WR400 header glows red hot


I have a 98 WR400F Euro spec, and am curious about the tendancy for the header pipe to glow red hot when on the cold choke.

A bit of background... I bought the bike 2nd hand a few months back, so have no idea what the jetting is etc.

Up until now I have only carried out basic maintenance, e.g. oil change etc. but am starting to think a bit more about other maintenance that I should be doing.

I noticed this behaviour (red hot glow) while preparing for an early morning winter ride a while back, but never thought much about it. Now, the engine is quite nosiy and I am planning to replace the piston and rings proactively, and was wondering if the red hot header could be a sign of a lean mixture and perhaps an accelerated wear of the piston etc... While doing the work to replace the piston I plan to take a look at the jetting etc. The bike is running a standard pipe right now, but had a free-flow pipe of unknown brand when I bought it. When I change dthe pipe for stock, I did nothing to the carb...

Any advice is welcome...



Wilts, UK

I had this same concern when I bought my 03 WR450.

I talked to my local mechanic at the Yahama dealership, (whom I trust, he's been racing and wrenching Yahama's for years), and he says this is normal due to the fact that the header on the Yahama is only a single wall pipe.

Most other dirt bikes have a double wall exhaust pipe, so you never see the outer pipe glowing red.

Although a glowing pipe is normal, I still wouldn't let the bike idle on choke for more then a minute, and/or idle without the choke for more then a couple of minutes.

Thats how I light my cigar.

So, do you think its running lean, or is it normal? Only reason I want ot be sure is that the bike has definitely increased it's engine noise since I took ownership, hence the new piston, rings etc that I will be putting in soon... Another detail is that if left idling, it starts boiling within about 3 minutes, which doesn't sound right to me...

Again, thanks in advance.

I personally don't let my bike idle for more then 3 minutes.

Your bike is mostly likely running toward the lean side. The bikes usually are set towards lean from the factory.

You can change this by changing the settings on your fuel/air screw. Set it so that it is 2 1/2 to 3 turns out. This will make it run less lean off of idle.

Your fuel/air screw is buried inside a small hole on the very bottom of your carburator.

Why would you let it idle for 3 minutes?

Go ride the thing.

does it pop on deceleration? If it doesnt, it's probably not lean on the pilot circuit. I'd investigate the engine noise before you replace the piston/rings. The clutch basket is possibly the cause of the noise.

Only occassionaly... I had the "noise" listened to by a couple of MX workshops, and the consensus is that it's "piston slap" or worn piston... hence the new piston and rings waiting to be installed.

Don't forget that a skid plate reflects the engin noise upwards. A WR will make more engine noise than a YZ because of that.

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now