Broken throttle slide plates/ aka direct METAL injection

Any more?

I just experienced my second carb slider disintegrating this past Saturday. Some of you may recall reading my post about the engine not coming back to idle properly, running up, etc., and it all started last September when my carb slider broke for the first time. The right side broke off the first time. The left side broke off last week. The one with the "L" on it is the newer one. There is a design difference between them. The original one seems thinner and stamped. The newer one is thicker and looks machined. You can see the extent of the damage on both but the original one has a big hairline crack in it that the newer one doesn't have.


The throttle stop is cut to about 9-10 mm long. I think that's how long it should be. I have a feeling that the original one broke due to old age. It had 15k miles on it at least. Now the new one, that's a real puzzler because it only has about 1k miles on it. But I have an idea: take a look at the detail I cut out from the above picture.


See anything different? Yes, the seals are installed the opposite way from each other. On the original one, the wide part of the seal is against the piece, while the newer one I had it turned around. I didn't make a note of the orientation of the old one when I pulled it out. I thought the manual would be explicit but it wasn't. So I installed the new seal on the new one facing the way it seemed logical. But that way doesn't seem to create as much of a buffer between the plate and the carrier as on the original. I'm wondering if it broke because the damping effect of the seal didn't work as well with it in like it is. And if it didn't seal properly, could that account for the intermittent running up? Could someone verify the proper orientation of the seal on a carb with the original slider plate?

BTW, I pulled the head and there was no damage. I found all the bits of both carb sliders in my muffler can. They were like little hunks of molten metal.

I think Rich is on to something here.

I haven't taken the rubber off of mine yet. The pic of Rich's on the left is installed the way mine was from the factory. The convex side towards the air filter. The rubber is gently U shaped and acts as a spacer/damper and has 3 contact points, the 2 edges on one side and the middle bump on the other side. In stock form middle the bump sits against the plate, the inside diameter edge sits in the little valley in the plate, and the outer high edge sits against the slide and gives it some spring effect. I just flipped mine over backwards and looked at it. Backwards, the only damping effect the rubber has between the slide and the plate is the thickness of the rubber itself. Both high edges sit against the plate and the high center sits in the little valley in the plate, so both contact points of the shock absorber are trapped in a solid object. The backwards rubber is not effective as a damper between the slide and plate, but whether or not that is causing the breaks, I can't say. It could be that, or maybe the slide bore is wearing in the carb creating clearance that allows too much vibration. Since the new one broke so soon there must be something else causing the problem, like the rubber.

Good call Rich. :thumbsup:


Well, took her out, almost reinstaleld and noticed a hairline crack. Houston...we have separation.

Both sides. Opened up with really not much pressure, really easy.

I guess when they get so much flex due to a crack that when you slam the throttle closed, the pressure from high speed air keeps the crack open slightly due to continued flow and when you blip the throttle, you relieve the pressure momentarily, the crack closes due to low pressure and then remains closed until you put it under extreme pressure again such as WOT to none. Am I making sense? I'm trying to understand why the throttle blip would temporarily cause the problem to disappear.


As for the seal, mine was installed as per original position diagrma and Rich's right photo.


anyway....first thing tomorrow (before I defend my strategic plan at work!!!) I'll be on the phone. I guess that you installed a stock slide Rich-Frostbite???

Yep i think we maybe about to discover a VERY IMPORTANT!,piece of technical info on plate instal here,does the new plate come with a new seal fitted (i`m thinking it don`t?)and if so a new seal with a new plate maybe a wise choice yes? :thumbsup:,but now that i have seen missile`s photo it looks very much like lamination cracking(what i mean by this is the metal is not fused and can be layerd leaving a weak spot)normally it is a result of dirty casting,an x-ray would tell for sure,a plate made from a billet may be the real cure here!

The depression is definitely the weak spot, but I'm thinking that for the original to last for years and then the replacement to last for months means there are other evil forces at work. Rich could have gotten a very badly cast plate that broke right away, but the backwards rubber may also have added. I ordered 2 plates for mine so I'm going to put the first one in with the rubber goofy and monitor it for cracks. Since I ride in extreme temps it may fatigue quickly. If it breaks, then I'll pop the other one in properly and monitor it. If that breaks then we'll know it must be in the carb body itself, like putting a new set of rings in a toasted cylinder. The rings get chewed up quick but it's not their fault. It would be nice if we could find folks who have broke and replaced plates that have lasted, and folks like Rich that have failed early, to test the O-ring theory so I wouldn't have to pull the top off my carb every week. With that in mind, I'm going to put up a new post to see if we can get more input.

