Hanging Idle problem

[CoastsideXS650]

I just purchased a 1980 xs650 Special II and I am having issues with a high idle speed.

you can see the thread here:
http://www.xs650.com/forum/showthread.php?t=40368&highlight=lucille

When riding, as I pull in the clutch, the idle shoots up to about 3-4000 rpm. When I kill it at high idle and start the motor again, it is at normal idle (1500ish rpm).

Motor:
stock TCI ignition
rebuilt from the head up. did not break piston seal.
valves and camchain properly adjusted.

carbs:
completely rebuilt with new butterfly seals and proper float height.
Stock jetting
uni pods
New JBM carb boots

I also have a chopper that was running fine with no such idle problems. So I took the carbs from the chopper and put them in this bike. Now it has much better idle. but after warming up a little down the street, the same issue kept popping up. Maybe it is a little less severe but its still there. The chopper carbs have a bigger pilot and main jet. It is tuned with uni pods and open exhaust with no muffler.

I sprayed carb cleaner all over the butterflies, boots, and intake. Nothing changed.

Anyone have any idea what could be going on here?

 

[TwoManyXS1Bs]

Does it have the clutch 'safety' switch? Will it still doit if that switch is disconnected?

Is the clutch cable routed in such a way that when it flexes, it pushes on the throttle cable?

That's all I got...

 

[CoastsideXS650]

There is no clutch switch.

The clutch cable is free and not binding with anything.

The throttle cable is also free and not hanging up on anything.

Can you see why this is such a mind effer? I can't figure out what is going on here.

 

[TwoManyXS1Bs]

Well, then. Time to think out of the box. The clutch disengagement mechanism, and the TCI sensor wiring, exist in close proximity behind the left engine cover. During disengagement, a force of about 400 lbs is pushing outward against the cover. Might peek in there, look for anything suspicious...

 

[pamcopete]

Try unplugging the regulator as a test. That will remove the current going to the rotor. It's possible that the magnetism in the rotor is interfering with the TCI pickup.

 

[retiredgentleman]

When you re-built the carbs, you should have done a "bench sync". Some people do it just by eye, but I like to use a piece of paper as a feeler gauge. That will get the sync very close, and then follow up with a manometer when the engine is running.

Its important that the butterflys are able to close fully in the carb bore. When the butterflys are installed onto the throttle shaft, the fastening screws are left slightly loose while you open and fully close the throttle shaft a few times. This allows the butterflys to self aline themselves into the bore, and then the screws are fully tightened.

BS34 carbs; Brass floats use 27 mm, Plastic floats use 22 mm ...........did you use the correct float height?

 

[CoastsideXS650]

Well, then. Time to think out of the box. The clutch disengagement mechanism, and the TCI sensor wiring, exist in close proximity behind the left engine cover. During disengagement, a force of about 400 lbs is pushing outward against the cover. Might peek in there, look for anything suspicious...

I have checked the wiring. Nothing is binding or being weird. I was very careful to make sure no wires were pinched when re-installing the clutch cover.

 

[CoastsideXS650]

When you re-built the carbs, you should have done a "bench sync". Some people do it just by eye, but I like to use a piece of paper as a feeler gauge. That will get the sync very close, and then follow up with a manometer when the engine is running.

The carbs are bench synced. Also the other set of carbs I installed and tested have been bench synced and using a manometer. So I don't think it is improper sync.

Its important that the butterflys are able to close fully in the carb bore. When the butterflys are installed onto the throttle shaft, the fastening screws are left slightly loose while you open and fully close the throttle shaft a few times. This allows the butterflys to self aline themselves into the bore, and then the screws are fully tightened.

I have rebuilt several sets of carbs before and learned this trick the first time out. I definitely made sure they seated and were centered before tightening it all down.

BS34 carbs; Brass floats use 27 mm, Plastic floats use 22 mm ...........did you use the correct float height?

Yup brass floats are at correct height. both sides. double and triple checked.

 

[CoastsideXS650]

Try unplugging the regulator as a test. That will remove the current going to the rotor. It's possible that the magnetism in the rotor is interfering with the TCI pickup.

If I unplug the regulator and it no longer has a high idle you are saying I have a bad rotor?

I used 1500 grit sandpaper and cleaned the rings of the rotor so the brushed would make good contact.

Or the opposite. unplug regulator to test for bad regulator?

 

[TwoManyXS1Bs]

No, Pete's talking about a possibly weak trigger magnet. Forum Google search on "rare earth magnet" will give you background on this...

 

[NONclow]

Shot in the dark here, the spring around the throttle shaft(s), could be wound one revolution tighter around before the cable bracket is installed......

