QuestionI recently rebuilt the engine for a yamaha XT 225. Reassembly went smooth with no problems. I also replaced the entire electrical harnass that was damaged. When I try to start the bike, the starter turns and the piston moves. Once the cyclinder is primed with the choke, iy begins to fire but will not start. There is ample air flow, fuel appears to be flowing and I have checked for spark witch is present. Any ideas why it will not start?
AnswerHi DJ,
The cause of the problem could be from a variety of sources. Start with the following and via process of elimination, work towards finding the cause.
Review the following tech file and perform the checks mentioned:
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. Crank engine no longer than 10-15 seconds at a time. This prevents the starter motor from over-heating.
Then do a wet comp test by adding a teaspoon engine oil 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.
Respectfully,
Mark Shively
