2002 YZ426 fuelling system possibly not set up right? Can you help?

2 replies to this topic
  • cjard

Posted October 26, 2009 - 11:39 AM


Hi guys

Got a few questions regarding a YZ426 I've just gotten to complement the WR400 I recently built.

The first few times I tried the YZ it was being an awkward start. The guy I bought it off claims it hasnt been used more than once or twice in the last 2 years and it certainly looks like it's been collecting some dust. I dumped the fuel and put in some fresh. Plug looked a little black(oily rather than sooty) but at the point I was examining it I hadn't actually gotten it running.

The spark seemed weak and intermittent so I took the tank and seat off the YZ to get better access to the wiring. The bottom if the seat is burned/melted and there is molten plastic in the airbox; possibly been on fire.
After tut-tutting at the truly horrendous mess that the after-bodged lighting wiring is I disconnected the whole lot of it to remove one variable from the equation.
Swapped in the coil and HT lead off the WR but no change to the spark. Swapped in the CDI but the WR's CDI wouldn't work at all. Cleaned up the YZ CDI connectors and the spark improved; it was still weak, but at least it was consistent.

Put all back and got it started after a few determined kickings, but I had to hold the throttle very (very) slightly open. I let it warm up and took the choke off but it didn't idle very well and seemed prone to cutting out. The idle also seemed high, but I havent measured it with a tacho to know what the rpm is. When blipping the throttle the revs would rise and fall in step with the throttle position, but would seem to hang high for a few moments before dropping to a more normal tickover (like something that should be sliding was sticking?). By "hang high" I mean that for example: the engine would be doing 2000 rpm (a guess). I'd blip it to 4000 and it would fall to 2500 and hang there for a few seconds before dropping to 2000. All RPMs are for illustrative purposes only :bonk:

The bike is fitted with an aftermarket FMF exhaust and there's an anodised knob with numbers on the side on the bottom of the carb where the pilot air screw should be (according to the manual)

Of most concern to me was that I had to shut the bike off after a few minutes of running because the header pipe was glowing bright red; yes the idling had been a lot faster than the WR's idle but I was surprised the exhaust had gotten that hot so soon..

I didn't check the plug after I shut it off (it was too dark; darn winter!)

A few questions:
Given that it's a bit more of a pig to start than my WR, and even when warmed up it wanted the choke knob on before it would go and it cooked the header pipe very quickly, do you think it's running lean, or has an ignition timing problem?
Is the apparent fire in the airbox going to be a result of something the engine did (this fuelling problem?), or the seriously shoddy electrical wiring for the lights?
If it's had some nugget fooling around with jetting, mixtures, air screws etc can someone suggest to me the way I go about setting up the carb properly?
I'm going to see if I can find a timing light and check that the spark is falling in the H mark on the flywheel, but how is the timing altered?
Can you recommend what to check to trace the issue of the weak spark? (Coil and its immediate connections have been ruled out)

Thanks in advance for any help. If I've left any detail out, please shoot me a question


Visit the ThumperTalk Store for the lowest prices on motorcycle / ATV parts and accessories - Guaranteed
  • grayracer513

Posted October 26, 2009 - 12:13 PM


A hanging idle and the need for the choke when warm indicate a lean condition, which could be tight intake valves as well as lean mixture.

Timing is not adjustable. If it's not right, the flywheel key is sheared or the CDI is defective.

  • phart2005

Posted October 26, 2009 - 03:52 PM


Glowing header is *mostly* due to long cam overlap, its just the nature of the engine. Imagine if the pipe were clear, you'd see a lot of flame coming out of the exhaust port. If its lean, that doesn't help but the header is thin and with the cam timing it will glow when sitting still too long.

All that being said, check for vacuum leaks.

Related Content


The "going back to the other darkside" adventure... by Monk

Dirt Bike   Make / Model Specific   KTM   250/350/450/505 SX-F & XC-F (4-Strokes)
  • Hot  113 replies

Wilson the Blue Privateer by jetfuel

Dirt Bike   Special Interest Forums   Pro Racing
  • Hot  50 replies

1998 yz400f will not idle. by trolley

Dirt Bike   Make / Model Specific   Yamaha   YZ 400/426/450
  • Hot  32 replies


Dirt Bike   Make / Model Specific   Yamaha   YZ 400/426/450
  • Hot  30 replies

Megabomb Fitment by 288yz450

Dirt Bike   Make / Model Specific   Yamaha   YZ 400/426/450
  • 1 reply

Join Our Community!

Even if you don't want to post, registered members get access to tools that make finding & following the good stuff easier.

If you enjoyed reading about "" here in the ThumperTalk archive, you'll LOVE our community. Come join ThumperTalk today!

The views and opinions expressed on this page are strictly those of the author, and have not been reviewed or approved by ThumperTalk.