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89 XR600R carb problems

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Hi, I've been strugling with the idling of my XR600R now for a while. I have rebuilt two carbs, the stock 89, and a stock 93, and they both are giving me the same results.

Basically, when I fire up the bike it idles nicely. If I just barely open the throttle slowly, the idle jumps up really high and stays their. If I blip the throttle, it tends to drop bike down to the normal idle.

I know it sounds like the slide is sticking, or the throttle cables are not pulling back all the way, but this is not the problem as far as I can tell. I have a bunch of slack in the cables (to test), and I am manually moving the throttle right on the carb. The slide inside the carb operates very smoothly and seats down nicely.

I have checked for air leaks around the linkage, boots, and top of carb (on both carbs), and no air leaks.

When the bike is in it's high idle state, I try pushing back on the throttle linkage (at the carb), tapping the carb body with a screwdriver, with no sucess in dropping the idle. The only way is to blip the throttle, and it drops back down. The idle adjust screw is always hitting the linkage in these states.

One other thing is when it is in it's high idle state, I can actually turn the idle adjust screw out and bring the idle back down again. But as soon as I blip the throttle, the engine dies, and of course the bike won't start unless I crack open the throttle slightly.

I have also played with the air screw and it doesn't seem to make much of a difference with the idle speed even going +/- 1 to 2 turns. I have it set at 2 1/2 out initially.

I thought maybe my stock 89 carb was worn out, but now that I am having the exact problem with the 93 carb I have no idea what to check. I did completely go through both carbs. I have rebuilt a lot of them, but for some reason on this particular bike, I am overlooking something.

Any ideas or comments? I'm willing to try just about anything right now to avoid having to spend $300- $400 on a new carb. But if it comes down to it I will. I just want to make sure I have checked everything and know that the problem really is with both of the carbs before I do that.

Thanks.

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Don't know really. Sounds like you've checked things pretty throughly. I've had a couple of older Mikunis (one flatslide, one roundslide) that do what you describe and the problem with the roundslide is a notch worn in the slide,,caused by a bad idle adjust screw...ie the end of it;s not good enough and jams the slide..The Flatslides just old and tempremental (Like me) and I've given up trying to get it perfect..I'd be re-lubing all cables, checking the actual handlebar accelerator mechanism and the routing to make sure it's all spot on..A small kink or wrong angle in any of those cables will lead to a sticking throttle.Specially if they have always been using the same route to the carb for 10-15 years and are now running a slightly different route due to your work...Make sure you have the 10mm gap between the handlebar and end of the accelerator mechanism or you will also get sticking..Also have a look at the actual handgrip on the accelerator mechanism and make sure it's not touching the housing..needs a gap of a couple of mm. I know you've probably had enough of checking the routing etc (Sounds like you know what your doing) but try it again for luck..and check the slide and the screw that rests on it to maintain idle if the later models use that system..I don't know which carb they use as I only own twin carb models and the XL500 which uses the Flatslide Mikuni.

Edited by Horri

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My suspicion is that the slide sticks, but there is too much slop in the internal parts to force it back down before the cable cam hits the idle adjuster.

I have a spare 600 carb that does the same thing and I attribute it to wear. I might investigate possible fixes when I'm done running E85 and put the good carb back on.

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Thanks for the great tips to look into.

One of the very first things I suspected was the slide. Sure enough, the slide in my 89 carb was completely grooved all around. So I took the slide from my 93 carb (which looked great) and stuck it into my 89 carb. This may have helped a little, but the problem still existed. That's when I decided to just rebuild the whole 93 carb and try that.

As far as the cables, throttle mounting, and grip, those are all set up pretty good. With the bike OFF, the throttle feels very nice. It pulls easily, and snaps back very repsonsive (from stop to stop on the steering). The slide inside the carb has a nice crisp snap to it when it closes.

As far as the cables, one of the first things I checked when the bike goes into this high idle state is to make sure the linkage on the carb is all the way closed up against the idle adjust screw. And yes it is all the way closed even at high idle. So I am pretty sure that it is not a cabling or throttle sticking issue. It feel like it is something inside the carb, or adjustment in the idle circuit.

It almost seems like when the bike is running and I give it some throttle, then let it off slowly, the slide is not going all the way back down (but only when the bike is running). Almost like the vibration from the motor is causing the slide to stick in the carb. Unfortunately I don't know of anyway to check this, or force the slide back down. When I do a blip of the throttle, I feel it is enough to force the slide back down. But when I slowly open and release the throttle, the slide seems to stick.

Thats just a theory I have without knowing how to verify this.

HeadTrauma, I think what you are saying may be the problem. There may just be too much slop on the internal parts, so the linkage is hitting the throttle stop, but the slide is not comming all the way down. Possibly due to the vibrations from the motor.

Though I find it kind of odd that this is the problem on both the 89 and 93 carbs I have, but they are both old.

I wonder if there is a way I can check for too much slop. Like I had mentioned before, when on the bench, and when the bike is OFF, the slide works great. Opens very smoothly, and snaps back very solid. Unfortuneatly when the bike is running I can't hear the nice snap of the slide, so it very well may be not snaping shut due to the slop and vibration of the motor.

