carb mixture screw

How to Improve Your Motorcycle's Start and Warm Up, Idling and Acceleration

AIR CUT-OFF VALVE

The Air Cut-Off Valve is an extremely important part of the fuel / air intake system that is frequently missed even by competent mechanics, most of whom are not even aware of its existence or how it functions. Over time, air cutoff valves will harden up, perforate and/or disintegrate leaving your motorcycle in a permanently lean, low RPM range (0-2200).

Most early model Honda, Kawasaki, Suzuki and Yamaha motorcycles use air cut-off valves. Every Honda CB650, CB750, CB900 and GL1000 has one, located in the air intake tube between the air cleaner box and the manifold. The GL1100 has four, located in the top air intake passage of each carburetor, and every GL1200 has one located in the plenum box below the air cleaner box similar to the GL1000.

These valves are "air blockers", along with the carburetor o-rings, serving to restrict air flow, increasing the fuel-air mixture ratio when the carb slides are in the lower RPM range, up to closed so that the machine will start and idle smoothly with virtually no choking and accelerate rapidly, with no stumble or hesitation.

When typical starting requires heavy and prolonged choking to keep the bike running, when stalling and hesitation occurs even while "warmed up" and when backfiring pops while the throttle is shut off while cruising, most people will start fooling with the air screws to try to overcome the problem, only making it worse along with reducing their gas mileage.

Running with dysfunctional air cut-off valves can cause severe engine damage or failure. When the valves are dysfunctional, the engine pulls nearly total air into the low RPM range through the carbs and on to the combustion chamber. The fuel-air ratio is extremely poor with pre-ignition long before the normal 38 to 40 degree firing point, generating an enormous amount of heat, with no power, which is trapped a longer period of time in the cylinder before the exhaust valve opens.

This heat goes mostly up through the cylinder head, gradually incinerating the head gaskets and allowing engine coolant (antifreeze) to seep into the engine. You can easily spot when you are in trouble by a puff of bluish-white or white smoke on first starting the engine. Don't leave it alone. Change the head gaskets immediately or you could take out your bottom end bearings, ruining the engine.

For those of you installing new exhaust systems, you will discolor your head pipes, either blue (lean state) or gold (rich state) if the signs are ignored. You an destain your pipes using Blueway or Blue-Off but be careful not to rub the chrome, just dab gently. These destaining compounds are usually found at a Harley dealer or Harley-oriented aftermarket shop.

Honda genuine pipes, like many Yamaha pipes, have a double wall for a portion of the head pipe which masks discoloration - but also masks a bad state of tune. Even if your pipes are original and still in good shape, don't think you're safe if your motorcycle has any or all of the dysfunctional air cut-off valve running and performing characteristics.

If your 1975 to 1987 Gold Wing is not getting 50 to 58 miles per gallon (20-23 kilometers per liter), you have one of the most obvious signs of trouble. It is likely your air screws have been backed out to try to overcome the effects of the poor running, stumbling, starting, stalling and quitting.

INSTALLATION GUIDELINES

Refer to your Clymer, Haynes or Honda Workshop manual for location and installation procedures.

Replace your air cut-off valve/s then resynchronize all four carburetors to the #3 carburetor vacuum level using a mercury carb sync tool (carb stick) or vacuum gauge synchronizer tool. The number 3 carb is the constant fixed flow carburetor set at the factory.

DO NOT attempt to set by ear or just by guessing as the results can be anywhere from pathetic to disastrous.

DO NOT deviate from the book settings of your Honda, Clymer or Haynes manual. Wiser people than all of us designed this very effective and efficient intake and carburetion system.

To further insure against any additional vacuum loss, replace your inexpensive carburetor manifold o-rings. Vacuum loss can occur there and mimic the effect of a bad air cut-off valve. Paste a new set in with vaseline and make sure they are sealed with an ether check.

