Engine falling off at idle comes right back then idles fine..

[Copterpilot_57]

Ok, the idle mixture screws are set 2-1/2 turns out.. Motor cranks fine idles ok.. After idles for a while it like sputters hesitates for a second then comes right back to normal idle.. The motor tune is great running down the road real strong no back fire on deceleration etc.. I'm thinking one carb looses fuel for a second?? Any thoughts? 1980 XS 650 motor, carbs all rebuilt 100 miles on it thus far..

 

[TwoManyXS1Bs]

Might be a throttle shaft seal problem. Have a look at these:

http://www.xs650.com/forum/showthread.php?t=29452
http://www.xs650.com/forum/showthread.php?t=11980

 

[Copterpilot_57]

Man that's some good info 2 many.. I know the seals work under a vacuum.. The weird thing is it will idle 15 - 20 minutes just fine.. Then fall out and return.. I noticed the photos when the bike was for sell that it had some brass looking six shooter baffles in the exhaust tips.. When I got the bike they had been removed?? Wondering if the bike was jetted with the baffles in and now without the back pressure the jetting may be off??

 

[TwoManyXS1Bs]

The exhaust baffling and 'backpressure' shouldn't affect idling, unless it's really plugged up.
With their long duration cams and lightweight flywheels, getting a stable idle on these bikes can be challenging, especially on the early models.

http://www.xs650.com/forum/showthread.php?p=417260#post417260

A couple of other things may influence idle stability:

Ethanol in the fuel, has different vapor pressure and latent heat of evaporation compared to the old fuels. Usually causes cold-start problems. But, if rejetted/adjusted to fix cold-start, could be too rich when hot.

I've found carb throat to throttle plate fitment issues in my carbs, as have others. I'm thinking that this was a contributing factor to idle instability. You can check my carb rebuild album for more details:

http://www.xs650.com/media/albums/2684/

Edit: Also check these threads:

http://www.xs650.com/forum/showthread.php?t=44079
http://www.xs650.com/forum/showthread.php?t=43752

 

[Copterpilot_57]

Your right 2 Many excellent point! The back pressure should not effect the idle.. Really interesting points you shared? I bought the bike from up north Iowa.. The guy ran only premium fuel in the bike he said to keep carbs less likely to gum up.. The ethanol you described is interesting on these style carbs.. Looking like I'm going to have to pull them and take look at them.. I'm curious also, do you have any experience with the round slide 34 MM performance carbs such as TC Brothers offers?? These carbs look to be easier to work on than the originals..

Thanks for taking the time to post these your expertise is so valuable and I really appreciate the direction.. This bike is real close to running perfect!

 

[gggGary]

After years here, we have found the statement "carbs rebuilt" to be near meaningless. Like 2M hints at a bit of excess fuel building up into a drip that gets sucked into the engine causing a "hiccup" was a problem with early carbs even when working as designed. I will guess you still have a minor issue with your 80 BS34s IF that's what is a still on the bike. Carb swaps year to year are VERY common. A pic of you carbs would help ID them as still being the chrome lid, brass float, no float bowl drain "side screw" BS34s that came on your 80 On those carbs a couple of common gotchas are cracked floats and leaking float valve o-rings. What air filters are you using?

 

[kevski]

Man that's some good info 2 many.. I know the seals work under a vacuum.. The weird thing is it will idle 15 - 20 minutes just fine.. Then fall out and return.. I noticed the photos when the bike was for sell that it had some brass looking six shooter baffles in the exhaust tips.. When I got the bike they had been removed?? Wondering if the bike was jetted with the baffles in and now without the back pressure the jetting may be off??

If your bike is idling for extended periods it will start to hunt as we say in the UK because there is a tendancy to run rich at idling speeds, are you running pods and an open exhaust system these and especially pods will add to any underlying fuelling problems, more information is really needed to be able to point out more positive points.

 

[XSLeo]

Shouldn't the BS34's use a 3-3 1/4 turns out on the mix screws?
If so you could be a bit lean at idle.
Leo

 

[Copterpilot_57]

There are no chrome lids I believe they are the original 34s.. I'm trying to figure out how to upload photos..

 

[Copterpilot_57]

Both carbs are 2-1/2 turns out seems to idle best there.. The motor is lean if anything there is no rich smell to the exhaust..

 

[gggGary]

I'd shut down and pull the plugs the second it hiccupped......