Motorcycle Repair: cycle not firing, honda shadow vt600, air fuel mixture

Question
i have a 1997 honda shadow vt600, thats not firing, what would be the most common thing to check for?  

Answer
Hi Freespirit,

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 13 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.

Troubleshooting
Engine Doesn't Start, Starting Difficulty:

1) Starter motor not rotating:

Starter lockout or neutral switch trouble
Starter motor trouble
Battery voltage low
Relays not contacting or operating
Starter button not contacting
Wiring open or shorted
Ignition switch trouble
Engine stop switch trouble
Fuse blown

2) Starter motor rotating but engine doesn't turn over:

Starter clutch trouble

3) Engine won't turn over:

Valve seizure
Cylinder, piston seizure
Connecting rod small end seizure
Connecting rod big end seizure
Transmission gear or bearing seizure
Camshaft seizure
Balancer bearing seizure

4) No fuel flow:

Fuel tap vacuum hose clogged
Fuel tank air vent obstructed
Fuel tap clogged
Fuel line clogged
Float valve clogged

5) Engine flooded:

Fuel level in carburetor float bowl too high
Float valve worn or stuck open
Starting technique faulty
(When flooded, crank the engine with the throttle fully open to allow more air to reach the engine)

6) No spark; spark weak:

Battery voltage low
Spark plug dirty, broken, or maladjusted
Spark plug cap or high tension wiring trouble
Spark plug cap not in good contact
Spark plug incorrect
IC ignitor trouble
Neutral, starter lockout, or side stand switch trouble
Pickup coil trouble
Ignition coil trouble
Ignition or engine stop switch shorted
Wiring shorted or open
Fuse blown

7) 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
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


Respectfully,
Mark Shively