 

[CoastsideXS650]

So I re-adjusted the camchain, set valve lash, and tried again. It is sitting fine when cold and idles great. But when the bike warms up it still wants to shoot up to 3000 - 4000 rpm when I pull in the clutch.

You can hear the revs shoot up when pulling in the clutch to shift as well. At stop lights I have been having to let the clutch out a little while holding the front brake and it quiets down but then shoots back up right away.

I have been adjusting the fuel screw richer little by little as I ride to try and fix what I thought was a problem of running too lean.

So some observations:

When shutting down the bike after riding for a while I get 2 small backfire pops from both exhaust pipes one after another.

It only idles high when warmed up.

I used yamabond to seal the brand new intake boots. Carbs have new seals throughout and new jbm diaphragms, And I did the xsjohn airbox mod. Exhaust is stock special headpipes with Emgo shorty reverse cone mufflers (installed using adapters to inner tube)

I am still so very stuck as to what the problem is. The motor runs great and strong (the clutch is a little tired but ok). I just put 160 miles on it yesterday. I am so stumped.

Anyone have any more ideas?

 

[chizler62]

Get the bike warm and idling high. Put on center stand. Start in neutral. Let it idle where it wants. Does the rear wheel spin in neutral?
Pull clutch in. Does it creep less?

It maybe that you have free play in cable but you're not actually totally disengaging the clutch when totally pulling the lever to the bars.

Definitely check clutch arm for free play itself to make sure clutch is totally engaged.
It maybe just the grooves in your clutch basket are worn to the point that when disengaging the clutch can't and grooves keeps clutch engaged slightly.
Sometimes you can file those grooves out of both the inner and out baskets. Be careful if you file the outer basket to much the ears can brake off.

Posted via Mobile

 

[TwoManyXS1Bs]

I've seen this happen on engines that get low compression when hot. Difficult to explain, but, a funny balancing act occurs when low rpm idle settings (enough to keep it running) become too much at higher rpm, because the "effective" compression at those higher rpms is also higher. Internal engine loading also changes with lower compression at lower rpms, reducing the natural loads that slow an engine down.

So, when it does this hot/high idle thing, you can try to immediately pull the plugs and do a compression test...

 

[meeeearcus]

You can hear the revs shoot up when pulling in the clutch to shift as well. At stop lights I have been having to let the clutch out a little while holding the front brake and it quiets down but then shoots back up right away.

Did you ever figure out your issue? I have the exact same behavior on my '80 XS650.

 

[Downeaster]

Hmmmmm.

I dunno if there's any connection here, but I just happened to think that my high idle problem went away at the same time I found the backfiring problem: A vacuum leak somewhere between the carbs and the petcocks on my vacuum-operated petcocks.

I just shitcanned the whole concept and am using "prime" as a manual petcock for now and capped off the ports on the carbs, so I don't know exactly where the leak was.

 

[meeeearcus]

Did you ever figure out your issue? I have the exact same behavior on my '80 XS650.

I think I just identified it down to one of the throttle shaft seals for me. Was able to recreate repeatedly by spraying carb cleaner on the shaft seals. Sorry to hijack the thread, just seemed to be all too familiar.

 

[XS1961]

I junked my TCi box last week and installed a Boyer ..All my 'carb' problems have disappeared now, she returns instantly to idle after blipping the throttle & idles smoothly at 1200 rpm. I struggled for ages with my supposed carb settings..... Worth a look

 

[Grimmith]

When you re-built the carbs, you should have done a "bench sync". Some people do it just by eye, but I like to use a piece of paper as a feeler gauge. That will get the sync very close, and then follow up with a manometer when the engine is running.

Its important that the butterflys are able to close fully in the carb bore. When the butterflys are installed onto the throttle shaft, the fastening screws are left slightly loose while you open and fully close the throttle shaft a few times. This allows the butterflys to self aline themselves into the bore, and then the screws are fully tightened.

BS34 carbs; Brass floats use 27 mm, Plastic floats use 22 mm ...........did you use the correct float height?

I'm having this high RMP issue to but it's right when I start the engine. I can try to do the bench sync, but setting that aside I don't have my butterfly's loose at all with just a tiny amount of space under them where you an barely see light. is this too much? maybe I'm getting too much fuel? the intake valves are .003, and the exhaust is at .006

 

[XSLeo]

The bit of light is a place to start. You need to further check the sync with a manometer. This is an easy thing to build and it's easy to use and extremely accurate.
Some clear tubing, a stick and some bright colored fluid.
A search on here will find more specific instructions on building and using.
Leo