yz400 questions

7 replies to this topic
  • camshaft_89

Posted January 25, 2010 - 01:00 PM


I've got a 99 yz400f I picked up about 3 months ago. As far as I know the bike only has an FMF bowerbomb exhaust but I am just assuming it has no motor work since I bought it second hand. The thing ran very poorly. So after jetting and rejetting I end up running a 205 main jet in it, do a plug chop and the plug looks perfect a nice light mocha brown. It's got a 50 pilot and I'm unsure of what needle it has in it but I had to go all the way up to the second groove from the top to get the midrange to clear out. It runs clean through out it's rev range now and I was happy with it.

I rode it mostly on the track for about two weeks almost every day with no problems and now I am getting this intermittent problem. It will be running fine and all of a sudden it's like I flipped a switch and I lose all my power, the thing won't even get out of it's own way. I take it back and pull the plug and it's covered in a black sooty mess. Looks like I fouled it????. So I put a new plug in it starts fine, I warm it up run another 4th gear pull to wide open kill the engine look at the plug and it looks perfect light mocha brown all the way down the porcalin. Take it back out to the track, after about 20 mins or so of riding it goes right back to the loss of power. I shut it down and let it sit for about 10 mins and start it back up and it runs fine for about 5 mins then loses power again. I searched the forums and found a similar situation and his problem was a bad CDI unit. I have yet to test mine but will as i assumed it was an electrical problem causing an intermittent weak spark.

However, when it happened first and I replaced the plug I also pulled the top cover off the cams to check my valves. If I pull the two plugs on my side cover and align the marks on my flywheel the cam lobes are pointing towards each other. I searched the forums and everyone finds thier TDC by putting the cam lobes facing opposite each other (the manual says this as well as aligning the timing marks) My question is, when I align the timing marks on the fly wheel are my cams supposed to be facing opposite of each other or facing each other?

Also, from what I read a 205 is a pretty large jet compared to what everyone else is running. Do I have a problem even tho my plugs look good and my bike runs great, (other than when it decided to run terrible)

Sorry for the extremely long post. and thanks in advance

  • grayracer513

Posted January 25, 2010 - 03:18 PM


Are you running unleaded fuel?

  • tek_01

Posted January 26, 2010 - 12:52 AM


Regarding the cam lobes. Turn the crank one more revolution and they will be facing outwards. Right now your on the exhaust stroke i believe (lobes in) where the manual wants you on the compression stroke (lobes out).

They seem like really large jets too

  • camshaft_89

Posted January 26, 2010 - 12:13 PM


I'm running shell 93 octane, when I bought the bike it had a 168 in it, and absolutely no top end, yet I never heard any pinging so I wouldnt think it has too much motor work done. But the jet size might be telling me different.

Thank you, for some reason I thought I somehow had my cam timing completely off...yet still ran ok :ride:

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  • grayracer513

Posted January 26, 2010 - 02:21 PM


If you are running unleaded fuel, you cannot use a spark plug read as a gauge of the correct mixture. It was the lead in the fuel that provided the color on the plug, and the same jetting that produces a nice light tan plug chop on leaded gas will come out bone white with unleaded. It's meaningless to read plugs with unleaded fuel.

At present, your main jet is way out of line with the normal range for these (175 is stock), and your plug is coming up sooty.

  • rmx250rider

Posted January 26, 2010 - 10:47 PM


whats the main on the 1998? the same

  • DeeCee

Posted January 27, 2010 - 12:57 AM


Stock main jet for the 98 is 175. I run my 98 at sea level with 165 main.

  • camshaft_89

Posted January 27, 2010 - 01:02 AM


Thanks greyracer. This makes sense, when I bought the bike it was jetted lean, I obviously over corrected this by assuming I could read the plug like I have done in the past. However in the past it was on a trx470r running vp-110. I'll drop my main jet size, about what range should I be running, stock with a full FMF exhaust. I was thinking somewhere around 185 or is that still too rich?

I am running close to sea level, about 400-1000ft range

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