yz400f wont start!

8 replies to this topic
  • yamaha rider 507

Posted November 14, 2011 - 02:25 PM


I just bought a yz400f for $400. bike is mint but i cant get it started! the guy i bought it from didnt know anything about it and sold it for a friend. he knew it sat for a year. i couldnt get the bike to pop off when i looked at it but bought it because of how good of a deal it was. when i got it home i took the carb off, and cleaned the bowl and drained the gas. put it all back together and a splash of new gas, still wouldnt pop. i know to put the chooke on and dont touch the throttle. i also know it has low compression , and probably has a bent intake valve. im now trying to get the spark plug out. i took it all apart again, seat and tank off, top motor mount off but cant get to the plug. do i need a special tool to get to it? any suggetions? also even if it does have low compression i should still be able to get it to fire, am i right?

  • RasmusDK

Posted November 14, 2011 - 03:22 PM


How low is the compression? Are there no hard spots when you kick it? Maybe the compression valve is stuck/hanging. Are there any spark?
I use a 3/8 plug hex, no problem

  • DeeCee

Posted November 14, 2011 - 06:57 PM


What do you mean by low compression, lower than you expected, or no compression ?

If you kick it over slowly you should feel some resistance. Assuming you have some resistance you have some compression and it shold fire.

Lack of compression could be the decompression lever is seized/stuck and jamming the exhaust valve open. If you have the decompression lever on the handlebar then make sure there is some slack in the cable and the lever on the cyclinder head is closing properly.

If no lever you may have a bike with a decompression exhaust cam, Sometimes (but rarely) these stick and can cause a lack of compression.

I would check these first before considering bent valves and the like.

Also if the bike has sat for a while chances are the smaller jets in your carburettor are gummed up. Make sure these are checked thoroughly.

Once all is correct, the bike should start OK.

There is a video in the sticky's at the top of the forum page which shows how to start the bike, take a look at these as well.

  • bonehead7101

Posted November 16, 2011 - 11:55 AM


look at it this way, if it has good compression it should dead stop like the motor is seizing if you dont hold the decomp, if you keep pressure on the kicker the compression will bleed off slowly, but you shouldn't be able to continuosly kick through the cycle without the decomp.

second, 400s are a bitch if you've never had one, i don't know if you've owned one before but ill throw this out there

heres my starting sequence:
gas on
choke on
hold in decomp and kick it 5 or 6 times to get fuel in the cylinder
twist throttle 3 times for a little extra fuel from the acc pump
release decomp
kick until you hit tdc (you can't kick anymore)
pull in decomp and kick just enought to get past tdc
bring kicker all the way back up
twist throttle 3 times for a little extra fuel from the acc pump
and kick through

it sounds like alot but once you get used to it its easy.also, while kicking do not twist the throttle like a 2 stroke your leg will pay for it

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  • grayracer513

Posted November 16, 2011 - 12:24 PM





The plug takes a 5/8" plug socket and a universal joint.

  • bonehead7101

Posted November 17, 2011 - 01:02 PM


awesome gray, i didnt know there where videos :busted:

  • eflyguy

Posted November 22, 2011 - 07:38 PM


I'm new here and to thumpers, but after I cleaned the carb and replaced the float valve seat O-ring, my ('99 YZ400F) bike starts first or second kick following the procedure in the video.

  • MrN2OBelvedere

Posted November 23, 2011 - 03:20 AM


back to basics...

fuel, spark, compression. If you have these, then she'll run.

My "drill"

gas on, choke on
kick through 4 or 5 times with compression release to clear things out
take hands off compression release and kick slowly until you find the hard spot, and by hard I mean you can stand on the kickstarter and it won't budge
pull compression lever and kick over only a very little bit
bring kickstarter all the way back up, and give er a kick with all you've got

When my bike is cold, it usually takes a few tries. Also, if you weigh less than 150, its gonna be tough I think. My brother is smaller than me (I weigh 165) and he can't start the bike no matter what.

  • yamaha rider 507

Posted February 18, 2012 - 11:18 AM


I just got this bike back from my mechanic yesterday. so the reason why it wouldnt stary is because it sucked a part in the carb through the intake and it was clanking around in the cylinder. luckily i didnt need a new cylinder, put in a new piston and got the valves lapped. my mechanic had it running and ready to go. when i got there he couldnt start it because he just got surgery on his foot. i brought it home and now i cant start it. i turn on the choke and gas give two small turns of the throttle and kick and kick and still nothing. im only 130 lbs but can kick it hard for the most part ( sometimes having trouble getting through the last ring of compression right before the footpeg). is the reason i can start it because i cant kick through that last ring of compression hard enough?

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