WR400F Problem

So I'm about ready to order a new slide plate from RonAyers.com and I've got a couple of more questions for you WR pros. Whne ordering the slide plate, do you think I'm going to need the seal that goes along with it? According the fiche at RonAyers, the part numbers are 5JG-14546-00-00 (plate) and 4FN-14997-00-00 (seal). I'm also going to order a head gasket and valve cover gasket, because I think I may take the head off and do a bit of an inspection.

cooper, thanks for the effort and help :)

the valve cover gasket is reuseable 100's of times.

You will get a few uses out of the head gasket too if you are lucky.

If there is any ridge left on the head gasket, it will seal up. just give it a light coasting of white threebond :)

sweet, being a college kid, i'm all about saving money :) Thanks for the tips Hamish.

I have the same problem with my carb slide. It has broken twice now and I don't ride every weekend. Does anyone know what a long term fix would be? I don't want to deal with this issue again. I see a few comments about auto decompression cams. Is this where the problem lies? Who sells them?

The best fix for the broken slide problem is an Edelbrock carb. Got tired of replacing mine. Search for threads in this forum.

The auto decomp cams have nothing to do with the carb problem. How long did your 2nd slide plate last? As per my PM, you may have installed the O ring backwards or the throttle slide itself has too much play in the carb body.

I am very curious as to whether or not this is a chronic issue or not. What exactly causes the failure? Are there any aftermarket replacements that more resistant to this failure? I'd love to try an edelbrock, but unless I find an awesome deal on one, that's not an option. It would cut into my rekluse fund as well :thumbsup:

Kieethes PM'd me today and here's what I sent him. :thumbsup:

I studied the wear patterns of my carb body, slide and plate. Just to clarify what I’m describing – the carb slide is the block that holds the needle and rides on the little wheels and the plate is the mostly flat piece of metal connected to the slide that is what breaks.

The slide plate shows most wear when the throttle is in the idle to ¼ position. When the slide is almost closed the bottom corners of the plate fit into a slot in the carb body that tend to hold the plate from moving back and forth. The carb slide itself has a bit of play and is free to move back and forth a bit in the slide guide. Since the plate is connected to the slide the plate moves back and forth with pulses of the engine but the bottom corners are held by the slots in the carb body. There is a bending force right at the weak spot - the indented section of the plate and cracks begin to form at the corners of the indented spots.

My best guess is that lugging the engine at small throttle openings and engine braking is causing the problem. With small throttle openings there’s not much velocity through the carb and engine pulses push back through the intake valves a bit due to the overlap of the cam, and push against the carb slide causing it to vibrate back and forth. At higher rpm’s with larger throttle openings the velocity through the carb is probably strong enough to prevent the engine pulses from popping back into the carb. I think the most damage is done at small throttle openings when the corners of the slide plate are trapped and engine pulses are rocking the slide back and forth.

There is quite a bit of play between my slide and the channel it rides in in the carb body. Maybe replacing the wheels would help, or maybe the body is too worn. I think that the less back and forth play the slide has in the body, the less chance of premature failure of the plate. I even thought about gluing thin feeler strips into the carb body to reduce the play.

One last important thing is the rubber O ring between the plate and the slide. I’ve heard of a few guys who broke a plate, installed a new one and it failed in a week or 2. Then they installed another one and it lasted for years. This is probably because they installed the O ring backwards. In the proper position it acts as a shock absorber between the slide and plate. If it’s installed backwards it does nothing and I think that would cause the quick failure that some guys experienced. Since they pulled it apart right away, they didn’t know which way the O ring was installed.

The YZ’s have less plate failures probably because they don’t do as much trail riding, putting along at small throttle openings.

Of course, I could be wrong :thumbsup:

Talk to BDMmotard on SMJ, he's been through this a few times, and finally found some custom billet plate. another solution I've heard is that a FCR2 from a later 426 will not have the same failures as the carb used on the 400's.

i've replaced my carb to a yz426f carb for close to 2 years n the slide plate still last till now. plus it gives a better responds compared to the stock carb. :thumbsup:

juz my 2 cents :thumbsup:

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