New Top End YZ450F, Won't Start!!


14 replies to this topic
  • etzeldude

Posted November 13, 2008 - 05:56 PM

#1

Just installed Athena big bore in 2006 YZ450F. Bike won't start. Have spark, gas, cams lined up at top dead center of compression stroke( verified with straw in spark plug hole ), bike ran perfectly just prior to teardown. Compression seems good but not great kicking it over. Absolutely nothing but occasional backfire. Is it possible for rotor in magneto to "slip" or be mistimed ? Any info will help. :thumbsup:

  • TD-3

Posted November 13, 2008 - 06:08 PM

#2

This happened to me and a few others in here. My issue was the valve shims not seating correctly. Just do a quick check of your valve clearance and this will tell you if that is the problem. Good luck.

  • desert-rat660

Posted November 13, 2008 - 06:23 PM

#3

+1 on the above statement. when u did the big bore did u replace the cam chain?

  • 642MX

Posted November 13, 2008 - 06:28 PM

#4

An engine needs 3 things to run, spark, air/fuel, and compression.... Your not getting all of them or it would run.

How long was it apart?...(long enough for the gas to gum up the pilot?)

Are you sure it doesn't have a fouled plug? (its common for a fouled plug to create a little back fire noise, but it won't fire completely)

  • todds924

Posted November 13, 2008 - 07:13 PM

#5

An engine needs 3 things to run, spark, air/fuel, and compression.... Your not getting all of them or it would run.

How long was it apart?...(long enough for the gas to gum up the pilot?)

Are you sure it doesn't have a fouled plug? (its common for a fouled plug to create a little back fire noise, but it won't fire completely)


Correct but "spark" at the right time! How you gonna set up your cam timing with a straw in the sparkplug hole...thats pretty crude.. Why not do it the proper way like it says in the manual. Cam timing is probably off....Check that and your valve clearance

  • cablemonkey334

Posted November 13, 2008 - 08:11 PM

#6

I had same problem , my timing was off , i used the wrong mark on the crank. Its kinda hard to see i finally took the cover off so i could see it.

  • Charlie755

Posted November 14, 2008 - 09:33 AM

#7

I agree with cablemonkey334 and todds924. If the cam timing is off it will backfire of and on while trying to start it. Just because the cams timing marks line up doesnt mean its timed right. I have had to set up timing on more bikes than I care to remember because the owner thought they had the timing right on only to find out the cams were lined up but the crank showed one tooth off. If the bike ran before you took it apart it should start just fine now. Definitly go by the manual to set the timing.

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  • desert-rat660

Posted November 14, 2008 - 06:31 PM

#8

u cant really time spark timing on these bikes well i guess u could if u modified the pickup coil to advance or retard it but otherwise there is no way to. its all controlled through the cdi, but if u ment cam timing that would make sense

  • todds924

Posted November 14, 2008 - 06:35 PM

#9

u cant really time spark timing on these bikes well i guess u could if u modified the pickup coil to advance or retard it but otherwise there is no way to. its all controlled through the cdi, but if u ment cam timing that would make sense


yes, cam timing

  • yamaha542

Posted November 15, 2008 - 05:24 PM

#10

Timing is off! if your cams are not timed right, it can seem like poor compresion upon kick, but its really just your cam timing off.....make sure your piston is at TDC by the hash mark or the flywheel (left side case cover) on the top of the stator cover is a hole with a plug in it, remove both plugs from cover and use a 17mm deep socket to turn the flywheel counterclockwise untile the first hash mark is seen. Then lighn up the cams both sides with one dot on top and one on the out side level with the head. if the top and side dots dont make a right angle to the head, there are a tooth off, or two, its easy to put each cam one tooth too far towards the head surface. imagine a right angle triangle from the center of the cam to the dots. use this as a reference. good luck

  • redlineblues

Posted November 15, 2008 - 05:55 PM

#11

If you confirm valve timing is correct then do as was I was told and that is bump start the thing down a hill in 3rd gear. I had tried everything and it still only backfired, push it down the hill and its been running ever since.

  • etzeldude

Posted November 17, 2008 - 12:19 PM

#12

TD-3 and Desert Rat, DING DING DING !!!!!!!! You are the winners. One intake valve shim was not seated correctly. ( OK, I admit it was on my workbench). Replaced it and bike started on 2nd kick. Thanks to all for the information !:thumbsup:

  • TD-3

Posted November 17, 2008 - 12:27 PM

#13

That's good to hear! :thumbsup:
On mine, kicked my ass off for 2 hrs before I figured it out.:moon:

  • Kasjok

Posted November 17, 2008 - 11:57 PM

#14

TD-3 and Desert Rat, DING DING DING !!!!!!!! You are the winners. One intake valve shim was not seated correctly. ( OK, I admit it was on my workbench). Replaced it and bike started on 2nd kick. Thanks to all for the information !:thumbsup:


I have had it happen too. But luckily I checked the clearances before putting the cover back on.

  • Jake92

Posted November 18, 2008 - 01:11 PM

#15

Precisely Kasjok, I always check my clearences one last time. I have had this same frustrating thing happen.





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