Topic: CB750 Air Screws - Question

[Simpson]

If you adjust the air screws, do you re-sync the idle screws?

What is your air screw adjustment strategy to dial perfectly?

They have an enormous impact or engine smoothness, fumbling and decel pops...

[markjenn]

Assuming the idle mixture screws aren't too far out of whack, I doubt they have much effect on carb sync, especially off-idle sync which is the most important thing to get right during carb sync.

On my bike, I have to compromise with an air screw setting that balances overrun "popping" and a off-idle stumble as I pick up the throttle - I accept a little lean popping to get a lean enough mixture to avoid the stumble.  Whether that's inherent in the beast, just 40-yo carbs doing their thing, or a PO screwed up the jetting, I don't know.

- Mark

[TwoTired]

Pilot screw operation varies with the model/year of 750.

[Simpson]

Assuming the idle mixture screws aren't too far out of whack, I doubt they have much effect on carb sync, especially off-idle sync which is the most important thing to get right during carb sync.

On my bike, I have to compromise with an air screw setting that balances overrun "popping" and a off-idle stumble as I pick up the throttle - I accept a little lean popping to get a lean enough mixture to avoid the stumble.  Whether that's inherent in the beast, just 40-yo carbs doing their thing, or a PO screwed up the jetting, I don't know.

- Mark

That's just the topic I was trying to touch on. Thank you for speaking up. Hitting "spot on" between decel pop and stumble (Lean to rich) is just a tiny adjustment but makes a HUGE difference in performance ---> IE. idle, acceration, engine vibration or smoothness. To me, following the Honda shop manual and using the Tach/exhaust pressure to fine tune just is NOT nearly enough. Pulling plugs is absolutely necessary while keeping in mind riding characteristics. I was chasing my tail for awhile and finally getting it right came with surprises. It would be nice to have an O2 sniffer. That would make A/F dial-in as clear as strobe light timing.

[markjenn]

Hitting "spot on" between decel pop and stumble (Lean to rich) is just a tiny adjustment but makes a HUGE difference in performance ---> IE. idle, acceration, engine vibration or smoothness. To me, following the Honda shop manual and using the Tach/exhaust pressure to fine tune just is NOT nearly enough.

On my K1, it was just a bunch of trial and error riding with a screwdriver.  I'm not sure a A/F meter would do much good as the carbs of this era weren't any paradigms of mixture control when new and now 40-years later, I'm sure the A/F would be all over the map.  Same with sync - I can get them real close one minute, but a few blips of the throttle and they're off again.  I just accept it as the nature of the beast.

- Mark