Carb Tuning Screws Do What?

Below is a picture of a carb similar to mine on my 97 Pacer. I understand the idle screw of course and I know what they are all for just not sure what tuning is taking place with the idle jet screw and main jet screw. Thanks for the help in advance : )

Carb shown: https://www.treatland.tv/spaco-SHA-13mm-carburetor-lever-choke-SUPER-tune-p/spaco-sha-13mm-lever-4200.htm

(edited)

Re: Carb Tuning Screws Do What?

Main jet screw doesn't adjust. It's just for access to the jet. Tighten it barely more than snug.

Idle screw adjust idle. In for higher idle, out for lower.

Air screw, or pilot screw, meters airflow. It's got a sharp pointy tip. Should be screwed in till it stops (lightly, so you don't damage the tip), then backed off 1.5-2 turns. That's the ideale setting if your carb is jetted right.

If the bike runs better with it screwed in all the way, your pilot jet is too big, if it runs better screwed out more than 3 turns, jet is too small.

Idle should rise as you unscrew it from seated, until it starts dropping. Then screw it back in a 1/4 turn or so and make sure your idle doesn't "hang," (meaning it stays high for a minute, then suddenly drips,) and that it doesn't hesitate when you blip the throttle.

If it does either, it's usually the wrong size jet.

Re: Carb Tuning Screws Do What?

Dirty30 Dillon /

There's a bit of misinformation ^^^.

"Idle Screw" is actually referred to as Idle Air Speed. This raises and lowers slide as you'd expect, increasing or decreasing slide opening. As stated, 2.5 turns out from bottomed out is a great place to start.

"Idle Jet" is just that, simply an exterior-mounted jet. I doubt you'll need to change it out, but definitely removal for accurate cleaning is great.

"Main Jet" is actually Idle Mixture Screw, and has nothing to do with the main Jet. This will adjust the amount fuel/air being allowed through the idle Jet so that you can tune your idle and tip-in feel. Typically starting position is 1.5 turns out from bottom.

Main Jet and needle valve are in the bowl, as per usual.

Re: Carb Tuning Screws Do What?

Makes more sense now thank you

Re: Carb Tuning Screws Do What?

Yeah, sorry, I didnt look at the carb in treats, I just assumed from your photo it had a main jet that could be accessed from outside the bowl, like an old bing.

The rest of my info is correct though. The fast idle screw is the one that raises and lowers the slide. I've never heard it referred to as an "idle air speed" screw, because it does more than affect the air. It also adds fuel. Raising the slide is the same as opening the throttle. More air, more fuel.

So the bottom screw is a fuel enrichment circuit.

The way to tell what screw does what, the rule of thumb is, if the screw is in front of the slide, it's air, and if it's behind it (or under it, in this case,) it's fuel. And the fast idle screw is obvious, because it's usually the big one that controls the slide opening.

Confused yet? ;)

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