Crack on carb slide on 400

I just finished rebuilding the top end on my 99 400 (piston and rings). As I was installing the carb I noticed that the cylinder side of the carb slide is plastic. This has a crack in it toward the bottom. Anyone else have this problem before? Would this cause a some backfiring when I chop the throttle at speed?

I have ordered a new one and plan to install ASAP. $75 for that little piece of plastic! The bike is totally stock, and yes I did adjust the air screw long ago and it rarely backfired untill later this year.

Thanks in advance!

If the crack is all the way through and allows the motor to pull in air with the increased vacuum caused when you chop the throttle then yes it could cause backfireing (due to the lean condition caused by an air leak).

That's a lot of money for such a small part!

I know, but what choice do I have?

about three months ago someone new to the site advertised carb plates home made. it'll take a bit of work but go look for it. i would go back to october.

good luck


This problem is quite common on the WR400,s that are ridden around here.(high mileage as they are used for commuting as well as recreation).A few of the slides have broken up and pieces sucked into the engine (EXPEN$IVE)!I check my slide every 6 months for cracking on my 99 YZ400 and so far OK.Apparently the slide is made from carbon fibre thats why so expensive to replace.

OK, I replaced that expensive little piece in the carb and today was the first chance I had to ride it. Big difference! I turned the air screw in about 1/4 turn from where I had it (the screw is now about 2 1/4 turns out). It runs like new - no backfireing at all. Moral of the story is...if your YZF has a lot of hours on it, check that little carb slide plate for cracks. It costs a lot ($75), but what can you do? Stop riding? I think not!

By the way, mine was cracked about 75% from the bottom. It's made of aluminium, and if a piece were to come off, it would go into the cylinder and you would a new problem.

Good thread. As an aside on this info, later model YZF's can benefit by simply adding a small washer underneath the Allen bolt that secures the airbox adaptor to the carb intake. Later models have a bushing insert that often fails. When this bushing fails, the adaptor slides away from the carb intake and allows air to seep inside causing an extreme lean condition. An easy fix for this is to simply add a washer underneath the carb adaptor Allen bolts. Yamaha really missed the mark here.

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