The 086a and 7A carbs found in the 750K6 and 750F0 bikes have a notch cut in the edge of the float bowl gasket's outer perimeter to indicate where to set the float gage. Most of the 086a floats are plastic, while many (not all) of the 7A floats were brass. Sometimes the opposite shows up in them, I have pictures of both, and this corresponds with Honda's scramble to make the K6 after the F0 sales fell flat by some 50+% of expectations. (The wiring harnesses reflect this often, too!)
The difference between the floats is: their performance in real riding. The brass ones will cause slightly deeper float depths in real-riding situations, while the plastic ones ride a little higher and make slightly shallower bowl depth WHILE RIDING, not while parked. This is due to the mass of the floats themselves: the brass ones will bounce downward on a road bump more readily than the plastic ones do (the plastic ones have a higher mass, and thus resistance to the sudden change of bumps), so the performance is a bit different.
-Only the purist need try to measure this: in between my wives, circa 1976-79, I did...
So, now beyond the theory-vs.-practice that makes these work...
When I rebuild these for folks - using Keihin's brass - given the physically lighter gasoline [and gas-ethanol mixes] we now have (yep, it is a factor now, too, as it has a lower specific gravity which provides more bouyancy for the floats) and given the stiffer float valve springs we all suffer since 2002 or so, I alter the float settings, period.
Honda called for 26mm from the bottom of the float to the flat zone by the notch on the bodies of the carbs for 1970s-era fuel. This was a 'strict rule' during the Carter administration's DOT (which I have explained in other posts) and this shallow setting let the bikes pass 'static' emissions tests easier, while also needing some hand-choke action to start up cold bikes. It also cut off the top end speed by almost 10 MPH (105 MPH was nearly impossible with most of the K6 bikes, usually 98-102 was all), but the powers that be thought that insignificant. Today, though, it matters again.
So, this calls for alterations if you don't want to be run over from behind by a semi full of rocks!
To get there, I first try, if at all possible, to use the softer float valves found in the older carbs. If they are done for, the next-best step is to both deepen and stagger the floats, if they are not already there. The staggering began because of the propensity of the carbs to dribble on hot days when the bike was on the sidestand: setting the 'lower' float 1mm taller (25mm) than the 'higher' float, referenced to the sidestand, increased the float force against the float valve and made them seal better while parked. (Note: this didn't work for choppers with 30-degree sidestand lean angles, though, important to remember...). This worked until gasahol appeared in the 1990s.
When the lighter-density fuel first appeared the bikes ran rich, all else being equal: we demonstrated it many times by just switching between the different fuels at the pump. Running the 10% alcohol mix would foul plugs quickly and make the bikes often dribble at long stoplights. When MTBE replaced the ethanol, this reversed. Personally, I think this is where the confusion began regarding float depths and performance: it had me for a while.
Then I had the chance to work with some K0/K1 bikes alongside the later ones (K4/5). The early ones worked better with the new fuels. They had the older 657a series carbs, which had the staggered brass floats (25/25mm and 25/26mm in K0 and 25/26mm in K1) from the factory: taking the lead from those, we went to 25/26mm in the later bikes (still with the softer float valve springs, though) and things got better.
So, I have stayed with the 25/26mm float depths, despite the stiffer float valve springs, in most rebuilds - although one caveat: mine are 24/25mm so I MUST shut off the fuel at all parking stops. This requires that the float bowl gasket be a good one, and gluing it to the carb body with a couple of dabs of gasket cement will help you when the carbs are back on the bike and you must remove a float bowl. I also open up the larger holes in the emulsifiers to 0.0375" on almost all rebuilds because it takes more air to burn the ethanol more completely without a catalytic convertor: this is especially important if the bike must pass emissions tests somewhere. It also helps keep the plugs cleaner, longer. Mine are all 0.039", which works well at 6000+ ft altitude.
Don't overlook the mainjet size, nor the octane ratings: understand today's fuels as compared to 1970s fuels. This means:
1. ethanol needs more air to burn, so a leaner mainjet and/or larger emulsifier holes are sometimes needed. Mine were #110 from the factory: today at 6000+ ft altitude I run 102.5 size since I switched to a K4 cam in 2013.
2. floats ride higher in ethanol than in gasoline, so higher depth (lower mm numbers) are needed, usually by 1mm.
3. fuel burn rates are much, much slower than in 1970s gasolines, so use less octane. Example: I run Regular grade in town since 2016, and Midgrade on the interstates (to 110+ MPH) to help keep the plugs cleaner longer and retain the high-end speeds. This burn rate has been slowed to help cars light off their catalytic convertors sooner: the warmed-up unburned fuel lights off in the catalysts quickly, and it needs to be able to do this in cold weather. Refineries blend for your altitude, so ignore the old rules of changing octanes yourself with high-altitude trips, it's just not needed since about 2016 anymore (USA rules, don't know about elsewhere).
