High Idle: can slow with hand


XS650 Enthusiast
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Sydney, Australia
Hey, it's another idle thread! I got my 75 XS650B on the road a few days ago and have been loving it. I took it on quite a long ride (6hrs), then some shorter ones across a few days. Still a few things to tune but mostly there. Today I took it out and when the engine got warm it started idling high, 1.5+ (it's also a hot day, 32 celsius).

I've twisted idle adjustment screw out but the throttle stop is no longer against the screw. I can push it down and lower the idle speed. There's maybe a 3-4mm gap between the throttle stop and the idle adjustment screw bracket(?).

I'm wondering if you guys think it's related more to the throttle stop, or just general carb issues. They've been cleaned but not since I got it running.

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Sounds like the throttle butterfly plates are binding and hanging open. If you back the idle speed adjustment screw off all the way, so much so that it's not touching the cable arm anymore, the butterfly plate should close completely and that cylinder should actually stall or "die". Did you have the butterfly plates out for a butterfly shaft seal replacement? If so, I don't think you centered them correctly when you reinstalled them.
Ah, that's good to know! Yeah if I back the screw out it stays at 1.5, I have to push it to get it to stall so that sounds like it. I didn't do the shaft seal replacement but I'll pull it apart and have a look.
What happens is when you assemble the butterfly shaft and put the return spring on it, that spring pulls the shaft towards it as the butterfly plate opens and closes. This can cause the plate to bind up against that side of the bore. To "center" the plate and avoid this, you install it loose then assemble the rest of the shaft, especially that return spring. Then you open the butterfly plate fully and let the return spring snap it shut several times. Then you can fully tighten the butterfly plate screws.
With the carb off, hold the air filter end up to a good light and look in the engine side at the butterfly plate. You'll be able to see if it's bound up against the spring side of the main bore, you won't see any light along that side
5twins is probably correct with his diagnosis. But if you can manually return to proper idle level by applying pressure to throttle shafts at the idle screw location, it could also be a throttle cable problem. Make sure internal cable(s) are not frayed inside housing(s). The cables for my '73TX were both frayed near the throttle grip which resulted in elevated idle readings...............
Is this what you're talking about 5twins? Also needs a good clean... Am I right that these are '77 carbs because of the O stamp on the floats? It's a '75 but I have no idea about the history. Not much has been touched on the bike in general, still a lot of original bits... I'll order rebuild kits but just want to confirm.


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Is this what you're talking about 5twins? Also needs a good clean... Am I right that these are '77 carbs because of the O stamp on the floats? It's a '75 but I have no idea about the history. Not much has been touched on the bike in general, still a lot of original bits... I'll order rebuild kits but just want to confirm.
It looks like the throttle plates are not centered, yes. But they are rubbing/ catching on the opposite side to what 5twins described. The fix is still the same though.
I would also check for any play in the throttle shafts/ bushings while working on the carbs. And of course check or replace the seals. Air leaks past these seals will cause poor low speed running.
I'll order rebuild kits but just want to confirm.
I wouldn’t order any kits, most are junk and you’re better off just giving it a good cleaning. as articXS said make sure there is no “wiggle” to those shafts and read the carb guide in the tech section. ‘75 was the last year for un-linked carbs. If yours are linked someone has swapped them out.
Yes, rebuild kits are not recommended. The '76-'77 rebuild kit in particular is bad, coming with the wrong jet sizes. But, you need to positively I.D. the year of your carbs first. Yes, it looks like you have '76-'77 float bowls, but bowls are easily swapped. You will need to look at or for some other attributes to positively I.D. them. If they are bolted together as a "bank" and use just one throttle cable then they are '76 or newer. To determine whether they are a '76-'77 or '78-'79 set, you will need to look at the intake bells and inner sides of each carb. On the intake bells, you will be looking for the number of round holes around the outside edge. A '78-'79 set will have 4, a '76-'77 set only 2. On the inner sides of the carb bodies, you will be looking for an external float chamber vent. The '76-'77 carbs have them but the '78-'79 carbs do not. On the later set, the vents were moved internally and are the 2 larger round holes around the edge of the intake bell .....

Is there slack in the throttle cable when it's at idle?? A cable binding at some pinch spot under the tank or because it's rusty is popular.
Yeah there was slack, I loosened it all the way on the throttle adjuster and could see the slack at the throttle stop but it was still stuck/idling high.

I was looking through the throttle plate forums and read that a little bit of light is ok(?).

The throttle plate stuff looks a bit tricky, and would rather avoid it, but I can see the screws on the throttle plate on one of the carbs have been rounded out a little. So its probable they haven’t been centred right. Here is a photo of both the carbs and the light that gets through.


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I also found the common problem that the pods that came with it were blocking off the air jets. Could this contribute to the high idle?

edit: could the high idle be from the carbs being out of synch? I'm just looking for a way not to take the plates off... I got it running after 5+ years of sitting somewhere and the idle was fine for the first few days but then was too high overnight.
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Okay, so I cleaned out the carbs and increased the jetting (it was stock). Mains up 2, pilot up 1. Put on some uni filters, and replaced the boots and clamp as that was broken and allowing air in. I've played around with the synch but it could be better, the right side is a little weaker and popping a bit more on decel.

I'm still getting the high idle so I think it's the throttle plates. I'll take them off and do that but it's kind of weird I can push the arm down and the idle goes down right? When I had the carbs off I couldn't push it back anymore, but I can when it's on the bike. It's like it's not getting forced all the way back, but I've tried it with the throttle cable detached and it makes no difference. After I let go it rises, it's a very small amount but I guess it is only 300 RPM.

I'm also wondering if it could be choke-related, as I can't seem to get the bike started with choke on. It dies/stalls after a second. If I have the idle screw in and start it with no choke it idles great, then when I get going obviously rises. Could air be getting in there somehow? It really does not like the choke on. I can only get the dust seal part here in Sydney, and mine does look cracked, but 'dust seal' doesn't sound like it affects air, is that right? Anyway I'll get around to the throttle plates eventually and see how it goes.
Looking at those pics it’s absolutely not centered. I’d also be a little suspect if someone had it out did they stake the screws or use loktite if they took them out. You will have to loosen those up to where the plate can be centered to allow both to shut properly.
Well, they are pretty near round, but the top and bottom edges are beveled at a slight angle. This allows it to seal to the bore better because the plate isn't perpendicular to it when closed, it sits at a slight angle. His plates are installed correctly, I can see it in his pic. When looking in at the plate, the Mikuni "square-in-square" emblem and the #120 should be right side up and below the butterfly shaft .....