QuestionHello, I have bought a 1983 yamaha xj650 special (maxim) japanese spec imported in to the uk back in 1994, my problem is with its running i have had the carbs cleaned and balanced every thing is ok, when the bike is reved while stationary there is no miss fire but when out on the road and under load it miss fires as if there is some fuel starvation could it be to do with the vacum pipe leading from the petrol tap to the carb inlet rubber have you heard of any problems with this xj model . many thanks mark.
AnswerHi Mark,
The problem may be from variety of causes. Basic troubleshooting needs to be performed to accurately diagnose the problem.
I suspect a restricted air intake or lack of fuel.
I return tech files via email. There was no email address for you as requested. I pasted a couple of tech files to help you. Obvisouly, you can tell why email is necessary.
Respectfully,
Mark Shively
Poor Running at Low Speed:
Spark weak:
Battery voltage low
Spark plug dirty, broken, or maladjusted
Spark plug cap or high tension wiring trouble
Spark plug cap shorted or not in good contact
Spark plug incorrect
igniter trouble
Pickup coil trouble
Ignition coil trouble
Fuel/air mixture incorrect:
Pilot screw maladjusted
Pilot jet, or air passage clogged
Air bleed pipe, bleed holes clogged
Pilot passage clogged
Air cleaner clogged, poorly sealed or missing
Starter plunger stuck open
Fuel level in carburetor float chamber too high or too low
Fuel tank air vent obstructed
Carburetor holder loose
Surge tank duct loose
Compression low:
Spark plug loose
Cylinder head not sufficiently tightened down
Cylinder, piston worn
Piston ring bad (worn, weak, broken, or sticking)
Piston ring/land clearance excessive
Cylinder head gasket damaged
Cylinder head warped
Engine not sufficiently warmed up after lash adjuster installation
Valve spring broken or weak
Valve not seating properly (valve bent, worn, or carbon accumulation on the seating surface)
Hydraulic lash adjuster damaged (worn, seizure, or spring broken)
Hydraulic lash adjuster oil passage clogged
Backfiring when decelerating:
Vacuum switch valve broken
Air suction valve trouble
Coasting enricher trouble
Other:
Igniter trouble
Carburetors not synchronizing
Carburetor vacuum piston doesn't slide smoothly
Engine oil viscosity too high
Drive train trouble
Final gear case oil viscosity too high
Brake dragging
Air suction valve trouble
Vacuum switch valve trouble
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Troubleshooting
An engine needs three elements to run: 1) adequate cylinder compression, 2) a spark at the exact time, and 3) correct air-fuel mixture.
With a running engine the first thing that should be done is a compression test.
With a well warmed engine remove the spark plugs and the air filter, if you have constant velocity (CV) carbs. Hold the sliders (throttle slides) up with a finger once the comp tester is installed, and hold the throttle grip wide open as you push the start button. Record this reading and do each cylinder this same way for as many cylinders as there are.
Then do a wet comp test by adding 6 to 10 drops of what ever engine oil you use to each plug hole. Spin the engine over one time before you install the comp tester to distribute the oil you just added. Repeat the same test procedure. On paper you will then have DRY: cylinder 1, cylinder 2, cylinder 3, cylinder 4 and that test reading. Under that will be WET: cylinder 1, cylinder 2, and so on for as many cylinders as you have. Then you'll have a known set of figures.
Next, test each wire and plug for visual spark. Using a plug plier's tool is recommended. You might not get zapped if you use them. In any case don't support your body with the bike. With the key ON hold the plug tight against the engine and push the button and watch the plug spark. It should be bright fat and blue. Set engine timing if it is possible. These days on many bikes this is not possible with out doing tricks. Older bikes can still be adjusted mechanically
If a plug fails test the plug wire first, as the ends tend to fail first. One way is to remove the wire when possible. Some bikes have built in sets with the coil these days.
With an ohm meter hooked to the ends of a wire wiggle it. A good wire will have a reading as book spec. A dead wire will not read. Check the wires to the ignition coils and to the ECU (electronic control unit) aka: CDI or TCI. Obviously if there is a compression problem the engine is in need of repair. If you get no spark or weak yellow spark there is a problem. Weak spark can be a weak battery problem. If you were able to crank the bike well 1-3 times as stated above, probably the battery needs no testing. If the cranking became weak and/or you get yellow spark, the battery needs a charge and some testing.
Compression, spark, and fuel should be tested in this ORDER. If compression and spark are ok, then fuel can be considered. For fuel related problems please see vacuum leaks.
Also consider a leak-down test in conjunction with the compression test.
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Bad Gas
By M. Shively
Gasoline goes bad with time and in as little as 3-4 weeks. This effect is known as varnishing. Jets and passageways within carbs become obstructed when varnishing occurs.
Liquid gasoline changes chemically into a gel like substance. Advanced stages of varnishing results with the solid gel changing into a crystal powder substance. Interior carb surfaces are etched in the process and may require carb replacement.
The choke and pilot circuits with most motorcycle carburetors share passageways. When pilot jets become obstructed, the choke circuit compensates and allows engine to start and idle with choke, but stalls without choke.
See the carb cleaning information file below. See microfiche parts, check parts prices, and order repair manuals at: "www.bikebandit.com"
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, o'rings, 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 each Venturi (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 o'ring. 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 by spraying cleaner and wiping clean. Other material type floats may require replacement if cleaning is necessary. Inspect the needle valve (float valve) and seat. Check needle valve's spring loaded pin. It should depress and return smoothly and without resistance. Check the needle valve's tip for a worn groove. Replace needle valve and seat if either symptom exists. These parts wear together and must be replaced as a set.
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).