A question for Rich - Did you make any other mods to your bike when you replaced the plate or just before the first one broke, like new pipe, swiss your airbox or rejet? When my bike's broke I tend to take the down time to fine tune and change things a bit. I'm looking at my plate and thinking that a couple of good backfires could speed up the flex process. The last few changes I made to my bike before the plate broke were to try running without the airbox lid, cracked my exhaust off right where the pipe enters the can, and then installed a new White ros E pipe. All of these made my bike run very lean and backfire. I didn't run long without the airbox lid, but had to ride 30 miles home with the broken exhaust. I put my biggest main in when I got the new pipe and even though it was lean and backfiring, I was so horny to ride after waiting for the pipe that I didn't care and drove it for 2 days solid. That's when the plate broke. Maybe it's not the age of the plate as much as it is the age of the bike. Bike gets old, muffler gets bad, bike gets paid off, extra money for parts, new header goes on.....ya never know.

It would be nice if we could find folks who have broke and replaced plates that have lasted,

Hate to beat an old dog but there is a post right here in this thread about a guy that replace 2 plates. After he fixed a modification he has had no more problems. BrandonW's post at the top of the 2nd page. Last I'll mention it, good luck, hope you get it dial in! :thumbsup:

Beat away, I'm all ears, or I guess in this case, I'm all eyes. Brandon did fix his stop and his 2nd new plate lasted. But - was it the stop that cured the problem, or had he installed the rubber O-ring backwards the first time like Rich did a bit higher on this page, and then install it correctly on the second one without knowing, and assume the throttle stop had cured it?

:thinking:I'm not ready to bet on the throttle stop just yet, but I'm open to all discussion.

Just to add a little to the discussion:

The Thursday before the carb slide broke I got my top end back together. I honed the cylinder, put in new rings and a new wrist pin and did some test rides to check operation. I did about 25 miles Thursday evening with no running-up, but it felt a little lean. So Saturday morning I upped the pj back up to a 38 and adjusted the pilot air screw and fuel screw accordingly. I was out on a test ride after the rejetting when it broke. I did a top speed run up to about 82 mph (GPS verified) for about a minute or so and was doing tests to see if I could get it to run up. One way to make it happen was to go to WOT and let off on the throttle completely. That has caused the running up on several occasions since September. The bike was responding just like I wanted it to (it would go back to idle and engine brake the bike). I did this about 10-15 times at about 55 mph when I noticed that it wasn't slowing down anymore. I let off the throttle completely and it maintained speed. Finally, I pulled in the clutch and braked to listen to the engine and it was running like I had the throttle wide open. The slide was cracked.

Could this have anything to do with the throttle stop? I don't see how, but I can see how the enormous pressures on it going from WOT to closed and back repeatedly could crack it. It still shouldn't happen, though. What is the proper throttle stop length, anyway?

Could this have anything to do with the throttle stop? I don't see how

The discussion I remember from a few months back was that by allowing it to go to far up into the throttle housing by having the screw too short it caused some problems. I can't remember what the conclusion was but if it gets too far up it could be several different things. The air flow across it could be unsupported, it could get cocked wierd by moving farther than it was designed for, ect, ect. FWIW I don't recall any YZ's having this problem and I look at that forum as often as this. Now I have been known to be wrong before so I could be this time too.

Not having a slide problem.....yet. However, having looked and listened to your accounts, I have a few questions that may shed some light.

1. With the carbs that have the shorter throttle stop installed, is the slider binding when in the WOT position? In other words, can you move or wiggle the slider when it is in WOT position? Does it move more or less or the same when it is in closed position?

2. If it is possible, can someone take a macro picture of the break area perpindicular to the break. I would be good to see the texture of the break area to see if we can determine if the break is a result of a porous casting or a fatigue failure.

3. The material appears to be aluminum, can anyone confirm what material this plate is made of?

My suspicion is that since these slides are breaking at the bottom, it is more likely that the breaks are occurring as a result of chopping the throttle from high RPMs. This creates high vacuum on one side of the slide and with atmospheric pressure on the other, it loads and unloads the tabs in a cyclic manner, which could lead to a fatigue failure. The amount of loading depends on the unsupported area of the tabs. Aluminum doesn't generally have good fatique characteristics. It will be good to have some more information.

Why a new one would crack sooner than the original is a bit of a mystery, but as Rich stated, the new one appeared to be manufactured differently.

I'm looking at my carb and checking the slide play at different openings. There is quite a bit of play but it is uniform from 1/4 to full throttle and above full the play doesn't seem to get worse. I don't have a snap guage and micrometer but the slide shaft looks like it is machined straight, so the play shouldn't get worse at the top, so I'm putting that on the B list.


I bought my bike new and didn't cut the stop. My slide completely clears the bore at full twist. I thought the shop might have cut the stop as part of the PDI but it looks factory to me, with a nice rounded top.


The only spot the play is different is below 1/4 throttle. The play is less down there. I checked to see why and see that there is a small metal block on either side of the center of the bore.


