Hanging Revs Issue! Revs decrease slowly. Help!

Nate A

XS400 Member
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I just recently bought a 1980 Yamaha XS400 Special II, and need a bit of help with an issue that I can't for the life of me figure out. I am by no means a mechanic, just started rebuilding bikes as a hobby a few years back. These threads have been an absolute godsend in my rebuild process, and I can't thank the members enough!

So here it goes.

The bike is 98% stock, plus or minus some relays. When I pull the throttle just barely faster than "creeping forward" the RPMs rise but slowly come back to proper idle. It, for some reason, does not occur when the bike is warming up, but only happens after 5-10 minutes of idle. The bike is driveable, but after I take it for a 15+ minute ride, the hanging idle only get worse. The revs will always fall back to idle, but shift changes are brutal, as you can always slow return of RPM's. The problem is most noticeable in 1st and 2nd after 2000 RPMs, 3rd/4th seems slightly better, and 5th/6th the problem is almost unnoticeable.

The bike is stock, the carbs have been THOROUGHLY cleaned 5-6 times, I have new throttle slides and diaphragms, brand new intake boots new gaskets and o-rings, all jets are pristine and match the factory jet sizes, float level has been meticulously set (they were not plastic, so I had to bend and balance each. Air filters have been replaced and all hose clamps are tight as can be. I have searched and searched for vacuum leaks but to no avail. The gas tank had very minimal surface rust which has been removed properly and effectively. I have two in-line filters and never find any evidence of crud in the carbs when I take them apart. The throttle cable has been replaced and adjusted to the right amount of free-play.

I have two-three concerns/ ideas that could pose as a threat and would love some feedback, as I am no professional.

1. After replacing the throttle slides, springs, and diaphragms, I manually tested the slide action. They work just fine when I raise and drop them slowly. But when I raise them quick and let them drop, there is a slight hesitate before reaching the bottom followed by a faint suction noise. For the most part, they arrive at the base at the same time but with that hesitation. I assumed the hesitation was the cause of the slowly dropping revs, which is why I replaced the unit, but the problem persists. Could it be the needle not seating properly in the needle jet?

2. A second concern is that I just simply haven't tuned the carbs properly. I seldom need a choke when starting, and she'll idle just fine. I have bench and vacuum synced the carbs and the idle sounds beautiful. This only occurs the throttle mixture screw is screwed nearly all the way in, any less and the bike dies. The air mixture screws are each turned out 2.5 turns. I have tried all settings in 1/4 increments from 1 turns to 3 turns, but the hanging revs remain. Factory settings are stated at 1 1/4 turns out. Could the setting of the throttle screw allow too much gap in the butterfly valves, resulting in the hanging revs? If so, how do I remedy this so I can find a steady idle with slacker setting?

3.I read somewhere that the clutch might also be causing the issue. Everything is adjusted accordingly and the gear shift mechanism was adjusted as well. The clutch spring free length, clutch plate, and friction disks wear limit are all up to specification, and the teeth on the clutch housing have no nicks or dings. My concern here is that I have not replaced the friction disks. The wear limit is fine, but no matter how much I clean them/ change the oil, they continue to leave a black greasy substance on their partner plates. Could their age combined with the residue cause clutch slippage, resulting in the hanging revs?

Hope ya'll can help me figure this one out! I've got nothing but time during my quarantine, and would love to take my bike out through some good ol texas hill country roads soon.

Thanks again!
I think the issue is going to be number 1. I have noticed no hesitation on my carbs when I mess with the throttle. Have you tried revving the throttle with the throttle cable disconnected as well? A seized or poorly lubricated throttle cable could cause hesitation. That said, I'm assuming you have tested it without the throttle cable since you had the carbs off.

Hopefully someone with more expertise can chime in and help out! I can only be it limited help since I believe our carbs are different (I have a 82 Maxim)
Thanks for replying! The throttle linkage snaps back properly when I tested them off the bike. The very first issue I had was a stuck butterfly valve. So I ended up having to install a non-rusted set from some donor carbs / spent hours making sure it works right.