© Saber Cycle®

<input id="gwProxy" type="hidden" /><input id="jsProxy">

About the Author

Eugene Bileski represents over 46 years in the motorcycle trade as a past importer of Bridgestone, Ducati, Marusho, Norton and Yamaha motorcycles and is founder of Saber Cycle Honda Gold Wing Parts and Accessories at http://www.saber-cycle.com

Inexpensive air cutoff valves for Honda CB650, CB750, CB900, GL1000, GL1100 and GL1200 can be found on our website, in addition to the carburetor synchronizer tools and manuals referred to in this article.

<input id="gwProxy" type="hidden" /><input id="jsProxy">

SPIDER MAN MINI CHOPPER CARBURETOR CARB 49CC 50CC 70CC 90CC 100CC 110CC 125CC
SPIDER MAN MINI CHOPPER CARBURETOR CARB 49CC 50CC 70CC 90CC 100CC 110CC 125CC

$39.95 (1 Bid)
Time Remaining: 3h 5m
Click To Buy Now
Bid now
Quick Fuel Holley 4160 Carburetor Rebuild Kit 390 600 750 CFM 1850 3310 9834
Quick Fuel Holley 4160 Carburetor Rebuild Kit 390 600 750 CFM 1850 3310 9834

$27.92
Time Remaining: 16d 1h 39m
Buy It Now for only: $27.92
Click To Buy Now
CARBURETOR KRAGEN ZOOMA 33CC 36CC GAS SCOOTER GOPED TY ROD II GO KART CARB NEW
CARBURETOR KRAGEN ZOOMA 33CC 36CC GAS SCOOTER GOPED TY ROD II GO KART CARB NEW

$19.95 (1 Bid)
Time Remaining: 3h 20m
Click To Buy Now
Bid now
MINI BAJA WARRIOR HEAT 163CC 55HP 196CC 65HP BAJA MB165 MB200 CARBURETOR CARB
MINI BAJA WARRIOR HEAT 163CC 55HP 196CC 65HP BAJA MB165 MB200 CARBURETOR CARB

$21.95 (3 Bids)
Time Remaining: 3h 25m
Click To Buy Now
Bid now
SS Super E Carb Carburetor for Harley Davidson 11 0420
SS Super E Carb Carburetor for Harley Davidson 11 0420

$277.99
Time Remaining: 21d 15h 56m
Buy It Now for only: $277.99
Click To Buy Now
Honda 305 CB77 Superhawk Carburetor Carb Rebuild Kit x 2 sets 1961 1968
Honda 305 CB77 Superhawk Carburetor Carb Rebuild Kit x 2 sets 1961 1968

$29.99 (1 Bid)
Time Remaining: 4h 1m
Click To Buy Now
Bid now
Honda CB160 CL160 Carburetor Rebuild Kit x2 1964 1965 1966 1967 1968 1969  Carb
Honda CB160 CL160 Carburetor Rebuild Kit x2 1964 1965 1966 1967 1968 1969 Carb

$29.99 (1 Bid)
Time Remaining: 4h 1m
Click To Buy Now
Bid now
Motion Pro Pilot Screw Adjusting Carb Carburetor Tool NEW 08 0119
Motion Pro Pilot Screw Adjusting Carb Carburetor Tool NEW 08 0119

$22.94
Time Remaining: 16d 6h 47m
Buy It Now for only: $22.94
Click To Buy Now
4x YAMAHA 1978 XS1100 CARB KITS XS1100E
4x YAMAHA 1978 XS1100 CARB KITS XS1100E

$64.00 (1 Bid)
Time Remaining: 4h 26m
Click To Buy Now
Bid now
CARBURETOR ECHO PB 260L PROLITE PB260L POWER BLOWER LEAF BLOWER CARB 10MM NEW
CARBURETOR ECHO PB 260L PROLITE PB260L POWER BLOWER LEAF BLOWER CARB 10MM NEW

$24.95
Time Remaining: 3h 19m
Click To Buy Now
Bid now
RD Flex Jet Fuel Mixture Screw Flexjet CRF TRX YFZ Keihin FCR
RD Flex Jet Fuel Mixture Screw Flexjet CRF TRX YFZ Keihin FCR

