99 yz400f question


5 replies to this topic
  • GANNAWAY88

Posted June 14, 2010 - 10:51 AM

#1

i just bought a 99 yz400f. it runs great but my question is why do i have to pull the decompression lever to start my kick and release it as I kick thru to start. the kick starter will not go down uless it gets help from the lever. Any thoughts?

Edited by GANNAWAY88, June 14, 2010 - 12:20 PM.


  • grayracer513

Posted June 14, 2010 - 12:38 PM

#2

In the first place, that's the wrong procedure:

http://www.yamaha-mo...troke_vid_a.mpg

http://www.yamaha-mo...troke_vid_b.mpg

The engine simply has too much compression to be kicked through the whole stroke. Therefore, the compression release is used to get past the first half of the compression stroke.

Compression begins when the intake valve closes as the piston starts up the bore at about 120 degrees before top dead center (TDC). So you push the engine up against the "hard spot" and take pressure off the starter lever. Pull the release lever in and then push the kick starter down about 1 - 1.5 inches more, then stop. Release the lever, move the crank back to the top, and give the engine a full, smooth stroke all the way through.

By doing this, you have taken the engine from 120 degrees before TDC to something more like 90 or 80 degrees BTDC. This "shortens" the compression stroke to a more manageable size and allows you to kick the engine past the ignition point with enough pressure to make it run.

  • GANNAWAY88

Posted June 14, 2010 - 01:09 PM

#3

I have an 03 yz450, which has the auto decompression.

sorry i didn't explain myself better.

to start my 99 i pull the lever in and let it get to tdc, then slowly kick it about an inch then release the lever and kick it thru. i can not kick the bike without ingaging the decompression lever. is that right? it seems odd, like the engine is locked up if i don't use the lever.

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  • jayh300

Posted June 14, 2010 - 01:35 PM

#4

with the manual decomp. you have to use the lever...
when i had mine, I would kick slowly, no decomp lever, until you hit the hard spot..
then pull the lever and kick the inch or so past the hard spot, release the decomp lever, return the kick start lever to the top and kick smoothly..
sounds complicated, but once you learn it, it is a piece of cake...
of course you could put a hot cams auto decomp cam in it and do away with the lever and make is just like your 450.. (or do the 450 cam mod also) there is tons of info on that here and a complete deal in the stickys at the top of this forum...

  • grayracer513

Posted June 14, 2010 - 01:45 PM

#5

I have an 03 yz450, which has the auto decompression.

sorry i didn't explain myself better.

to start my 99 i pull the lever in and let it get to tdc, then slowly kick it about an inch then release the lever and kick it thru. i can not kick the bike without ingaging the decompression lever. is that right? it seems odd, like the engine is locked up if i don't use the lever.

You explained it fine, and I understood you completely. My answer is correct the way I gave it to you. You can't kick the bike thorough without using the lever, and the way you described using it is incorrect.

Watch the videos.

  • GANNAWAY88

Posted June 14, 2010 - 05:37 PM

#6

You explained it fine, and I understood you completely. My answer is correct the way I gave it to you. You can't kick the bike thorough without using the lever, and the way you described using it is incorrect.

Watch the videos.


thanks guys, i bought this bike to race in the decade classes here in TX. And i got worried when i couldn't kick it.

Thanks for the videos, unfortunately i watched them after I responded. Shoulda watched them first. Also thanks for the hotcam advise, i'll look into it.





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