The plate is kind of floating with the slide until the bottom edge of the plate slips below the top of the metal block. Then it is supported snugly by it's bottom edge while the slide is still free to float. This seems to be making sense now. The bottom edge is locked in tight and the rest of the slide is vibrating back and forth due to the play in the slide shaft and finally wiggles it enough to crack the metal. For the guys who first had intermittent problems, maybe the bottom edge of the plate was cracked and would hang up a bit on the metal block since there's not much clearance. A couple of blips of the throttle could line it up so it would drop in.

I think the damage is being done at idle and just above. Take a look at the wear pattern on the plate. It is being worn when it is at the bottom of the bore. If it was taking a beating at full twist the wear pattern would be the opposite way, like a rising sun, wearing against the top of the bore.


A couple of guys broke their second plate pretty quickly. Rich noticed that his rubber O-ring was in backwards. That lessens the damping between the plate and slide and could give it more of a beating. Installed properly it acts as a cushion. If the boys changing them don't realize this, they may have installed the O-ring correctly the second time by accident, like Rich did, and the plate held up. They also changed other stuff so they figured that was the cure.

I'm putting my money on the O-ring, and am thinking that a new one should be installed since the old one may be dried out and dead and not offer much cushion between the plate and slide. Why aren't the YZ's breaking theirs? I dunno, I looked at a parts slide and the plate/slide setup looks identical. Maybe they don't make as much power as we do down low and have to keep it whacked to have any fun.

Just my 2c. Of course I could be wrong, I'm just making this up as I go.

My slide plate also broke I had removed the throttle stop

Is this a coincidence :thumbsup:

My dealer replaced my slide plate a few weeks ago:

My bike ran fine for the first couple of outings but is now idling fast, backfiring and will not tick over again (as it was prior to the slide replacement). I guess the plate has broken again.

I would like to disassemble the carb, remove the plate and take a look for myself.

As I am not that confident with mechanics: could any of you kind people provide me with simple instructions on how to remove/replace the carb slide plate.

from memory so all details might not be correct

- undo the throttle cable holder on the handlebars - this frees the throttl cable allowing you to pull the carb further out than normal.

- undo the screws/bolts on the large rubber tubes that lead in (air in) and out (air out) of the carb body

- turn gas off an disconnect the gas intake pipe from the tank

- Disconnect the hot start mechanism attached to the frame and let it dangle

- pull the rubber pipe of the back (air intake) and wedge it behind the frame. Will take quite some force

- gently wiggle the carb till it comes free

- pull it out slightly, feeding the throttle cables as you do

- undo the top plate of the carb body

- undo the centre brass colored hex bolt

- using a pair of tweezers grab the top of the needle and pull it out

- VERY IMPORTANT OR YOU WONT SUCCEED: there's a shaft going througg the side of the carb and holding the carb slide/shuttle in place, with s spring on its outer end you need to loosen this and pull the shaft out slightly to allow you to rotate the mechanism you see, up and out of the carb body. There is a spring on the shaft which you need to fight against, its quite tricky.

- the shuttle and slide will then lift out if you have done it right

- you then need to check the slide under a bright light for hairline cracks and make sure the rubber seal is installed the correct way round. see thread.

good luck

Thanks very much. I will give it a go tonight.


I took the slide plate out:

The ruber seal was installed the rong way round but I cannot see any hairline cracks?

The black coating/paint is coming off the plate though. Could the bits of coating be blocking the jets maybe.

I'm out of ideas now :thumbsup:

Any help would be much appreciated.

Black bits coming off ?? if they are, they are likely going stratight into the engine and NOT likely to blocking any jets. They would have to enter the fuel line for that.

Check very closely under a bright light, use a magnifying glass if youi have to. They are loacted along the bottom edges of the curved portion of the rounded/stamped profile of the slide. They are difficult to see. Try GENTLY flexing the edge of the slide to see a bit better. Hope yours is intact and you can save some £££'s

Also...check the fuel petcock. Someone here had an issue with the petcock being fouled and gas not flowing create a vacuum in the fuel line and all sorts of problems.

Whilst you have your carb out clean the jets anyway and do the "Taffy Mod" (Search) on your squirt!!! :thumbsup: Seriously though, if you havent done it you should. Also clean the accelerator pump cos its likely all gunged up.

Note well. Be very careful when taking off the bottom casing of the carb, and especially with the fuel screw which you should remove to clean as well...there is a tiny rubber o ring and washer and spring which you will lose if you are not careful.

Also check the air jets for blockage. There's a metal pipe diameter 1.5 inches on the intake side. Remove this and you'll see them.

Use a compressore to blow air through the jets but ensure all seals and rubber bits are well hidden or you'll be cursing all night.

Finally, if you can get a rear derailleur mountain bike cable protector use this to protect the entrance to the accelerator pump where the actuating arm enters it.

On reassembly, make sure you align the actuating arm of the accelerator pump or you'll finish the job and see that you have to start over !!!

good luck

we also have a few 98 wr400 with the broken slide,

i was told the post2000 wr4 carb will solve this problem,

anybody can confirmed this?


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