Thanks again for the info. I've been rebuilding carbs and motors since the early 90's when I started racing motocross, and this is definitely something new that I have never seen before.

Terry

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Well, after my last post, I went into the garage to take another look.

I think I just proved to myself that indeed the slide is sticking inside the carb when the throttle is opened slowly then released slowly.

With the bike running, I cracked open the throttle, got the bike stuck into the high idle state. It would just sit there for a while until I gave it one or two good blips of the throttle, then it would drop back down to the normal idle.

I then put it back into the high idle state, and took a large screwdrive and started banging on the side of the carb over and over again (right where the slide is). Sure enough, the idle slowly started to drop until it almost reached the normal idle. It was still idling a little high, but nothing like before I started banging on the carb. I verified that the throttle stop was seating against the idle screw during all these tests. I would push hard on the linkage at the carb against the stop and it made no difference in bringing the idle down.

So it looks like the slide is sticking in there due to the motor vibrations (because it definitely seats well with the motor off). When I start to bang on the carb I must be causing the slide to slowly drop bringing the idle back down.

I did this multiple times with the same results every time. A good blip brings the idle back down to normal, and 5 to 10 good bangs with a screwdriver also brings the idle back down close to normal.

Now is there a fix for this? Maybe a small adjusment I can make inside the carb? At least now I feel that if I do fork over the cash for a new carb it should run just fine.

I still want to give the stock carb a chance before I chuck it. I'll pull it apart again and see if there is anything I can figure out.

Terry

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Also keep in mind that these big thumpers pull a lot of vacuum behind that slide. That's why a lot of people, including me, have trouble with them being tough to modulate right off idle. I find that oiling the slide periodically makes a noticeable difference.

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I never thought of oiling the slide. Mine is definiely as oil free as can be. I have to stop thinking two srokes.

I'll pull the slide out and give some oil a try.

Thanks.

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I started off by just pulling off the top cover, but then decided to pull the whole thing off anyway. Only takes a few minutes. I wanted to make sure I got everything oiled well and double check the slide and linkage in there.

Anyway, oiled everything really well, checked out the linkage and all looked good. Put it back on the bike, but still the same issue.

I went back though the same diagnostics of bringing the bike up off idle, then realasing the throttle having it stick at a high idle. I first used a screwdriver and pushed back on the throttle linkage at the carb just to make triple sure that it wasn't sticking open. It was 100% up against the idle screw with no change in idle.

Then I started banging on the side of the carb, and sure enough the idle started to drop again.

It all makes to much sense now. Blipping the throttle brings it back to normal idle, and banging on the carb does the same. In both situations the linkage is at the stop, so the only other thing it can be is the slide inside the carb. At least that's all I can think off.

Oh well, I'm probably trying to fight a lost battle with these carbs.

And to top it off, I decided to change my oil. Being tired and a bit frusturated, I managed to only pull the case plug, drain the oil out, then start to fill it up. Well, 3/4 of a quart and the oil starts overflowing all over the frame and engine. Ooops.. Never done that before!!

Tank came off again, had to pull off the skid plate, and clean up the mess. Draining the oil again and giving it a second try. What a day!

Thanks for all the help and advice so far.

Terry

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Mine does exactly the same thing.

Mine seems to be better for a while after cleaning the slide and carb with carb cleaner and a tooth brush. Oiling the carb works for a while as well, but soon the fuel washes the oil away. It's got to be wear on the carb body and/or the slide. The vacuum causes the slide to get hung up resulting in the high idle. Well that's my theory at least.

One thing I tried a few rides ago was some two stroke oil. While getting ready for a ride I watched my friend fill up his KX500. He almost drained his premix and was kind of bummed that he mixed up just a little too much. I ended up using the last of his premix fuel to top off my bike. No measures, but I think I put a little under 2 quarts to my tank that had about 3.5 gallons in it at the time. The oil ratio was something like 250 to 1 so I thought it wouldn't do anything. I was wrong, it made a big difference to the hard to modulate throttle.

The real solution is a non worn out carb. Now how to do this without dropping $400....

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Well I've done both with the carb. Cleaning it very well with carb cleaner (slide and carb), and now oiling the slide. But neither seem to have made much of a difference. Since the fresh 30W I just put all over the slide didn't make much of a difference, I don't think the 2 stroke oil will help much in my particular situation. I think the carbs (both the 89 and 93) must be worn out.

At least I am not puzzled anymore. I don't ride the bike all that often. Only put about 300 miles on it in the last 5 years (boy thats sad). The only thing that really bothers me about the idle is when I pull up to a red light with cars all around, and I'm sitting there bliping the throttle over and over to get the idle to drop from what sounds like a 1500 to 2000 rpm idle. With a super trap on there, it's kinda loud. Plus if there are any cops around, they definitely look. But when I'm on the trails, it really doesn't bother me at all.

Sorry to hear others have to deal with the same thing, but at least I have a good idea of what I need to do now. I'll be shopping around.

Thanks.

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