$28.99
Time Remaining: 17d 3h 13m
Buy It Now for only: $28.99
Click To Buy Now
ROCHESTER TRI POWER 2 BBL 2GV 2GC 2 JET 3 BIG BORE  3 SMALL BORE CARBS + PARTS
ROCHESTER TRI POWER 2 BBL 2GV 2GC 2 JET 3 BIG BORE 3 SMALL BORE CARBS + PARTS

$99.00 (4 Bids)
Time Remaining: 1d 8h 49m
Click To Buy Now
Bid now
1980 87 suzuki gs1100 gs1000 gs850 gs750 gs550 mikuni BS carb O RING GASKET KIT
1980 87 suzuki gs1100 gs1000 gs850 gs750 gs550 mikuni BS carb O RING GASKET KIT

$24.50 (2 Bids)
Time Remaining: 1d 23h 34m
Click To Buy Now
Bid now
Motion Pro Motorcycle Carburetor Pilot Screw Adjusting Tool Carb Mixture 08 0119
Motion Pro Motorcycle Carburetor Pilot Screw Adjusting Tool Carb Mixture 08 0119

$22.95
Time Remaining: 4d 19h 21m
Buy It Now for only: $22.95
Click To Buy Now
1978 83 yamaha xs1100 xj1100 special mikuni BS CV carburetor O RING GASKET KIT
1978 83 yamaha xs1100 xj1100 special mikuni BS CV carburetor O RING GASKET KIT

$23.99 (2 Bids)
Time Remaining: 1d 23h 34m
Click To Buy Now
Bid now
1978 83 yamaha xs1100 xj1100 special mikuni BS CV carburetor O RING REBUILD KIT
1978 83 yamaha xs1100 xj1100 special mikuni BS CV carburetor O RING REBUILD KIT

$12.99 (1 Bid)
Time Remaining: 1d 23h 44m
Click To Buy Now
Bid now
AED Holley 4150 Rebuild Kit Double Pumper Carbs 750 650
AED Holley 4150 Rebuild Kit Double Pumper Carbs 750 650

$47.32
Time Remaining: 13d 18h 41m
Buy It Now for only: $47.32
Click To Buy Now
1980 87 suzuki gs1100 gs1000 gs850 gs750 gs550 mikuni BS carb O RING REBUILD KIT
1980 87 suzuki gs1100 gs1000 gs850 gs750 gs550 mikuni BS carb O RING REBUILD KIT

$12.99 (1 Bid)
Time Remaining: 1d 23h 44m
Click To Buy Now
Bid now
VINTAGE ANTIQUE HARLEY PRO REBUILT OEM BENDIX ZENITH CARBURETOR W OEM KIT
VINTAGE ANTIQUE HARLEY PRO REBUILT OEM BENDIX ZENITH CARBURETOR W OEM KIT

$204.99 (3 Bids)
Time Remaining: 2d 13h 36m
Click To Buy Now
Bid now
Quick Fuel Holley Carb 3310 Carburetor Vacuum 1850 Rebuild Kit +
Quick Fuel Holley Carb 3310 Carburetor Vacuum 1850 Rebuild Kit +

$39.95
Time Remaining: 16d 1h 39m
Buy It Now for only: $39.95
Click To Buy Now
Weber 48 IDA carbs with EMPI intakes HEX BAR linkage dual port VW Baja bug
Weber 48 IDA carbs with EMPI intakes HEX BAR linkage dual port VW Baja bug

$500.00 (7 Bids)
Time Remaining: 3d 4h 54m
Buy It Now for only: $1,250.00
Click To Buy Now
Bid now
CARBURETOR HONDA HELIX CN250 CN 250 SCOOTER COMPLETE CARB ASSEMBLY 1986 2001 NEW
CARBURETOR HONDA HELIX CN250 CN 250 SCOOTER COMPLETE CARB ASSEMBLY 1986 2001 NEW

$54.95
Time Remaining: 3h 10m
Click To Buy Now
Bid now
HOLLEY CARBURETOR 1850 3310 390 600 750 CFM REBUILD KIT
HOLLEY CARBURETOR 1850 3310 390 600 750 CFM REBUILD KIT