Oooh good point. I didn’t even think of that. I did clean the cable and it slides through the sheath with ease. But I’ll have to put the carbs on, start the bike, and try it without the cable. Maybe there’s a kink in the cable that just happens to be in a bad spot.
Did you replace the shaft seals? Have you synced the carbs with a manometer? On a stock bike 3 turns out for the pilot mix screws is a good start. Post some pics of the bike.
I did sync with a manometer but I haven’t replaced the shaft seals, they seemed like they were in good condition. But that very well could be the problem. I did spray starter fluid around them (among plenty of other places) to check for vacuum leaks, but didn’t find any change with the idle upon doing so. I’ll order those tomorrow and try them out as soon as I can.

Do you guys have any recommendations for where I can get specific Mikuni parts ? I’d rather do those than the cheap eBay knock offs.

I pulled the stock plugs out on these carbs to access the air mixture screws, and it looks like they had been factory set at 1 1/4. Is 3 turns out a good starting spot because of the age of the bike?
Here’s a most recent pic. Days upon days of scrubbing to get her to look half decent lol.


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Three turns is a good start. jetsrus.com is a good place for jets. Checking a carb sync often might be needed as compression changes and should be done after a valve set. Do it with a warn bike and at 1200 rpm idle. What are your floats set at? Should be around 26mm with brass floats. Make sure to remove the bowl gasket and do both sides of the float.
Just a thought, I had a similar issue because one of the vacuum ports on the boots where the carbs mount to the engine wasn't plugged. Make sure those are sealed or else your air-fuel mix will be all messed up in that cylinder.
Awesome, thanks for all the suggestions!
@xschris, i've got the floats set at 26.5 +- .5mm. The brass floats were bent out of shape when I got them, so I bent them back so they're level.
I just finished cleaning out my gas tank and as I was putting on the petcock, I came to realize that it wasn't stock. The guy before me put on a high flow Pingel Petcock. I took it apart again and cleaned it thoroughly just to realize there's no diaphragm or anything like that. Could that be a factor at play? There was no leak or anything, but could the differences in pressure have this kind of affect?
Post a pic of it. Make sure your vacuum ports are plugged. In your pic the crank case breather hose that attaches to the intake H-pipe is missing. This will cause issues. You need to hook it up.
^ I reckon this would cause an excess amount of air, leading to a lean mixture. I've heard lean mixtures can be the cause as I'm hunting for the solution to my hanging idle as well. I've got holes in my exhaust, though, so that's a different story. Good spot, xschris. Are you a jedi?
Taking note of the post #1 carb slide description of manually lifting it and letting it go. If the diaphragm is not dead supple and soft like it needs to be it can cause issues. The rubber has to fold itself basically inside out and if it has sat in closed position long enough to heat form the rubber to a different tension, at flexing it that resistance to folding the other way may be causing the idle hang. Why it might hesitate at almost all the way down. Why I loathe rubber CV diaphragm carbs and prefer to work with labyrinth type like Honda uses with no diaphragms at all. Nothing to get old to change your slide operation.

You might try a little experiment to see if it weeds out anything. Warm the bike up and get it to where the problem occurs. Then on purpose screw the idle speed screw back to low enough the engine will not idle, it tries to die, you will then have to keep it running by hand but not hard to do. In doing so let it idle as slow as you can get it to by hand and watch for the same hanging idle problem while up in rpm and shifting gears like normal. The only difference being the idles are way backed off. Tell us if anything changes.

It's a test to see if your normal idle setting has the throttle plates open enough to induce the CV diaphragms to hang open due to slight flow through the open butterflies. Usually caused by engine wear at ring and valve, having to open the throttles slightly too much at idle can cause the slides to hang slightly even if they have new rubber on them.

Of course the slides cannot drag on the carb body either, if there is enough wear you may need to dress any rub points with some #600 sandpaper to make any drag drop way off.
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FYI, if anything the Pingel petcocks are known for leaking when turned off, they do not seal inside very well and on Hondas that issue has leaked enough fuel out into carbs to run into engine to hydrolock and break rods.