A/F screw function?

[mitchelljd02]

I've searched and searched but I cannot find the answer?

Does our A/F screw increase/decrease the air mix at idle or the fuel mix at idle?

Also, I read that the mikuni choke actually enriches (adds) fuel to the carbs for cold start.

Stage 1 jet, UNI filter, and UFO 4-2 street pro exhaust. My bike will start and run with the choke on, but starts to die with it off. Throttle roll (blip) is severely delayed also.

Any ideas?

 

[davesax36]

Watch this video.
http://www.youtube.com/watch?v=wyspAHrMbb8

The way I understand the screws is this:
The A/F screws actually adjust how much of the mixture gets into the intake. So it's sort of adjusting both the air AND the fuel. Screw them in too far and you don't get enough fuel to fire. Unscrew them too far and you get way too much and you're just wasting gas. The float height allows the proper mixture based on generated vacuum.

Yes, our choke is actually a "coasting enrichener," which means it allows more fuel in for starting instead of reducing the amount of air available (that's the way I understand it, at least).
when you say delayed are you talking about the needle goes up slowly or after you twist the throttle open or it returns to idle slowly?

Slow return usually indicates lean condition, return to idle with dip below set idle speed usually indicates rich condition.

To check where your A/F screws are set, take a small, but tight fitting screwdriver and turn them in SLOWLY until they stop. Count the number of turns in for each, return them to where they were, and post the numbers here.
Cylinder order is (on the bike)
Left rear=1
Left front=2
right rear=3
Right front=4

Most of us are between 2.25 and 3 turns out. I have a sneaking suspicion that you may need an adjustment both of A/F screws and float height depending on who did your install.

You might also consider reading this thread -- http://162.210.36.140/showthread.php?t=3809&highlight=carbs+bible

It will melt your brain, but you may also want to read this thread -- http://162.210.36.140/showthread.php?t=16981&highlight=carbs+bible

We'll help you figure it all out. There are tons of guys on here that know way too much about this bike.

Welcome to the board.

 

[mitchelljd02]

Well that will give me a lot of info to chew on.

All screws are 2.5. I did the install.

When I say slow on the throttle, I mean I'll roll it open and it will take about 1 sec to rev and when I release it's slow to return to idle. Before I did the install, everything was running good and functioned well. I guess it's not so much plug and play like EFI mods.

I swear this stuff is like a black art.

Thanks for the input and info. I appreciate it.

 

[Rusty McNeil]

Yes, it controls how much fuel can flow from the idle circuit Pilot Jet. It "fine tunes" the pilot jet. So out is richer in is leaner, as it either adds more or less fuel to the air flowing through the carb.

I don't know why we, including me, call it an a/f screw, I think a more accurate name would be an idle mixture screw.

And our choke is an enrichment circuit as it adds fuel instead of restricting air like a traditional choke.

In truth though the "Coasting Enrichener" is a different item than the choke, its the little diaphragm the size of a nickel in the side of the carb, whose function is to enrich things when the throttle suddenly closes, which helps control annoying lean back fire on decel.

The symptom your describing, if I understand correctly, sounds to me like a clogged idle circuit. You can try the "pea shooter" and the "shotgun" and it may help, but every time I've had one that would not run at idle without the choke, and resisted being revved up, I ended up having to totally tear them down for a thorough cleaning.

The links in the earlier post are very good ones....

 

[mitchelljd02]

All is great info, and I appreciate it.

Maybe everyone can relate, but I'm at my wits end with taking them completely apart for the 3rd time.

I guess I will try the "shotgun" approach first.

I have another bike, so I'm about ready to just drop it off (trailer is) somewhere and let someone else with more experience deal with it.

It just bugs the piss out of me something so simple on paper is whooping my butt.

 

[davesax36]

Have you drained the bowls into a jar/white cup? Do you see flecks of stuff in there? You may be chasing rust back through your fueling system if you do. ( Again, don't ask me how I know).