$29.99
Time Remaining: 19d 15h 59m
Buy It Now for only: $29.99
Click To Buy Now
 USED Honda ATC 200X carb carburetor bbr TTR 125 DRZ 125 CRF 150 Keihin 26mm
USED Honda ATC 200X carb carburetor bbr TTR 125 DRZ 125 CRF 150 Keihin 26mm

$0.99 (1 Bid)
Time Remaining: 3d 23h 5m
Click To Buy Now
Bid now
Honda  Shadow 1983 honda shadow vt 500 project bike
Honda Shadow 1983 honda shadow vt 500 project bike

$410.00 (7 Bids)
Time Remaining: 4d 2h 3m
Click To Buy Now
Bid now
HOLLEY LIST 7508 350cfm CARBURETOR 2 BBL SERIES 2300 ANNULAR BOOSTERS AUTO CHOKE
HOLLEY LIST 7508 350cfm CARBURETOR 2 BBL SERIES 2300 ANNULAR BOOSTERS AUTO CHOKE

$69.99 (1 Bid)
Time Remaining: 4d 2h 47m
Click To Buy Now
Bid now
Holley Brass Idle Mixture Screws
Holley Brass Idle Mixture Screws

$1.99
Time Remaining: 11d 17h 2m
Buy It Now for only: $1.99
Click To Buy Now
FORD58 MERCURY119 AUTOLITE 4100 4 BARREL CARBURETOR CORE
FORD58 MERCURY119 AUTOLITE 4100 4 BARREL CARBURETOR CORE

$250.00
Time Remaining: 4d 2h 57m
Click To Buy Now
Bid now
S  S SUPER E SHORTY CARB  COVER SHOVELHEAD PANHEAD KNUCKLEHEAD HARLEY EVO
S S SUPER E SHORTY CARB COVER SHOVELHEAD PANHEAD KNUCKLEHEAD HARLEY EVO

$285.00
Time Remaining: 4d 4h 1m
Click To Buy Now
Bid now
NEW ADJUSTABLE HONDA GCV160 CARBURETOR HRB216 HRS216 HRR216 HRT216 HRZ216 CARB
NEW ADJUSTABLE HONDA GCV160 CARBURETOR HRB216 HRS216 HRR216 HRT216 HRZ216 CARB

$18.25
Time Remaining: 8d 21h 47m
Buy It Now for only: $18.25
Click To Buy Now

carb mixture screw


SPIDER MAN MINI CHOPPER CARBURETOR CARB 49CC 50CC 70CC 90CC 100CC 110CC 125CC
SPIDER MAN MINI CHOPPER CARBURETOR CARB 49CC 50CC 70CC 90CC 100CC 110CC 125CC

$39.95 (1 Bid)
Time Remaining: 3h 5m
Click To Buy Now
Bid now
Quick Fuel Holley 4160 Carburetor Rebuild Kit 390 600 750 CFM 1850 3310 9834
Quick Fuel Holley 4160 Carburetor Rebuild Kit 390 600 750 CFM 1850 3310 9834

$27.92
Time Remaining: 16d 1h 39m
Buy It Now for only: $27.92
Click To Buy Now
CARBURETOR KRAGEN ZOOMA 33CC 36CC GAS SCOOTER GOPED TY ROD II GO KART CARB NEW
CARBURETOR KRAGEN ZOOMA 33CC 36CC GAS SCOOTER GOPED TY ROD II GO KART CARB NEW

$19.95 (1 Bid)
Time Remaining: 3h 20m
Click To Buy Now
Bid now
MINI BAJA WARRIOR HEAT 163CC 55HP 196CC 65HP BAJA MB165 MB200 CARBURETOR CARB
MINI BAJA WARRIOR HEAT 163CC 55HP 196CC 65HP BAJA MB165 MB200 CARBURETOR CARB

$21.95 (3 Bids)
Time Remaining: 3h 25m
Click To Buy Now
Bid now
SS Super E Carb Carburetor for Harley Davidson 11 0420
SS Super E Carb Carburetor for Harley Davidson 11 0420

$277.99
Time Remaining: 21d 15h 56m
Buy It Now for only: $277.99
Click To Buy Now
Honda 305 CB77 Superhawk Carburetor Carb Rebuild Kit x 2 sets 1961 1968
Honda 305 CB77 Superhawk Carburetor Carb Rebuild Kit x 2 sets 1961 1968

