Fuel Screw Vs. Air Screw

atv4life95 · 12-10-2011, 10:06 AM

Installing a set of Boyesen Power Reeds on a friend's RM, I noticed that this carb has an "air screw" instead of a fuel screw like on our carbs. Fuel screws I know lean out/richen the pilot circuit. But what really is an air screw? When I was working on the bike, I started turning the screw outwards thinking i was richening up the pilot circuit, but then all of a sudden the bike started revving out of control (LEAN!).. So, I turned the screw all the way in, to the richest setting, and now it idles normally. How does an air screw work? Where is it drawing the extra air from? Is it only for idle/low rpm riding (similar to the fuel screw)?

CFM-Z440 · 12-10-2011, 10:13 AM

Many 2 strokes are this way. It's not quite as precice as a fuel screw, but does the job. You do the same thing, just find the best idle with the air screw, and run it.

zookie400 · 12-10-2011, 07:03 PM

4 strokes pull much less air (that ultimately draws the fuel from the pilot circuit) so they have the fuel screw that supplements the fuel supply.

2 strokes pull a lot of air at idle, so they are actually able to draw too much fuel. 2 stroke carbs get an air screw that bleeds extra air in without fuel.

the adjustment methods are basically the same although opposite for in/out = rich/lean.

fuel screw out is more fuel which is richer, or with an air screw it is more air which is leaner.

atv4life95 · 12-10-2011, 11:43 PM

Thank you for clearing that up for me..

beaverclever02 · 12-11-2011, 03:19 AM

And this is why I love this site. I would have never known that.

CFM-Z440 · 12-11-2011, 09:46 AM

Not all 2 strokes use an air screw though. Some have a fuel screw, some even have both. You can tell if it is a fuel screw or an air screw by which side of the carb its on. If its before the bowl, it's an air screw. If it's after the bowl, it's a fuel screw.

atv4life95 · 12-11-2011, 03:39 PM

Quote:

Originally Posted by CFM-Z440

Not all 2 strokes use an air screw though. Some have a fuel screw, some even have both. You can tell if it is a fuel screw or an air screw by which side of the carb its on. If its before the bowl, it's an air screw. If it's after the bowl, it's a fuel screw.

This one was on the side of the carb, right next to the throttle adjustment. The throttle is a mechanical slide (those old keihin carbs where the throttle cable comes through the top of the twist off cap and the throttle return spring is directly underneath it.) The screw would remain attached if you removed the bowl.

zookie400 · 12-12-2011, 12:26 PM

that sounds like the idle adjuster screw, that lifts or lowers the slide.

sandman74 · 12-12-2011, 01:22 PM

Don't forget the adjustments are backwards from the fuel screw....at least on the PWK Kehein on the most of the RM's (Out is lean / In is rich)

atv4life95 · 12-12-2011, 03:59 PM

Quote:

Originally Posted by zookie400

that sounds like the idle adjuster screw, that lifts or lowers the slide.

Yeah, I meant to say idle adjustment

12-10-2011, 10:06 AM	#1 (permalink)
atv4life95 :::LT500 Rider::: Join Date: May 2011 Location: Westchester NY Posts: 1,245	Fuel Screw Vs. Air Screw Installing a set of Boyesen Power Reeds on a friend's RM, I noticed that this carb has an "air screw" instead of a fuel screw like on our carbs. Fuel screws I know lean out/richen the pilot circuit. But what really is an air screw? When I was working on the bike, I started turning the screw outwards thinking i was richening up the pilot circuit, but then all of a sudden the bike started revving out of control (LEAN!).. So, I turned the screw all the way in, to the richest setting, and now it idles normally. How does an air screw work? Where is it drawing the extra air from? Is it only for idle/low rpm riding (similar to the fuel screw)? __________________ 2003 Suzuki Z400 - SOLD Fully Worked & Race Ready, '98 Yamaha YZ125 - My New Toy

