What am I missing?


XS400 Addict
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Spring Hill, TN
I figured I put this question in it's own thread...
1981 XS400 H.

Carbs synced after engine warm-up. New air filters. Carb cleaned, new butterfly shaft seals and washers, new diaphragms. Floats at 26mm. All stock jetting and exhaust. My mixture screws are now at only one turn out.

Bike starts on first kick and idles beautifully. No air leaks around carb/boots/etc. However, I'm STILL getting the hanging idle. It's not a throttle cable issue; it happens regardless of the cable being connected to carb. If I barely open the throttle and release it, just a few hundred RPM's, it'll settle back down to idle. If I do more, it'll just hang and rise up to 3500-3900 and stay there. Whilst at that (high) idle, the left cylinder will sputter maybe once or twice. If I lower the idle by way of the idle speed screw, it'll come back down to (perfect sounding) idle.

Never needs choke. In fact, if I engage the choke at idle, it dies. So it seems to be running rich. I put a new set of plugs in, and after about 5 minutes of idle with a few hanging revs as well, there was some slight black film on the right side plug. Not covering it, just a few mm on the rim that surrounds the electrode. Left plug looked new still. I'll have to ride it a bit to get an actual plug reading.

I did notice that, while the carbs are synced at idle, the balance is upset when I increase engine speed.

This hanging throttle/rev is killing me... What am I missing? Thanks!
Yes. They are brand new but yes; still checked them. I made sure they went on well; didn't want to have to replace the rubber again. Index point is correct and the diaphragms drop down as they should when manually raised.
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dont know about the hang up but only 1 turn out on mixture screws doesnt sound right , normally 1 1/4 is rough setting sounds like running a bit rich , did you replace your needle and seat ? also check your float levels
Did some more checking etc. I turned the idle screws all the way in (fully seated) and the bike starts first kick and idles fine. So that doesn't seem right :banghead:

It does rev higher when I open the idle screws back up... but I'm still getting the hanging throttle. Time to take the carb out again and double-check everything. I'm bound to be missing something simple.

@flea thanks for the ideas; float levels are at 26mm. Needle is original/stock size.
what was it running like before you worked on the carbs , id double check on the carb manifold inlet for leaks as sounds sus . what air filters are you using , orig or pods , xs650 cv carbs dont like pods but they wouldnt be all the prob but try running without the air filters and see what happens even if they are the orig ones
@flea Again I appreciate the thoughts. This bike is something we've been building for a bit; if interested you can see it here. It has always idled well, and pulls well also. Upon initial start and test-ride, I had a hanging throttle and a few oil drips. All cables are new; throttle cable is smooth and kink-free. Stock everything as far as exhaust/intake/carb goes. The oil leaks have been addressed; new clutch rod seal and new neutral sensor o-ring. But the hanging continued.

It was suggested I check the sync, the butterfly shaft seals, and of course the diaphragms. Carbs are synced, all four shaft seals replaced with Yamaha parts, and new diaphragms (carefully) installed. I also checked around the carb for air/vacuum leaks and found none.

Still the hanging continued. I checked one thing I hadn't... the idle mix screws... and they were both WAY open, as in 3-4 turns. they were still under the brass plug and showed no sign of prior adjustment. So I first set them to factory 1.5. Still had the hanging idle. Today I experimented... I turned them completely closed and the bike still started first kick and idled great. My carb theory/knowledge is limited. I'm fairly sure the slides aren't hanging due to any mechanical issue, whether cable related or friction in the slides themselves.

The bike doesn't need choke; in fact, the choke, even on the first detent, makes the bike sputter and die. So it's running rich... at least I think.

I am interested in trying it w/o the filters; maybe I'm restricting the airflow somehow. I replaced the original OEM elements with new elements from Hi-Flo.

The idle will hang anytime it reaches about 2k+... then rise to around 4k and sit there unless I reach down and turn the idle screw down. So somehow the diaphragms are opening up and then staying there.

The idle mix screws have little effect on the hang; they do increase the revs as they should, letting me back the idle screw back down. But then I am creating the rich condition.

I'll do the "no-filter" check next and see what effect the unrestricted airflow has on the situation. I'll check the vacuum again, especially around the carb manifold/boots etc.

Otherwise... the bike has proven fun already just blasting around the neighborhood. If I come to a stop sign, I can "force" the engine back down to idle in first gear, but without that the engine just sits at 4k at a stop. No beuno.
all can suggest if cant fix the prob is to go over to xs650.com as you have a couple of real experts there . griz for eg who though can be a bit grumpy can and does help people out with carb probs and knows what he /they are talking about
If you were able to turn the idle mixture jet screws all the way in and it still idled, then you have to be idling off the main circuit and not the idle mixture jet circuit. I would start with turning the idle speed screw down and turn the idle mixture jet screws out. I think I have mine set to like 2.5 turns out. Just keep turning the idle mixture jets out and when the idle gets too high, turn the idle speed screw down. Eventually you'll get to where turning the idle mixture jet out lowers the RPM so turn it back in 1/2 turn, adjust the idle to 1200 and you should be good. Don't worry about being near to that initial 1.25 turns out setting. That is only a starting point and with carbs, your local elevation above sea level will require adjustment from the inital.
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@tstidham Ok this makes sense and I've learned something as well from your description. I just (literally) pulled the carbs to inspect so when they're back in... I'll do the above. Thank you.
If you have the carbs back off, double check the pilot jet (idle mixture jet). On my carbs, they were sitting for years and I had to take a single strand of some multi-strand copper wire (i think it was 14 or 16 gauge wire) and use it to clean out the pilot jets. I also cleaned out every passage and jet I could with it including removing the idle mixture jet screws and cleaning the passageways. Just spraying the pilot jet didn't get it all the way clean. I struggled with having the mixture screws make any change in idle speed. I also did the same with the emulsion tube, that pops up and out after you remove the diaphragm, slide and needle.
Yes; I completely disassembled the carb BUT did not clean the idle mix screws as they were still under the brass plugs. I'll give it another cleaning while it's out. I did clean the jets separately but I'm no professional. Operator error is definitely a possibility.
There is a good carb cleaning thread on here somewhere where they detail what jets to remove and all and to not attempt removing the air bypass screen in the front air horn, but I still ran the wire through that and spray cleaned the bejeezus out of them. Also, use brake cleaner, not carb cleaner. Works better!
Butterflies control air. If they were not closing, you'd be running lean.

This sounds like too much gas or too little air. I would check air jets for clogs, the vacuum pistons for sticking, and the fuel level. If your floats aren't right, you can get overfueling.
I figured mine out, it was aftermarket carb rebuild kits, with undersized needles. No matter what I did to the pilot screw it would run rich.. Fortunate for me the previous owner (who installed the kit) had saved all the original parts... I cleaned them and reinstalled the original emulsion tube, needle and jet. Tuned up and runs great.

I think the gas was making it through the emulsion tube and around the needle causing the rich condition even with the pilot screw closed down.