Air screw adjustment

[ph0010421]

Hi
I'm chasing a non-firing cylinder on my 350/4.
Happy with timing and suspect carb setup.
Pilot jet is clear so...
I'm going to follow the float height FAQ here and the bench-synch. Great guides.
The manual tells me the airscrew is 7/8 turn out plus/minus 3/8.
Which spans 1/2 turn all the way up to 1 1/4 turns.
I know in is richer and out is leaner, but I don't really know what I'm aiming for. Anyone got a suggestion please?
Thanks guys

[Nurse Julie]

The basics of it are that you are aiming for uniformity of colour of all 4 spark plugs, not too dark and sooty and not too white. Not all 4 mixture screws will be ultimately set at the same turns out to achieve parity of colour. Keihin put the mixture screw on the carbs to achieve critical adjustment of each carb. Start with all 4 screws at 1 turn out and then adjust accordingly after riding the bike, assessing colour of plugs, clean plugs, adjust mixture, ride and repeat until you are happy with colour. Running slightly rich is preferable to running lean.

[TrickyMicky]

If you have only one cylinder not firing then it's nothing to do with ignition timing, they fire in pairs, 1 with 4, and 2 with 3.
  If it's not firing at all then I doubt it's anything to do with the mixture screw, that will only affect how it fires ie. weak or rich.
  Are you getting a spark? If not then check plug/cap/lead. Is the plug wet or dry after running engine? If wet with fuel, then no spark. If dry then no fuel, and may need a firm tap on the float bowl to free up a possible sticking float or needle valve. Definitely not unknown!!
   If of course you suspect it might be a plug/cap/lead issue, then swap them over with their firing partners, 1 with 4, and/or 2 with three.

[K2-K6]

Honda put a really neat method in some of the manuals (certainly in 750) that has the setting of idle air screw explained.

In effect it asks you to (after everything else is verified) to back each air airscrew out until the tachometer shows maximum rpm (done one cylinder at a time) this is effectively searching for air / fuel ratio probably about 14 to 1 -ish,  then going back in to make it richer by dropping the rpm by 100. So in essence "divining" leanest,  then bringing back from there a touch (as julie describes) to run just a wisper rich from absolute.

Then, reset overall tickover speed on the main tickover throttle screw, then do the next cylinder same,  followed through all four. It's this routine that's setting combustion parity and may end up with the variance included in the absolute set point of each screw one to another.

The bench synchronisation method published by Bryan has the advantage of no uncontrolled influence (gauges, plugs cylinder psi and condition) which I view as very accurate.  It's primarily for setting slide opening parity of full torque and rpm and doesn't have the resolution to control tickover (although there's many a word written to the contrary) tickover is just a byproduct,  but the above airscrew tuning given by Honda is the refinement of low speed even firing.

[ph0010421]

Thanks for the wise words everyone. I'm back on it today.
Definitely getting a spark. It got me a few times 😄
I now see that the airscrew won't stop it firing if it's close enough, so I'm not at that stage yet.
I'm quite a dab hand at taking these carbs off now.
All the best

[ph0010421]

It's a bittersweet moment burning the palm of your hand on a exhaust header that previously wasn't firing

[Nurse Julie]

It's a bittersweet moment burning the palm of your hand on a exhaust header that previously wasn't firing

Oh dear 😬😬😬  The problem is sorted though? What was causing it?

[ph0010421]

Hello Julie
I'd love to give you a definitive answer!
I had the carbs off again and took the top and bowl off.
Compressed air all over it and ran thinners through it. Did a bench synch and checked float height per your guide. Thanks...heard the term, had no idea what it was.
TrickyMicky mentioned ht leads and caps. The offending cylinder had a bit of an insulation breakdown right at the cap. Spark was fat, but guess this wasn't helping when in situ. Remade it off just in case.
Anywho, one or all of the above sorted it.
A burnt hand and a million electric shocks is a small price to pay.
Oh, and refitting the airbox. What joy!

Still a bit hesitant when blipping the throttle, but I'll have a think on that.

Really appreciate your help