Broken throttle slide plates/ aka direct METAL injection

If you have an older WR I'd suggest you remove the throttle slide and check the flat plate that sits in front of it carefully. Last week I backed off the throttle after a long WFO stretch and my bike acted like the throttle was stuck open a bit. It was idling very high and I had to blip the kill button to get home. I did a quick check and found nothing wrong with the throttle, cables or airbox. I put up a post here and someone suggested that the slide may have come apart. Thanks to aremean_troy TT Bronze Member who told me not to try to start the engine and check the slide. I took the carb apart today and Troy was right. At first glance it looked like only one corner of the slide plate had broken away. On closer inspection I could see a crack running around both sides and it came apart with a little wiggle. The piece that broke away while I was riding didn't seem to do any damage to the engine, maybe because I was full throttle and it had enough velocity to get shot clean through. I read another post where someone wasn't so lucky and chewed up his top end. Some guys suggest that the break may be caused by the slide moving too far in the carb due to modified throttle stop. By looking at this plate and where it sits, I'd say that it vibrates back and forth like crazy with every pulse of the engine. The metal gets fatigued and cracks at edge of the circular indentation. My bike is a 99 and I ride 12 months of the year and have a lot of miles on it. I suggest that it may be worth your while to do a little preventative maintenance and check yours now. Even though it broke, the metal is not soft and is about the thickness of a dime. Better to pull it out the top of the carb than to shoot it through your exhaust.






i ride allot. and my bikes tend to do strange things after lots months of riding them every day. but i have never have a carb slide fall apart.


nice pics. thx.

ewwww did you finde the broken shard of plastic or is it stuck in your motor?

Yeah i have seen others with this problem!thanks for reminding us that it is wort checking this out once the bike has done a bit of work,yep i would guess it is more likely vibration fatigue thas causes the slide to crack!

The plate is not plastic, it's fairly thick metal. It must've been ejected out of the exhaust pipe.

I thought I remember seeing a post in the past that this may be caused by cutting your throttle stop to short. Don't remember for sure but you may want to measure it to be sure.

I checked mine before I had the top end off the bike last weekend and without disassembly the thing look just fine. I think however I will open it up completely and have a good look under a strong light. Could you see the break without dissasembly or did you only see it once you disassembled ?

My throttle blipping was just the same even after I had reassembled my top end so I'm hoping you are correct.

It would be nearly impossible to see the crack without removing the slide and probably using a magnifying glass. The crack around mine almost looked like a dark line in the coating. I wiggled it a bit since it was broken anyway, and the crack opened up. Maybe you could even do a leak test like we used to check combustion chamber/valve sealing. Lay the plate down flat and pour a bit of varsol into the depression. Wait a few minutes and see if it seeps through to the other side. The simplest and safest thing would probably be to just replace it if you've got a few miles on the bike.

With that in mind I looked at the carb and the slide and I don't think that would affect it. The piece broke off of the bottom, so if it was a stop it would make more sense that the idle screw was the problem, letting the plate slam shut. but I don't think it's that either. To make this a bit less confusing, I'll call the flat part that broke the plate and the main piece the slide. The flat plate sits in a pocket in the slide and is supported and protected by the slide everywhere except the bottom, especially at each bottom corner. The throttle slide is basically like a gate valve limiting flow to the engine. The engine wants to run WFO all the time like a garden hose, and the slide cuts off the flow, like holding your thumb over the end of the hose. There's not much pressure on your thumb if you cut off half the flow, but there is a lot more as you try to completely cut off the flow, like the closed slide. Does that make sense to you? I'm thinking that these unsupported corner tabs flex a bit with each pulse of the engine, and there's probably more pressure on them at 1/4 throttle and lower since the restriction is small. At full throttle there's probably a lot less pressure. The corners flex and cracks gradually develop at the weak spot, the stamped indentation, and the crack gradually works it's way along until pieces start coming off.

I'm guessing that the cut throttle stop has nothing to do with the problem. It's a normal wear item, like your cam chain that eventually could fail. Of course, I may be wrong. I just making this up as I go along. :)




Nah i would say you on the money with your assumption that the cut stop is not the problem nor the idle screw(unless you can see any sign of contact on the carb where the broken corner sits when the slide is closed?)i would guess that you are right that this may be a result of high vacume pressure in the inlet tract tearing the weakest point of the plate appart,IE:when flat out high vacume,then shutting down to no throttle where you start to hold the motor back by engine braking!,result is the motor still wants air but cant get it so it tears at every thing in it`s path,i guess the corner of the plate where the stamp is is the weakest part so it gets drawn in first,seems more like a design flaw than a modification problem to me? :)

How careful were you in cutting the throttle stop? I think they call for trimming that daddy down by 18 mm. If you cut it down shorter than that, the slide can actually go above the "throat" of the carb, and that causes some fluctuations that allow the slide plate to vibrate.

I have gone through two of these slides. First time, I crossed my fingers, replaced the slide, and hoped for no damage. Second time it happened, I freaked out, and pulled the piston, head and cylinder. Everything looked to be in tip-top shape, so I again replaced the slide, fixed my throttle stop, and have had no problems since.


so I again replaced the slide, fixed my throttle stop, and have had no problems since.

Ok I knew I heard that somewhere. Thanks for posting.

I didn't cut mine. I bought the bike new, rode it for a while, heard about the long stop and thought I was in for a big power boost. I pulled the airbox boot back from the carb and stuck my finger in to feel the slide travel. It went all the way up. I don't know if the dealer did it or if Canadian bikes are different.

It doesn't seem as obvious to me, but if you've replaced yours 3 times and the stop seemed to cure the problem that may be it. I'm gonna pop my slide back in the carb to see if it goes above the bore and if it's looser up that high. There may be turbulence created in the slot if the slide goes up too high. That's why this site is so great! Lots of different eyes and brains working together, thinking outside the air box. :)

So what is the REAL reason these slides are breaking? I've recently bought a carb from a '98 Wr400 to put on my S model Drz. I purchased it from a person in Singapore who has replaced the stock slide with a cnc'd metal slide. I just took the carb apart to inspect the slide, the slide's held up fine, the material is very resilient. I'm a bit concerned about wear on the literal body of the carb from the newer/harder metal. If the throttle stop is the definitive cause of the breaking, I needn't worry about it with my drz... Well atleast that's what I'm thinking, won't know just how far it'll pull untill I get her on the bike.

I've also heard theories on backfiring and the likes causing the breakage. My bike will eventually have 13.5:1 compression, so if the backfiring is even an issue, I may stick with the aluminum slide.

So is there any real consensus as to the cause of the slide breaking?

p.s. - I examined the body around the slide and it has visible wear but nothing out of the ordinary. Not sure how long the previous owner has used the carb with the new/harder slide, it may not be an issue.

I'm going to do a bit more digging into my carb for more clues.(Since I have the time now that I won't be riding for another few weeks). :)

Everyone so far suggested the broken metal piece blew out your exhaust. Do you have a spark arrestor? If so, it wouldn't have gone far.

It may not solve the mystery, but at least you'd know if it's still in the cylinder/head causing problems...

I just installed a new White Bros E pipe a couple of days before it broke. It may not have made it through the plates. I'll check tonight.

Ok guys just one thing,has everyone who has had a plate fail,have the same corner shear off??

Ok guys just one thing,has everyone who has had a plate fail,have the same corner shear off??

Not 100% sure, but I think so.

Looking into the carb from the airbox side, my bottom right corner broke away.

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