$29.99 (1 Bid)
Time Remaining: 4h 1m
Click To Buy Now
Bid now
Honda CB160 CL160 Carburetor Rebuild Kit x2 1964 1965 1966 1967 1968 1969  Carb
Honda CB160 CL160 Carburetor Rebuild Kit x2 1964 1965 1966 1967 1968 1969 Carb

$29.99 (1 Bid)
Time Remaining: 4h 1m
Click To Buy Now
Bid now
Motion Pro Pilot Screw Adjusting Carb Carburetor Tool NEW 08 0119
Motion Pro Pilot Screw Adjusting Carb Carburetor Tool NEW 08 0119

$22.94
Time Remaining: 16d 6h 47m
Buy It Now for only: $22.94
Click To Buy Now
4x YAMAHA 1978 XS1100 CARB KITS XS1100E
4x YAMAHA 1978 XS1100 CARB KITS XS1100E

$64.00 (1 Bid)
Time Remaining: 4h 26m
Click To Buy Now
Bid now
CARBURETOR ECHO PB 260L PROLITE PB260L POWER BLOWER LEAF BLOWER CARB 10MM NEW
CARBURETOR ECHO PB 260L PROLITE PB260L POWER BLOWER LEAF BLOWER CARB 10MM NEW

$24.95
Time Remaining: 3h 19m
Click To Buy Now
Bid now
RD Flex Jet Fuel Mixture Screw Flexjet CRF TRX YFZ Keihin FCR
RD Flex Jet Fuel Mixture Screw Flexjet CRF TRX YFZ Keihin FCR

$28.99
Time Remaining: 17d 3h 13m
Buy It Now for only: $28.99
Click To Buy Now
ROCHESTER TRI POWER 2 BBL 2GV 2GC 2 JET 3 BIG BORE  3 SMALL BORE CARBS + PARTS
ROCHESTER TRI POWER 2 BBL 2GV 2GC 2 JET 3 BIG BORE 3 SMALL BORE CARBS + PARTS

$99.00 (4 Bids)
Time Remaining: 1d 8h 49m
Click To Buy Now
Bid now
1980 87 suzuki gs1100 gs1000 gs850 gs750 gs550 mikuni BS carb O RING GASKET KIT
1980 87 suzuki gs1100 gs1000 gs850 gs750 gs550 mikuni BS carb O RING GASKET KIT

$24.50 (2 Bids)
Time Remaining: 1d 23h 34m
Click To Buy Now
Bid now
Motion Pro Motorcycle Carburetor Pilot Screw Adjusting Tool Carb Mixture 08 0119
Motion Pro Motorcycle Carburetor Pilot Screw Adjusting Tool Carb Mixture 08 0119

$22.95
Time Remaining: 4d 19h 21m
Buy It Now for only: $22.95
Click To Buy Now
1978 83 yamaha xs1100 xj1100 special mikuni BS CV carburetor O RING GASKET KIT
1978 83 yamaha xs1100 xj1100 special mikuni BS CV carburetor O RING GASKET KIT

$23.99 (2 Bids)
Time Remaining: 1d 23h 34m
Click To Buy Now
Bid now
1978 83 yamaha xs1100 xj1100 special mikuni BS CV carburetor O RING REBUILD KIT
1978 83 yamaha xs1100 xj1100 special mikuni BS CV carburetor O RING REBUILD KIT

$12.99 (1 Bid)
Time Remaining: 1d 23h 44m
Click To Buy Now
Bid now
AED Holley 4150 Rebuild Kit Double Pumper Carbs 750 650
AED Holley 4150 Rebuild Kit Double Pumper Carbs 750 650

$47.32
Time Remaining: 13d 18h 41m
Buy It Now for only: $47.32
Click To Buy Now
1980 87 suzuki gs1100 gs1000 gs850 gs750 gs550 mikuni BS carb O RING REBUILD KIT
1980 87 suzuki gs1100 gs1000 gs850 gs750 gs550 mikuni BS carb O RING REBUILD KIT

