrocarbons" [sic], with the result that our engines, which don't run as hot as car engines in the combustion phase, end up with leftover unburned fuel. This ends up on the plugs, which you see.
A question: have you drilled out the emulsifier tube holes? That also helps, especially on the pre-K3 bikes.
One fuel out there that helps to mitigate this is the stuff found at Shell and Phillips. Their combustion is damped with nitrogen, which absorbs the leftover oxygen from the ethanol and turns it into a scrubbing compound, nitrous oxide.
It probably improves the humor of the guy behind you, or maybe it accounts for the drivers we see on the roads, but that's another topic...
The biggest side-effect of the ethanol is the way it artificially increases the "octane rating" of the gas. It slows down the combustion (below 200 PSI compression pressures, anyway) and reduces spark knock with this method, and increases the temperatures in the exhaust system, partly to increase the efficiency of catalytic convertors. Which we use on all our SOHC4 bikes...
In the end, what this means for us is: we can use less octane by a stretch from the old pump numbers. In engineering terms: the K0 bikes were set to 95 octane (old numbers) before the (R+M)/2 method came out. When that octane rating appeared, the equivalent fuels were 92 octane (sea level ratings, here...). The Regular gas went from 88 to 85 octane under these ratings. This was in 1984 or so. Starting in the 1990s, ethanol permeated everywhere and the octane numbers had to be adjusted again, but not so correctly this time. The old 92 octane became "midgrade" fuels with a pump rating of 88-89. About that time, high-performance engines were popping up in cars with 11:1 and higher compression ratios, and some HD trucks needed it, too, so the 'new' premium became 93 octane (with about 12% ethanol by 2004 or so, at least here) which would foul the plugs in both my bike and my 1967 LTD fastback with hi-perf 390 CID engine. I dropped to midgrade for both, and the problem stopped.
Then came 2009. The fuels have changed again, and I have so far found that the EPA is not forthcoming with the engineering specs about it. Here, the pump ratings are so old that the labels are fading off the pumps, and the way it acts in the engines is simply outrageous. (rant...) While the midgrade pretty much mimics 91 octane (93 at sea level), it is hard to start when hot in my non-EFI cars, and in my EFI cars (all Fords) it gets worse MPG than Regular (i.e., 85 octane here, 87 at sea level). This means it is not burning completely at the compression ratios involved.
So, the moral might be: before you switch the jetting any further, try switching the fuels. Go down in octane and run it a couple of tanks to see what happens? Lower octane on any given engine increases the head pressures earlier in the combustion stroke, but does not burn as long. In the undersquare 750 (or 350F) this means a slight loss of power above 6000 RPM, which many folks never realize.
In my 750, I run Regular in town, Midgrade for commuter-freeway riding, and premium only when I hit the open road for full tanks of fuel at a time. This combo has let me run a whole season (sometimes longer) on a single set of plugs, since 2009's change. Before that mysterious change, I could ride 2-3 seasons on a set by using this octane swapping business.
