What ultimately killed the Airhead was the EPA. (I'm not bitter, just stating a fact.) In order to pass ever more stringent EPA regs, the engines had to run leaner, and leaner each year, which made them run hotter and hotter. These days most road going motorcycles have gone to water cooling, not because it's technically wonderful, but because they had to. The Oilhead survives because they pump lots of engine oil to both heads and then pass that same oil through a large oil cooler. So the Oilhead engine is still technically "liquid cooled". Now you understand the background.

What an Airhead engine wants is to run cool. Part of that cooling is the evaporative cooling effect of the fuel entering the combustion chamber and vaporizing. So a slightly richer mixer is what the engine craves in order to 1) run good, and 2) perform. Now you understand the need.

Now here are my comments...
► It would be a very rare carb rebuild where new Main Jets were required, simply because they experience no physical abuse. If they get clogged, simply twist a wooden tooth pick soaked in carb cleaner in the orifice and call it done.

► Yes, I personally run the next larger Main Jet in both my Airheads. #135 is probably standard in the 38mm carbs. What I would like is a #140, but one may not be commonly available, and so #145's may have to suffice.

► However, the Main Jet only meters fuel between 3/4 throttle to WFO. In road speed terms, that's 80 to 125mph !! Most Club members don't ride those speeds that often. 😛 So what I suggest is that you also need to look at your Needle Jet. This not only because that's where most of your riding is done, but also because the Needle and Needle Jet suffer physical abuse during use, which ultimately destroys their ability to meter fuel correctly.

I believe both the 38 and 40mm carbs run a "2,64" Needle Jet. If you'll look in your manuals at the jetting of the European models (that is to say "non-EPA models"), most of them were running "2,68". So then, new 2,68 Needle Jets is what I put in both my 1979 RT and my 1988 RS, along with new Needles.

So "Yes" I agree with using the larger Main Jet, but don't stop there. Hope this helps. 😛

Thanks. Replaced both needles and needle jets. Seem to be aluminum at this late date

Needles that is, not jets

Needles that is, not jets

The Needle Jets are easily replaced at any time by simply popping off the float bowl and then extracting the Main Jet holder. The Needle Jet will then fall straight down.

If you find 2,64 jets, then replace them with 2,66. If you find 2,66 jets, then replace them with 2,68. You won't believe what a difference this makes in performance.

I agree with your general comments on jetting.

I think you meant to say 32 mm. A #135 main jet is fairly common for the later R100 airheads. A case in point. My 95 R100RT runs 32 mm #357 & #358 carbs with #135 main and 2.66 needle. That seems a bit lean to me as that applies both to the 800cc & 1000 cc motor. So I've upped the needle to 2.68 and will run it for awhile before I up the main, likely to # 138.

I recently went through the Bings on my 1983 R100RS. I gave them a good cleaning and replaced the needles and main jets, among other things. I've for years been really pleased with the performance of the old girl (over 100 k miles on the clock) as she consistently returned mpg in the mid-40s at highways speeds. On a whim, I raised the needles when I went through the Bings. It was only one notch, but I was amazed at the change it made. It improved acceleration (seat of the pants test) and the engine just seems to be more tractable over the rev range. I should have raised those needles long ago.
With warmer weather I'll check for a change in the mpg.