The F bike has the stock F muffler and air box. I'm using the same UNI filter as you.
Are you over oiling the foam filter? Describe your oiling process. Are you squeezing out as much oil as you can after application?
Checking the two outer pilot screws (since they're the easiest to get to right now), one is at 1 3/4 turns, one is just shy of 2 turns out. They were all set for best running/idle with the bike warmed up and running.
These carbs are not properly set by that process since idle mixtures are set for throttle response behavior off idle RPM rather than max lean conditions. (The over rich idle setting is the primary reason why this design didn't survive the EPA mandates invoked in 1978. )
I submit that if the P.O. put a Kerker on it in 77, he also must have changed (or had the shop change) the carb internal adjustments, as the bike would run too lean without the F's native exhaust pressure. The bike can tolerate the extra lean setting of the pilot screws because other fuel metering pathways are larger than the carbs had when showroom delivered. Until you verify the carb's internal settings and adjustments, it's just a meaningless guessing game.
Carb details that effect fuel delivery. (assumes parts are clear of fuel residue deposits and corrosion) ------
• Carb bowl fuel height. A shorter distance between fuel level and carb throat allows more fuel delivery under the same pressure level differences.
• Needle jet position and taper angle/profile.
• Main jet size.
• Emulsion tube parameters; Needle jet orifice size, size and number and placement of emulsion air holes.
• Mains and needle air jet size, or passageway restrictions.
Tires are 100/90-19 and 110/90-18. Same tire sizes on my 550K that's getting 45-50 mpg.
Show us the spark plug deposits on all four. We need to see the ground strap and as much of the center electrode insulator as possible in a strong light.
Describe the dominant throttle position you use while operating.
Do you practice a stationary warm up period with choke applied?
Regarding 76 model trait assumptions...
Honda was trying small mods to bring the bike's emissions profile closer to what the EPA wanted. Honda was attempting a "good faith" gesture to placate the EPA bureaucrats. With the "deadline" of 1978 approaching, 76 models would tend to be leaner than prior models. Possibly why the spec sheet that Delta found, stipulated solid tipped pilot screws on both the 087a and 069a carbs. I haven't been able to locate a known unmolested 087a carb set. I speculate that the bike ran worse with the compromising mods and owners or dealers simply reverted to 022a internal parts and settings after importation to make the bike work as it did when it was first popularized. The F's exhaust pressure profile allowed the leaner settings to perform well. However, the K model used the same exhaust as the the 500s and 74-75 K models with much less back pressure than the F exhaust system. (Note that the 77-78 K model exhaust runs at higher pressures than earlier 4-4 exhaust systems.) Leaner mixtures (or less hydrocarbons out the exhaust) would be welcome by the EPA, but not so much by operator performance demands. In the US, there was a lot of #$%*ing and moaning about what the EPA was doing to motor vehicle power profiles in that era, with many altering systems in the hope of getting to older model power responses, ad getting widely varying levels of success.
Anyway, proclaiming a 35+ year old survivor is unaltered from new, is an assumption not easily verified.
