Temperamental YZ400F

9 replies to this topic
  • 400ex38317

Posted August 26, 2008 - 09:07 PM


I've got a big problem, and it's been keeping me from riding this summer, so any help would be awesome.
When first kick the bike (98 YZF400) it idles just awesome, but when I start moving (or just twist the throttle) the bike loses a SUBSTANTIAL amount of power. So much in fact that it hardly pulls me on flat ground. The bike Backfires, and bogs, and even burnt my White Brothers pipe to a crisp.
Here's the thing though! the bike runs like BRAND NEW every 25 or so times I start it up, but if I turn it off it will almost always go back to it's poor performance. The first time it started doing this was after I did a run up Chokecherry hill at St. Anthony Dunes in Idaho and turned it off. After 2 times of kicking it over it ran great the rest of the day. Now that I'm back in oregon it hasn't run.
At first I thought that it was a simple dirty airfilter, so I cleaned it. Still ran like crap. Next thing I did was clean out the carburator... a few times. Same thing. Next I got into the head, checked the valve clearances, adjusted the valves (one intake was a bit off), and noticed that the timing was a little funky. The actual cams lined up with eachother, but the H shaped timing mark on the flywheel was off by a few degrees, so Rotated both the cams counter clockwise one tooth. Now the timing seemed to be closer (on the other side of the timing mark) But still a little bit off (stretched Timing Chain?). So Decided to put it back together, and take it on the street for a test run. The bike runs AWESOME! Better than EVER! I pull in the driveway, turn it off, come back not 2 minutes later, and guess what... It's got the same problem.
:thumbsup: not too thrilled about riding the 400ex around while I know a perfectly good (sometimes) YZ is sitting in the garage. Please help me diagnose this problem!!!

  • DeeCee

Posted August 27, 2008 - 01:38 AM


The timing seeming to be a little out is not a problem. Have you rechecked the valve timing again to establish whether it has slipped again ? If yes then definitely need to change your cam chain.

Have you tried a new spark plug ?

Otherwise I would suspect the carburettion in some form or another. Check the rubber intake manifold on the inlet side of the carburettor as maybe there is a leak there. Also make sure all the passages in the carburettor are clean - take out all the jets and blow them out thoroughly.

Finally check your gas tank and make sure it is clean inside and there is no water there.

Good luck

  • jayh300

Posted August 27, 2008 - 05:04 AM


the only time i had any trouble with my 99 running bad, the gas cap was bad and not venting. pretty easy to check, just open the cap up about 1/2 a turn. You could hear pressure escaping out of mine, then it would run fine.
deecee has some good tips to.
good luck

  • grayracer513

Posted August 27, 2008 - 05:47 AM


Water was one of the things I thought of at first, too. Look for it in the bottom of the jet access plug. It will separate from the gas in beads.

If that's not it, see what happens when you disconnect the TPS. Doing this will put the CDI in a "base timing" mode in which it will map the advance curve only by RPM. If that seems to fix it, test the TPS.

  • 400ex38317

Posted August 27, 2008 - 07:36 AM


Hey thanks for the help. I'll check the chain again and while I'm in there replace the spark plug. I'll also look into the tps.

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  • Michael Cahill

Posted August 27, 2008 - 05:48 PM


sounds like somthing simple to me.

start with the easiest first :
1. spark plug, look at its condition, it will tell you alot about how the motor is running
2. water in your gas.
3. somthing in your carborator. (dirt, gummed up fuel etc.)

if all else fails, start with the electrical stuff that guyracer mentioned. :thumbsup:
good luck! :worthy:

  • motojoe_23

Posted August 27, 2008 - 05:54 PM


With it going from horrible, to perfect intermittently it sounds more electrical to me, like maybe a bad ground on th ignition or something.


Posted August 28, 2008 - 01:34 AM


Yep.. sounds electrical to me also. Its usually the culprit anytime you have intermittent issues like this.

  • 400ex38317

Posted December 25, 2008 - 04:14 PM


It was the CDI. Now I wish this snow would go away so I could ride it...

  • rdefonce

Posted December 25, 2008 - 07:46 PM


It was the CDI. Now I wish this snow would go away so I could ride it...

Congrats on finding the problem.
About the snow . . . all in good time my friend:ride:

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