QuestionHi Mark,
I have a Yamaha '78 XS 650 2F0..(manufactured Jul '78. For the last few years I've been trying to find out out about left side carb/petcock leaks. 1st) I had to coat the tank to stop the rust clogging the jet and/or the the petcock and carb. filters 2nd) I found leaks at petcock and rebuilt it. 3d) Seeping/leaking from carb (from the drain plug and maybe elsewhere) accompanied by hard starting sometimes. Checked all seals and replaced membrane. 4th) Gas is getting through ok to both cylinders. Sometimes the left plug is dry, sometimes it's wet, (doesn't change anything.) 5th) I checked the timing and points to make sure starting was not hampered by either. 6th) Gas sometime makes its way to the air filter. 7th) My last step was to change the tang clearance as suggested in my Clymer book. It seems to have made the starting, as well as (see next item) sputtering, worse.
Finally, the longest lasting problem has been the sputtering and hesitation anywhere beyond 2000 rpm. On occasion I have drained the bowl from the plug below, but this doesn't necessarily eliminate the sputtering problem, and sometimes the gas flow from the drain hole won't stop even when the bowl should have emptied long ago. Whew!... this was all a mouthful. Think you can help? Thank you, -Guy P.S. If you think it would be helpful, you can certainly post this on another site as well.
AnswerHi Guy,
I suspect needle valves (float valves) are sticking or worn and is causing fuel to flow to air filter (and to crankcase). Check engine oil for pressence of gasoline. If in doubt, change oil and filter. Do this after carb problem is repaired.
Adjust float heights to Clymer specifications by bending the tangs and measuring with Verneer/dial calipers.
Leaking drain plug may be caused by one of the following: Worn drain screw seal or damaged threads
Float bowl overfilling
Split (hairline) brass overfill tube (difficult to see)
Worn float bowl gasket
The hesitating and sputtering is due to overly rich air-fuel mixture or restricted air intake.
Remove spark plugs and compare them to those on NGK's website. Once the page opens, look to the left menu and click thru the "Plug Problems" links.
www.ngk.com/sparkplug411.asp
Visit Yamaha's website to see microfiche parts on line. Do the same at Bike Bandit.
www.yamaha-motor.com (enter info)
www.bikebandit.com (click OEM Parts and enter info)
Check cylinder compression with warmed engine if possible. If not, follow cold engine compression testing procedures. If readings are not within service limits as found in Clymer manual, perform a leak-down test to determine specific cause of cylinder leakage.
Look into this information and let me know what you find. I know it's a bit involved, but things to do. Most local car parts stores rent or loan testers as I've described.
Send me an email address. I'll return several information files that will assist you. Free email servers such as Yahoo and Hotmail do not accept my files. See a couple sample files below.
Respectfully,
Mark Shively
Carb Cleaning 101
By M. Shively
The elements of internal combustion engines are: correct fuel/air ratio, spark at right time, adequate cylinder compression.
There are many passageways and openings to check and clean. All are important in function and when obstructed or not working properly, have subtle to radical effects on engine performance. Vacuum leaks and carburetor synchronization also have effects on performance and should be inspected and adjusted following the below procedures.
Carb Cleaning 101
Warning: Remove all rubber parts before you begin. These parts usually include vacuum diaphragms, needle valves, orings, hoses, and other parts. Spray cleaners will damage these parts. Do not disassemble individual carbs from the carb bracket.
Air & Fuel Passageways: Trace and learn individual fuel and air circuits from beginning to end. Machines can only drill straight through the cast passageways. To change direction, another angled passageway must be drilled. The union is plugged with a brass or bronze bead. Inspect and clean each passageway with spray cleaner, brushes/pipe cleaners/etc, and compressed air. Remove any discoloration and debris. Look for spray cleaner to exit from one or more passageways.
Jet Cleaning: Inspect jets by holding to light and look through them. You should see an unobstructed round hole. Clean the jets with one or more of the following: jet cleaning wires, soak solutions, carb spray cleaners and compressed air. Re-inspect jets after cleaning and install when clear of obstructions. Some main jets have paper-like gaskets. Most have metal spacers between the jet and the emulsion tube. Some screw directly into a brass emulsion tube which is machined for a 7mm wrench at its float chamber exposed base.
Inlet Fuel Valve: Inspect the needle valve & spring. Press down the tiny metal rod that protrudes from the butt or float end of the needle valve. The spring should move freely and return the rod to its location. Check the needle valve's seat area for a groove or other wear. It should appear highly polished. Some needle valve seats are rubber and wear may not be visible. Inspect the needle valve jet seat. You can clean the jet seat with Q-tips and semi-chrome polish if necessary.
Carb Body Castings: Blow air through the atmospheric vent holes located on the dome of each float bowl chamber. Air should exit via hoses or brass nipples. Inspect the emulsion tubes and passageways (cast towers that jets thread into) for discoloration and debris. Clean interior emulsion towers with a soft bristle gun cleaning brush. Clean the Venturi's (main carb bore).
Needle Jets & Jet Needles: Clean the needle jets, jet needles, and passageway or tower that needle jet screws into. Clean the emulsion tube (pipe between needle jet and main jet) (Main Jet may screw into emulsion tube). Jet needles are part of the throttle slides. See below…
Throttle Slides: There are several types of throttle slides: Mechanical linkage, vacuum, diaphragm, and cable. Disassembling the jet needle from the slide is not always required for cleaning. If you have vacuum piston type throttle slides (large diameter solid metal slide), avoid cleaning the lubrication from sides and caps. If piston type check cap vents and passageways with air. Clean if necessary and re-lube. If you have rubber vacuum throttle diaphragms, inspect for dry-rot, defects, and tears by gently stretching rubber away from center. Do this until all areas around diaphragm have been inspected. Replace any defective part as described above. Clean carb body areas around diaphragm including air passageways and air jets. Diaphragms have a locator loop or tab fabricated into their sealing edge. Observe this locator upon reassembly. Avoid pinching the diaphragm when reinstalling caps.
