Idle hover

[TwoFitty]

I was experiencing an idle hover and carb popping when starting on my 77 xs. I checked the advance weights and they were not returning a they should so I cut a loop of of each spring and greased the rod. They seem to be returning when I push them out. It is starting and running well but I pulled up to a stop sign today and with the bike in first and clutch in the idle did not return until I let the clutch out a bit. Then after stopping and putting it in neutral I revved up to 4000 rpm and it stayed there. Any ideas? I also just put new carb boots on. Could it be caused by a lean main jet?

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[scabber]

Sounds like the advance is still not working properly. You need to check with timing light. Over advancing will hole a piston. If that checks out. The idle hover might be cured by a mixture screw adjustment.

 

[TwoFitty]

Is there a difference between over advance and stuck advance? And is it possible to check for with a multimeter? I don't have a timing light. I planned on checking timing with a multimeter but obviously you can't do that with the engine running.

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[resto]

Here's something you might want to try to see if a sticking advance mechanism is your problem. Jam some cardboard or equivalent between your advance weights and the ears of the mounting plate. This will prevent any advance at all. Rev the engine a few times, if your idle doesn't hover any more then you more than likely had a sticking mechanism.

Note: if you were to drive the bike with a disabled mechanism, it would be really sluggish.




resto

 

[TwoFitty]

Just out of curiosity and so I can understand what is going on a little more, if it is the advance sticking or an over advance why would releasing the clutch cause the idle to drop?

 

[wherearewe]

The ATU can stick for pretty much 2 reasons - worn springs or binding somewhere. Give everything a good service. Heres a couple of good threads:

http://hughshandbuilt.blogspot.co.nz/2012/05/how-to-pamcopete-277-ignition-install.html
http://www.xs650.com/forum/showthread.php?t=19482

Then fire the bike. I found you can actually see by eye if the advance is sticking open. Simply gun the motor and you can see the bob weights expand and contract. If you still think its not working correctly you can try the trick suggested by Resto but i found that the card or whatever you jam in flies out. I recommend getting some new springs from Mikes and shortening them or finding some new OEM.

The ATU is a pretty simple device. Its function is to control the advance curve and limit the range. Curve is essentially controlled by spring resistance and bob weights, while range is limited by the stops. A timing gun is a handy tool to have when messing with the advance.

 

[TwoFitty]

I found you can actually see by eye if the advance is sticking open. Simply gun the motor and you can see the bob weights expand and contract. .

I did this and I saw the weights expand but when the idle hovers they stay out. How do I know if them staying out is the cause of the hover or if the hover is happening for another reason and the weights stay out because the RPMs are still high?

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[wherearewe]

Its a bit of a what came 1st, the chicken or the egg type thing isnt it?

From my experience its likely to be worn springs. The increase or decrease of engine speed increases or decreases the forces of inertia on the bob weights. When you rev the motor the advance opens. If the springs are too weak to overcome the interia, then the weights stay open. This causes the idle to hover. If you kill the motor, the inertia forces stop and the weak springs now have enough strength to close the weights. When you restart the idle will be normal again. At least thats what i found when i had a worn ATU.

Try shortening 1 spring, or if you dont want to do that, bind the weights in the closed position to test.

A lean condition can also cause a rev hoover. This could be from an air leak somewhere (such as the carb boots or some such). Try spraying some WD40 or something around the carbs and see if the revs change.

 

[retiredgentleman]

I recommend you replace the needle jet O-ring on each carb. If they are old, they can allow extra fuel to be drawn around the needle jet.

Other things that can cause high rpm hovering:
Air leakage around carbs.
Carbs not synchronized.
Air/fuel mixture screws not adjusted correctly.

 

[TwoFitty]

So I bought a timing light and set the timing (points gaps were way off). This seemed to fix the problem and the bike is running great. Next issue is that when in neutral if I rev it up to a high rpm, say 5000 or so, I get white smoke coming out of the left exhaust. Any ideas what could be causing it and how big of an issue is it? I have not got a chance to sync the carbs or set the idle mixture, which I plan to do. Is white smoke a symptom of a lean condition? Thanks

 

[scabber]

White smoke we have a new Pope. Sync the carbs and mixture screws. If the bike is running good. I would take it out for 50 mile ride. It could be unburned gas in the muffler. Do a plug read when you get back.

 

[5twins]

White smoke could be a symptom of 120,000 miles.

 

[5twins]

Dude, what planet are you from? Or are you stroking out or something, lol.

 

[Travis]

5twins, I deleted that post and banned the account. That was one of the new spam bots. They take an existing post in the thread and change certain words. Several days later they'll come back and add their spam links hoping no one will notice since the thread is now a few days old.

In this thread, the spammer took scabbers post:

Sounds like the advance is still not working properly. You need to check with timing light. Over advancing will hole a piston. If that checks out. The idle hover might be cured by a mixture screw adjustment.

and created this:

Appears to be like the enhancer is still not functional. You need to check it with light. Over improving will be opening by a aide. If that really assess out. The nonproductive float might be treated by a combination of attached modification .. .

advancing = improving
idle = nonproductive
hover = float
etc..

They tend to add a few words before and after the original post. This spammer did this in 6 threads in just a few minutes.