Hi guys

When I first put my wr400 back together, I'll admit, I didn't check the valve clearances. That said, it ran great - would tick over for hours, smooth power when cruising, no backfiring..

Recently I notice it getting worse for problems like flame-out (ticking over, then there's just a "phut" sound and it dies), a horrendous amount of backfiring and popping on overrun (e.g. coasting downhill with the throttle closed, but also eevn just revving it at a standstil, as the engine returns to idle it pops.

Bogging down isnt so bad, rarely does it but sometimes if I tweak the throttle at idle, there's just a "phut phut" and it dies

What should I be looking at first? hotstart air leaks? pilot jet blockage? valve clearances?

Many thanks!
cj

Backfiring during decel under compression braking conditions with the throttle closed has typically been an overly lean condition of my fuel mixture screw.

It won't feel strong off-idle either, because it's too lean.

Check valve clearances first.
Adjust fuel screw for max idle.
Determine if more or less than 1.5 turns out, and adjust pilot size, or look for lean air leaks.
Check apump for even-strong stream squirt, and no dribble on the slide. That will cause havoc with jetting.

Fuel/Air screw good place to look first as others stated. Sometimes mine will pop like crazy at higher altitude till I adjust it.

Is your air cleaner clean?

Typically your 400 or 450 will get hard to start if the valves are getting out of adjustment.Good indiactor you need to adjust them.

Popping and backfiring upon close of throttle can sometimes be due to a bad gasket between the head and pipe. If it can suck air there it will pop like crazy. It is a copper crush gasket and usually you can get away using the old one for several head pipe removal/lnstallations but once they start to leak, that is it.THIS is a real easy fix!

I just went through all sorts of problems with mine tying to get it to run good again. I had parked it for about 6-8 months (this was two years back) as I had some knee work done.

Well I got it going again about a year ago after it was showing obvious signs of green carb from no/little use. Hard to start, would not idle unless choke was on, dies or ran real crappy at idle, etc.

I went through the carb cleaned and inspected all but it still ran crappy. I rode it two different times about 2-3 months apart, filling up each time with fresh gas. I ALWAYS doctor up my fuel with Stabil, Seafoam, Startron, and sometimes even a little Marvel. Carb was clean but fuel did not smell real "good" due to some of the gas being so old (I know some of what was in there had to be close to two years old) and it was running/acting lean and getting so hot at idle it would start to smoke my carbon fiber heat shield on the head pipe!

I had gone through the carb (two more times) and found nothing.

I completely changed out the fuel for new and that fixed it! Wish I had done that BEFORE I went through the carb the last two times! Du-OH!

So that said, I would check or replace your gas first! Also the 400 is supposed to be run on 95 octane. Highest I could find was 91 so I added some octane booster too.

Good luck, keep us posted!

Just re-shimmed it; all valves on the lower limit, except the kicker side intake valve which was 0.05 too tight..

Not hugely out but theyre all back at the biggest gap allowed by tolerance now.. I'll see how it goes. Gas in this bike is regularly renewed, and it's never been a hard start wince learning the technique.. But my YZ426.. having re-shimmed that also I think i'll be putting my time into setting up the carb properly cos that one won;t idle for toffee