No vacuum sync on the carbs recently...
If it needs it, you'll be happy with nicer idle smoothness.
Air filter is a foam element that has been in the bike. I cleaned and oiled it recently. Probably couldn't hurt to replace it as it's looking a little rough.
Bleed screws are set at about 1 3/4 turns
The foam filter presents less pressure differential across its membrane than the stock paper type. This pressure change reaches all the way into the carb throat to the pilot circuit and throttle valve exit ports and slightly leans the mixture. Another leaning effect would be the gas used, if it has ethanol, you are looking at yet another leaning cause. (They also call this gas "oxygenated" because of the leaning effect.)
I am using the UNI NU4055 foam filter in my Cb550F bikes too, primarily for economic reasons. (Cleaning and oiling is far cheaper than total replacement.)
The stock pilot screw setting is listed as 1 1/2 +/- 1/2 turn (using the stock paper filter). You should see improved throttle twist response by closing the pilot screw to find the correct setting for the modifications that were made. I think I arrived at 1 1/8 turn when the foam filter was used. But, note that I am at sea level +100 to 300 ft, where oxygen is most plentiful in the atmosphere.
My last tune effort was when gas was all gas (no ethanol).
Lately, I've noticed a bit of hesitation at 3-4K RPM now that ethanol fuel is mandated here in California. So, I'm probably due to change the pilot screw settings again to compensate for the ethanol. The hesitation does seem to improve somewhat as I ascend into mountains and air gets "thinner".
I understand the pressure differential issue that occurs when the slides are snapped open. I guess what I'm sort of asking is if the factory setup off idle is tuned more for economy and standard drivability rather than maximum response. If that is the case, then I'm happy to live with it...or at least try to compromise somewhere in the middle for the enrichment.
I believe there was some compromise between the two.
By 77, Honda was getting "import allowance" pressure from the EPA. It was the last year of production for this model. The EPA was quite happy to trade power for economy (or less emissions at whatever the cost). The exhaust has higher back pressure than prior K models, this allowed smaller mains; #98 rather than #100. And the slide needle was lowered to further restrict fuel flow. I do believe the stock F model was on the raggedy edge of lean at cruise throttle settings. The new/later fuel certainly did not help that, and there is no "automatic compensator" for enrichment.
With the combination of air filter and gas changes, the bike may need to be altered with both a pilot circuit adjustment and lifting the slide needle a notch or changing the slide needle profile. (I don't have access to a dyno to put the fine edge on the tune.)
So far, I have only done the air bleed screw modification. But, the F bikes were always good, quite possibly better than my 74 CB550 K with fuel economy. I've not scrutinized with micro numbers, though. The tell was how fast my wallet was emptied for fuel on a regular basis with commuter duty.
I shut off at 9K because I knew it wasn't going to be making any more peak power beyond that point and I had just put the motor back together, so I thought I would be respectful and stay out of the red. Nobody likes to get punched in the face as soon as they wake up from a peaceful rest, afterall...right? That and I don't actually know the mechanical limit of the motor...so I was being cautious.
If the engine was going though another break-in cycle, you were correct to be cautious.
If it had just been sleeping, a good slap in the face will just make it annoyed and roar at you. They'll do 10K + some, before the valves start to float, or if the points have weak springs on them so they float.
You're right though, the power peaks are well before that, so not much information gained running up too high.
Cheers,
