6 replies to this topic
  • wskek

Posted September 26, 2013 - 09:42 PM


Hello everyone,


recently I have picked up a 1999 YZ400F. I took it out to the track one weekend and it ran good, idled fine and had crazy power. Then it sat for a few days and when I tried starting it again, nothing happened. So my first guess was maybe the carb, So I took it off and examined it, opened the throttle and noticed that it wasn't squirting gas. I took it apart and turned out my accelerator pump diaphragm was shot. So I replaced it and it squirted gas perfectly every time I opened the throttle. But when I put the carb back onto the bike and cranked it a few times, I still didn't budge. At first it felt like it wanted to start but then after numerous kicks it just didn't want start. I checked the spark and it was there. The compression was also there. And the accelerator pump was now good as new. My pilot screw is set to standard (1 3/8) turned out, got a brand new spark plug, great compression and fresh gas.


Anyone have this same issue. PLEASE any advice would be greatly appreciated!!! 

  • Thommo_400f

Posted September 28, 2013 - 02:41 AM


Sounds like a blocked pilot jet.

  • antanddionne

Posted September 29, 2013 - 12:32 PM


These bikes are infamous for getting warm and then not starting because the colits at the top of the intake valve pop out but do not worry instead of taking it to bits try pulling the bike with a car or another bike and do a bump this will force the valve to move and the colit will pop back in its happened to mine several times and this fixes it everytime 3rd gear bump is best mate hope this helps

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  • wskek

Posted October 01, 2013 - 06:59 PM


Thanks a bunch!! I got the bike running again!! But what are these collits you were referring to? Are they the 2 little pins that fall into the retainer under the bucket and shims to keep the springs tension?

  • grayracer513

Posted October 01, 2013 - 09:48 PM


"Collets" is the word, but Yamaha calls them "cotters", and most of the U.S. auto industry calls them keepers. 


While I'm not surprised that the bike started by bumping, I don't buy the collet theory for a second.  If you spit up a keeper, you'll drop a valve, simple as that. 


The two most likely causes are valves that don't have adequate clearance due to wear, or any kind of carbon or gum fouling where the valve stem meets the guide.  More rarely, dry piston rings can cause this, but they have to also be pretty tired. 


One last thing that turns up once in a while is rusty valve seats.  The seats are iron, and if you park it for long enough with the valves open in a coastal or sub-coastal climate, they can rust just enough to keep them from sealing until it runs for a bit.  Not good for them on the whole, though.  With thumpers, you can avoid that by rolling the engine up against compression after you shut it down.

  • wskek

Posted October 04, 2013 - 05:39 AM


Actually I pulled the carb out and clean it to the best of my ability. Possibly the blocked pilot jet theory was the most accurate to my problem. Then kicked it a few times and got it running. But im still not satisfied with the way its running. It seems to idle just fine, but the second I hold in the clutch and shift it into first, it dies.... Even with my throttle screw turned all the way right to max, it still tends to do this. I have to ref it a lot so it doesn't die and to even get it moving. Is this normal or not? Also when I start to ride and then come to a stop, it has the tendency to just shut off without me pushing the kill switch. I almost have to hold the throttle a bit to keep it alive.

  • Thommo_400f

Posted October 04, 2013 - 01:24 PM


Your fuel screw may have a perished o-ring or is not set correctly.

Should be around 1 1/2 to 2 1/2 turns out from bottom.

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