Mixture Screw Revelation

[Fox]

I saw this video that said that if the mixture screw is on the intake end of the carb then it meters the amount of air entering the idle mix and if it's on the outlet end it meters the amount of fuel. So what I take away from that is that they work opposite each other. One screw makes it richer if you turn it outward while the other makes it leaner if you turn it out.

I'm guessing they are right but not sure.

[jmkjr72]

yep that is why it is also called an a/f screw
but i did read an article about arecche carbs that say theres are backwards
eventhough its on the manifold side meaning its a fuel screw it works like an air screw

[Fox]

Actually it's called the Idle or Low Speed Mixture Screw. It adjusts the air fuel ratio at idle. It has very little to do with the high speed ratio contrary to what a lot of guys think. I see posts where guys will say they have a black plug and they try turning the mixture screw in vain to change the plug color.

What I didn't know was that they have two ways of doing it depending on the type of carb you have. I always thought they all worked the same way.

[thunderkat59]

I think it depends how you ride your bike. In my experience on 2 and 4t bikes and how I ride, a juicy idle mixture and raised needle have given me all the juice I need. Too much juice make the bike hard to start and prone to fouling though . . . Last week when I popped my EPA plug and adjusted my mixture for cold mornings, I got it to start in 3 kicks and the off-idle response was 100% better. A plug-pull yielded a darker shade of brown than I wanted, but we are going to drop about 30 more degrees before the winters over, so it should be just right for the winter.
In any event, I think if people go up small--in increments, you cant really go too far wrong.

[90GTVert]

It's definitely correct in most cases. We've said that around here before. DellOrtos tend to have the screw on the outlet side and they adjust fuel.

The mixture screw will affect WOT, but it's really insignificant. You should never look to the mixture screw when you are having problems WOT. Smooth idle, starting, and throttle response are what you should be trying to improve when messing with the mixture screw whether it controls fuel or air. The needle position affects WOT more, but still not much. I've only had one setup where I could tell real a difference at WOT between 2 different needle settings. That thing was so picky though. Way beyond anything else I've tuned.

You might be able to get your scoot to run how you want by messing with the mixture screw and needle clip in some situations, but IMO there's no substitute for the right main jet.

[Fox]

Yeah I suppose that it will affect the transition from idle to 1/4 throttle or whatever so your engine doesn't stumble when you open the throttle. Anyway, I learned something new today.