new member's 400 won't idle

I just purchased a 98 yz400f today. I think that I will love it once I get the starting drill down. Any tricks? More importantly, once I get it running it won't idle. I've tried adjusting it but even all of the way up it still dies. HELP!

You need a valve job.

I think pulling the carb apart and dipping it and doing a good job of blowing air in every jet would be alot cheaper. A carb job does wonders for yzf's. Since you just bought it perhaps it's safe to say it sat for a while, and with today's gasoline it's only good for about 3 weeks. My carb was gummed up when I bought my yzf and also would start but not idle. I bought a gallon of carb dip and soaked it for an hour then blew air in every place possible and put it all back together and it ran great. Also check to see what jet's are installed and call around to make sure their the right ones for which ever pipe is installed. Good luck! Jeff


A word of warning, from personal experience most rubber o-rings and seals will well up in carb cleaner. You may want to separate these from the carb parts before you dip it.

If the carb job doesn't do it then you may also want to do a compression test to see if everything is ok. Valves that are too tight can hang open a tad and can cause idleing problems too (but you would likely notice a loss of power as well).

Maybe a little more information will help. This bike was a test bike for Cannondale when they were developing there fuel injection system, so the carb was never on it until recently. Also, I don't have any idea how it is jetted (I assume it's stock). I ride in the mtns of NC so any jetting suggestions would be helpful.

Before you rebuild the motor :) try to clean the carb.

As far as making jet recommendations, you'll should provide altitude and bike modifications.

good luck



99 WR400f, YZ timed, MX-Tech suspension, Scotts steering damper, White Bros E-Series (12 discs), tapered header and a/f. Kouba T-handle for the fuel screw. Works Connection billet throttle tube and frame guards. Cycra Pro-Bend, triple clamp mount handguards. Thumper Racing rad guards, Renthal Jimmy Button highs, YZ Tank and IMS seat, YZ number plate, odo removed, EKP, 48PJ, 175MJ at 500-1000' Thanks James Dean!

My bad. FMF header and FMF Power Core and as far as I know that's it. Should stock jetting be OK? Oh yeah altitude is approx. 3000'. Thanx!

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