$12.99 (1 Bid)
Time Remaining: 1d 23h 44m
Click To Buy Now
Bid now
VINTAGE ANTIQUE HARLEY PRO REBUILT OEM BENDIX ZENITH CARBURETOR W OEM KIT
VINTAGE ANTIQUE HARLEY PRO REBUILT OEM BENDIX ZENITH CARBURETOR W OEM KIT

$204.99 (3 Bids)
Time Remaining: 2d 13h 36m
Click To Buy Now
Bid now
Quick Fuel Holley Carb 3310 Carburetor Vacuum 1850 Rebuild Kit +
Quick Fuel Holley Carb 3310 Carburetor Vacuum 1850 Rebuild Kit +

$39.95
Time Remaining: 16d 1h 39m
Buy It Now for only: $39.95
Click To Buy Now
Weber 48 IDA carbs with EMPI intakes HEX BAR linkage dual port VW Baja bug
Weber 48 IDA carbs with EMPI intakes HEX BAR linkage dual port VW Baja bug

$500.00 (7 Bids)
Time Remaining: 3d 4h 54m
Buy It Now for only: $1,250.00
Click To Buy Now
Bid now
CARBURETOR HONDA HELIX CN250 CN 250 SCOOTER COMPLETE CARB ASSEMBLY 1986 2001 NEW
CARBURETOR HONDA HELIX CN250 CN 250 SCOOTER COMPLETE CARB ASSEMBLY 1986 2001 NEW

$54.95
Time Remaining: 3h 10m
Click To Buy Now
Bid now
HOLLEY CARBURETOR 1850 3310 390 600 750 CFM REBUILD KIT
HOLLEY CARBURETOR 1850 3310 390 600 750 CFM REBUILD KIT

$29.99
Time Remaining: 19d 15h 59m
Buy It Now for only: $29.99
Click To Buy Now
 USED Honda ATC 200X carb carburetor bbr TTR 125 DRZ 125 CRF 150 Keihin 26mm
USED Honda ATC 200X carb carburetor bbr TTR 125 DRZ 125 CRF 150 Keihin 26mm

$0.99 (1 Bid)
Time Remaining: 3d 23h 5m
Click To Buy Now
Bid now
Honda  Shadow 1983 honda shadow vt 500 project bike
Honda Shadow 1983 honda shadow vt 500 project bike

$410.00 (7 Bids)
Time Remaining: 4d 2h 3m
Click To Buy Now
Bid now
HOLLEY LIST 7508 350cfm CARBURETOR 2 BBL SERIES 2300 ANNULAR BOOSTERS AUTO CHOKE
HOLLEY LIST 7508 350cfm CARBURETOR 2 BBL SERIES 2300 ANNULAR BOOSTERS AUTO CHOKE

$69.99 (1 Bid)
Time Remaining: 4d 2h 47m
Click To Buy Now
Bid now
Holley Brass Idle Mixture Screws
Holley Brass Idle Mixture Screws

$1.99
Time Remaining: 11d 17h 2m
Buy It Now for only: $1.99
Click To Buy Now
FORD58 MERCURY119 AUTOLITE 4100 4 BARREL CARBURETOR CORE
FORD58 MERCURY119 AUTOLITE 4100 4 BARREL CARBURETOR CORE

$250.00
Time Remaining: 4d 2h 56m
Click To Buy Now
Bid now
S  S SUPER E SHORTY CARB  COVER SHOVELHEAD PANHEAD KNUCKLEHEAD HARLEY EVO
S S SUPER E SHORTY CARB COVER SHOVELHEAD PANHEAD KNUCKLEHEAD HARLEY EVO

$285.00
Time Remaining: 4d 4h 1m
Click To Buy Now
Bid now
NEW ADJUSTABLE HONDA GCV160 CARBURETOR HRB216 HRS216 HRR216 HRT216 HRZ216 CARB
NEW ADJUSTABLE HONDA GCV160 CARBURETOR HRB216 HRS216 HRR216 HRT216 HRZ216 CARB

$18.25
Time Remaining: 8d 21h 47m
Buy It Now for only: $18.25
Click To Buy Now