12-10-2011, 10:13 AM	#2 (permalink)
CFM-Z440 Super Moderator Join Date: Sep 2006 Location: Southern Minnesota Posts: 6,437	Many 2 strokes are this way. It's not quite as precice as a fuel screw, but does the job. You do the same thing, just find the best idle with the air screw, and run it. __________________ 03 LT-Z 470 Max Power 12.5:1 94mm Piston, Hot Rods +5 Stroker Crank, Web Cams 539 (+6) IN / 539 EX, with Hot Cams sprockets, Ferrea +1 Valves, LTZ/DRZ springs (IN)DRZ springs (EX), DRZ Manual Decomp. Ported head, FCR 41 with Noss adaptor, Home made intake, Modified HMF Full System, Home made header, Trailtech flywheel, Dyna Programable 54hp 36ft lbs

12-10-2011, 11:43 PM	#4 (permalink)
atv4life95 :::LT500 Rider::: Join Date: May 2011 Location: Westchester NY Posts: 1,245	Thank you for clearing that up for me.. __________________ 2003 Suzuki Z400 - SOLD Fully Worked & Race Ready, '98 Yamaha YZ125 - My New Toy

12-11-2011, 03:19 AM	#5 (permalink)
beaverclever02 :::LT500 Rider::: Join Date: Feb 2011 Location: Holt, Mo Posts: 1,742	And this is why I love this site. I would have never known that. __________________ 03 Z400, Fly Aluma Steel Bars, Blob removed, fly kill switch, ASV levers, Power Madd hand gaurds, MQ park brake block, DG O-series pipe with Curtis sparks header, Pro Armor full nerfs with heel gaurd, CST Ambush Front and Rears, GBC Sand shark and shark fins on douglas blue label, No lid,pro design air filter, no snorkel, 6000k HID conversion, procom cdi, 06 ressie shocks, FCR39 tuned.

12-11-2011, 09:46 AM	#6 (permalink)
CFM-Z440 Super Moderator Join Date: Sep 2006 Location: Southern Minnesota Posts: 6,437	Not all 2 strokes use an air screw though. Some have a fuel screw, some even have both. You can tell if it is a fuel screw or an air screw by which side of the carb its on. If its before the bowl, it's an air screw. If it's after the bowl, it's a fuel screw. __________________ 03 LT-Z 470 Max Power 12.5:1 94mm Piston, Hot Rods +5 Stroker Crank, Web Cams 539 (+6) IN / 539 EX, with Hot Cams sprockets, Ferrea +1 Valves, LTZ/DRZ springs (IN)DRZ springs (EX), DRZ Manual Decomp. Ported head, FCR 41 with Noss adaptor, Home made intake, Modified HMF Full System, Home made header, Trailtech flywheel, Dyna Programable 54hp 36ft lbs

12-10-2011, 07:03 PM	#3 (permalink)
zookie400 :::Post Whore::: Join Date: Aug 2007 Location: Connecticut Posts: 2,683	4 strokes pull much less air (that ultimately draws the fuel from the pilot circuit) so they have the fuel screw that supplements the fuel supply. 2 strokes pull a lot of air at idle, so they are actually able to draw too much fuel. 2 stroke carbs get an air screw that bleeds extra air in without fuel. the adjustment methods are basically the same although opposite for in/out = rich/lean. fuel screw out is more fuel which is richer, or with an air screw it is more air which is leaner.

12-12-2011, 12:26 PM	#8 (permalink)
zookie400 :::Post Whore::: Join Date: Aug 2007 Location: Connecticut Posts: 2,683	that sounds like the idle adjuster screw, that lifts or lowers the slide.

12-12-2011, 01:22 PM	#9 (permalink)
sandman74 :::Vinson Rider...wtf?::: Join Date: Jun 2009 Posts: 705	Don't forget the adjustments are backwards from the fuel screw....at least on the PWK Kehein on the most of the RM's (Out is lean / In is rich) __________________ Trinity Stage 4 full exhaust, UNI filter w/ DIY powerlid, 400EX airboot, FCR39, R&D fuel screw, DRZe kit, Yoshimura Cams, Motion Pro Twist throttle, Pro Taper bars, YFZ calipers, TRX master cylinder, KX clutch perch, KTM lines, Acerbis hand guards, TrailTech Vapor, aluminum radiator shrouds, aluminum heal guards, bossbearing DLR conversion, ZX6 LED tail-light, full PRM skids and bumper, ITP's w/GNCC's, Laegers +2, Houser +1.25

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