Mmcarp - Great advice, but I just wanted to add this one concern.

I've had problems using compressed air on carb passages since it can force debris to pack down into the channels of a passage, causing partial or full blockage.

The best way to clear a passage is to use carb clean with a tube that fits into the passage. This will flush out any debris instead of pressing it in. Wear eye protection when doing this since it can shoot back out at you.

You can however use compressed air if you can get into the backside of the channel, but it depends on the air fitting (aka a needle) and size and design of hole.

I have had carbs continue to fail after multiple re-builds due to this concern. In the earlier days, soaking or boiling a carb would help with this, but....don't think I would do that with today's carbs.


302 - I assume you have tried replacing the spark plug? This is the only thing that has made mine run bad.


98 WR400F, YZ'd cam, YZ muffler

i have problems with the bike idling correctly(See post under wierd idle problem). I was reading through some of the archives and I can across somebody talking about there tps. I have checked my tps using an ohms meter and it seems fine and within spec. Although if the bike is running and i disconnect the tps there is no difference. If anybody knows what a good running yz400/426 runs like with a disconnected tps let me know. Or if somebody had a similar problem.


If it were me, I would take the carb off, remove the covers, and clean the heck out it.

Verify float level height while your're at it. Spray solvent and compressed air into all jets and passages.

I would do this not just because you're having a carburation problem, but because you never know if the last owner EVER cleaned it.

If that doesn't work, which I think it will, turn your fuel screw out a tad. No adjustments on this screw can combat a dirty carb/pilot jet, so don't get carried away. About 3 turns out is the max because it could pop out on you.

i use a snap-on airjet with the rubber tip. in it i screw a valve for pumping up footballs (soccer) & rugby balls etc. the 1/32" valve is 1 1/2" long & it can get everywhere.



Have you checked over your hot start circuit? Is it bar mounted or in the stock factory location? Make sure it is closing off.

Do BOTH your throttle cables have a a little slack? Any fraying or wear on the cable or rough movement?

Is the grip binding on the bar and sticking?

What is the pilot jet and how many turns out on the pilot screw? (you know that turning out is RICHER on these) You may need to go out to 2+ turns.

Someone mentioned the black manifold between the carb and head can sometimes develop air leaks, but tipping the bike should not have any symptoms with this.

Keep looking, this is an unusual problem.


thanks guys for your responses they are appreciated greatly in my quest to figure out this problem. The plug of course is new and the cables are routed correctly, and i have cleaned the carb so well i could use it as a beer mug. if i forgot to mention in my original post, the bike is completely stock besides the 180 main and the clip in the fifth position.

On the black rubber manifold there is a black bump that is generated from the factory. Is this used to position the clamp? Because the clamps have an area for this ridge to seat. According to pictures found in the manual the screws on the clamps go on the top of the manifold. if i seat the ridge in the clamp it is at the bottom.

I went out and examined my intake boot just to refresh my memory. If you are referring to the rubber manifold that fits between the carb and cylinder head, then we are on the same track. This boot has a groove on both ends to ensure that the boot is positioned properly. The cylinder head and carb throat both have little tabs machined into them that fit into these grooves. This boot will install one way...and one way only. Otherwise, you would never get the carb to mount properly. As far as the clamps as concerned, I just intalled them so that I can get the Allen head wrench on them easily.

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