wont start


8 replies to this topic
  • tf705

Posted May 14, 2008 - 07:19 AM

#1

i just put the motor back in my bike and it wont start now. it back fired once and that is it. do you think my timing is off? by the way the bike is a 03 450.

  • tf705

Posted May 14, 2008 - 07:48 AM

#2

well i just pulled the head cover off and the timing is spot on. these motors arent like cars motors are they. because i can turn the flywheel one rotation back to the "I" mark again and the marks on the cam arent right then. does it matter what stroke you are on the "I" mark. hope you understand my question. i checked the spark and it fine. this bike ran fine before i tore it down. i didnt touch the carb when i pulled the motor so i wouldnt think that was the problem.

  • usmcpaul

Posted May 14, 2008 - 08:52 AM

#3

I am pretty sure it must be at TDC on the compression stroke. So bring it to TDC using the screwdriver method, and make sure all the valves are closed by checking for some slack in the rocker arm. If the marks are lined up, it is in time.

Don't overlook the obvious, fuel and spark. (forgetting to turn the fuel on happens to the best of us eventually, especially when you are in a hurry)

  • RJB

Posted May 14, 2008 - 08:56 AM

#4

I am pretty sure it must be at TDC on the compression stroke. So bring it to TDC using the screwdriver method, and make sure all the valves are closed by checking for some slack in the rocker arm. If the marks are lined up, it is in time.

Don't overlook the obvious, fuel and spark. (forgetting to turn the fuel on happens to the best of us eventually, especially when you are in a hurry)


Yeah, tell me about overlooking the obvious. Last week cleaned the airfilter and box. Left the TwinAir cover in while drying out, rushed off to go riding this weekend and could not start. Kicked and tried bump starting before the penny dropped and I realised my mistake. By then, it was well flooded and took another 30 mins effort to get her going. Did I feel like an idiot or what :thumbsup: :rolleyes: :rolleyes:

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  • grayracer513

Posted May 14, 2008 - 09:10 AM

#5

well i just pulled the head cover off and the timing is spot on. these motors arent like cars motors are they. because i can turn the flywheel one rotation back to the "I" mark again and the marks on the cam arent right then. does it matter what stroke you are on the "I" mark. hope you understand my question. i checked the spark and it fine. this bike ran fine before i tore it down. i didnt touch the carb when i pulled the motor so i wouldnt think that was the problem.

On the contrary, they are exactly like cars. Four-strokes take, oddly enough, four strokes of the piston to complete a cycle. That's two revolutions of the crank. Since there is only one intake and exhaust event during those two revolutions, the cams turn at half the speed of the crank.

When checking the timing of an assembled YZF engine, the proper TDC to use is at the top of the compression stroke. There is no particular need to identify this stroke by removing the plug. The cam lobes will be facing outward and upward at this point. Align the TDC ("I") mark, and check the cams.

When assembling the engine, it makes no difference which TDC is used, since the cams are what makes one the top of the compression stroke, and the other the top of the exhaust stroke. The ignition is different than most cars, in that it is triggered directly from the crank. That means there is a spark on every TDC, instead of every other, so the ignition doesn't need to be sync'ed with the camshafts.

  • tf705

Posted May 14, 2008 - 09:29 AM

#6

thanks gray. in that case then i have it timed perfectly. but all that does is confuse me more why it wont start and back fires every so often. also why do they put on dot on the flywheel also and not use it as a timing mark. i think that is weird.

  • tf705

Posted May 14, 2008 - 10:41 AM

#7

it seems to kick kind of easy. is it posssible for the decompression system to fail and stay open and not let it build enough compression? im not real sure how it works.

  • grayracer513

Posted May 14, 2008 - 11:14 AM

#8

It is possible to dislodge the flywheel key while assembling it. Use a screwdriver or whatever to probe through the plug hole to find actual TDC. With the piston at the top, the flywheel mark ("I") should line up. If it doesn't, you have your problem.

  • grayracer513

Posted May 14, 2008 - 11:16 AM

#9

As to the decompression system, no, it can't stick open, etc., but if the cam is incorrectly timed due to the flywheel being mis-indexed, it can close too late and relieve too much compression.





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