Fuel Screws: Fuel screws have sharp tapered ends. Carefully turn one fuel screw in while counting the turns until it seats lightly. Warning: These screws are very easily damaged if over tightened into their seats. Record amount of "turns-in" and remove the fuel screw, spring, washer, and oring. The fuel screw is part of the enrichment (choke) circuit...clean passageways as described above. When carbs are assembled, spray low PSI compressed air into diaphragm air vents located at intake side of carbs. Throttle slides should rise, then fall when air is removed. Lightly lube external moving linkages. Reinstall carbs and follow through with carburetor synchronization.
Throttle Cables: Lubricate cables periodically. If cables are disconnected from carbs or removed for replacement, etc . . . remember cable routing and ensure proper reinstallation routing. Avoid bread-tying, sharp bends, and pinching cables. Adjust cables so throttle grip has about 5mm of play or throttle slides or butterfly valves may not open completely (full throttle)(wide full open).
Float Bowls: Inspect float bowls for sediment, gum or varnish, crystallization, and defects. Clean all pipes, tubes, passageways, and embedded jets with cleaners and compressed air. Remove and clean the drain screw and area. Inspect bowl gasket and replace if necessary. Clean and inspect overflow pipes and tubes, look for vertical cracks.
Floats: There are several types of float materials: plastic, brass, black composite, tin, and others. Handle floats carefully. Avoid bending, twisting, denting, or other means of mishandling. Most floats are adjustable by bending a small metal tab near the float axle end. Do not change the float adjuster tab unless tuning fuel service levels. Clean metal floats by soaking or spray cleaners, if necessary. Replace other type floats if cleaning is necessary. Clean the float axle or pin.
Synchronization: This is a fine adjustment performed usually and preferably with the carbs installed and the engine running. The unusual part is performed with gauged wire with the carbs on the work bench. Carburetor synchronizing balances Venturi vacuum at the exhaust side of each carburetor, resulting with smooth idling and optimized performance at all throttle openings. Synchronization is checked using a set of gauges which are either air vacuum type or liquid mercury type. The gauges are connected to vacuum ports on the intake manifolds via nipple tubes or if sealed with screws, sync gauge adapters will be needed. With the engine running at temperature, and with a fan or means of forced convection aimed onto the engine, the carbs fuel screws and idle are adjusted, then the synchronization is adjusted via adjustment screws on the carbs. A reserve fuel tank is recommended for convenience of accessing carbs during this procedure. See gauge instructions and repair manuals for detailed use of synchronization gauges.
Notes: While carbs are apart, record the jet sizes. Look for a very small number imprinted on the body of the jets. Verify that numbers are the same for all jets on models with in-line cylinders. A few transverse-4 models and V-engines, the inner and outer carbs use some different size jets and it's important to not mix them up. If you have dial or veneer calipers, measure and record float heights. Perform measurements with floats just touching needle valves, though not depressing the needle valve rods. Replace fuel and vacuum hoses. Be sure to use fuel rated hose for fuel. Install or replace in-line fuel filters. It's a good time to remove and clean interior petcock fuel filters. Inspect carb manifolds for dry-rotting, inspect all clamps and air ducts. Inspect, clean, lube, and/or replace air filter(s).
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To adjust float height:
1. Remove float bowl orings or gaskets stuck to carburetor body. If they separated with bowls, leave alone.
2. Tilt the carbs at about 45 degree angle, or just enough to allow floats to touch the float valves (AKA: fuel or needle valves).
3. Do not allow the floats to compress the float valve pin & spring when measuring.
4. With the metal adjustment tang on the float just touching, but not compressing the float valve spring, place the setting gauge or dial caliper over the highest part of the float.
5. In most applications, if the floats are different heights on the same carburetor (right to left); use the highest float (the one farthest away from gasket surface).
6. To correct the float height setting, bend the small metal tang, then recheck the measurement. Bend the tang a little bit at a time until you get the desired height. Floats must be within a .5mm (1/2mm) or .020" range of each other.
Why adjust the float height?
A carburetor is just a fancy siphon. The piston creates a vacuum that siphons gas out of the float bowl through the various jets. If the level is low, the bike will run lean because the gas must siphon too high on a given vacuum. If the level is high, it siphons too much and runs rich.
If the level is erratic (sticky float hinge) it can act lean, then rich, and back again driving you nuts.
If the level is way too high, the floats may not stop gas flow and the engine and air box may take on fuel. Gas may flow until the tank runs dry. Some carbs have overflow tubes so it just runs out on the ground.
Changing the float height changes the level of the fuel in the float bowl. Fuel height adjusts: full throttle, 2k-3k rpm or partial throttle cruise. To give a scale of change, if the bike runs well when cold, but gets a bit sloppy when fully warmed up, lower the fuel level 1mm (i.e. go from 15mm to 16mm float height - remember the float measurement is "backwards").
Setting the fuel level on a carb with adjustable float heights is the way to jetting perfection. A too high or too low of a fuel level will cause the engine to run too rich or too lean at low rpm cruise and at full throttle / 3k (on sport bike) or 2k (on cruiser).
When do I adjust the float height?
When installing a Carb Recalibration Kit
When rebuilding carbs (if necessary)
To adjust low rpm areas of throttle operation that isn't pilot jet, fuel screw, or needle height related.
When a bike starts running rich at low RPM
