Carburetor butterfly not even when wide open?

[Woodstock1976]

1979 xs650 with bs38 carbs I switched to gravity petcocks and non vented air intake boots, so I’m trying to bench sync the butterfly valves on the carbs and noticing that when I get them to close at the same time they are at different angles at wide open throttle. One will open till flat and the other is not open quite as far, very slight difference but enough to notice with the eye. I imagine this would be a problem at highway speeds? How would you correct it?

 

[Kevin Werner]

What is the chance that one butterfly is slightly off-center thus corrupting the static sync?

 

[mike in idaho]

More important, they're even on the bottom, that affects off-idle response.

 

[Woodstock1976]

What is the chance that one butterfly is slightly off-center thus corrupting the static sync?

How would I check this? It looks centered to me but I could check with feeler gauges maybe?

 

[Kevin Werner]

How would I check this? It looks centered to me but I could check with feeler gauges maybe?

Maybe check with a bright light from the back side? But, @mike in idaho has a good point, it is the off idle position that is important. But tell us just how far off at full open are the plates? Now another point would be, are they off at half throttle? I myself seem to use no more than ~1/2 the twist grip.

 

[Wingedwheel]

While the screws are slightly loose release the throttle a few times and let the plate seat itself. Then tighten the screws and stake or loctite.

 

[Jim]

Bench syncing the carbs is just a rough sync... enough to make it run. The "real" sync comes from tuning so both sides pull equally. You can do that by ear and feel... or with a homemade or store bought manometer or sync gauges. So... set them (for idle) on the bench, install 'em and then do a proper sync.

Far as idle and WOT (wide open throttle) being off from each other... there's an argument to make in favor of biasing towards WOT. Being out of sync means one side is pulling stronger than the other (duh ). In a 360° twin, that means more vibration. I'd argue that syncing them at the throttle setting you use the most is where you want to be. If most of your time is spent out on the highway at 60mph, why not sync the carbs so they pull evenly there? Of course, that means using sync gauges that you can tape to the handlebars and a deserted road you can (safely) make your runs on to check and adjust the sync.... but it's doable.

On the other hand, if all you do is putter around town... a bar hopper, setting for sync at idle would be your best bet.

If it's a race bike, set 'em for WOT.

 

[Jim]

Forgot to add...
Just about every set of BS carbs I've worked on showed a slight variation 'tween idle and WOT. To be honest, as long as the butterfly's are properly centered, I never really worried about it.

 

[Woodstock1976]

Finally reassembled with the carb flys synced by eyeball at closed and it runs smooth. I guess a little variation at wide open is no big deal. I took it down the road and probably got up to about 80-85 no issues. Thanks for all the responses

 

[AUTOMAN]

Maximum airflow at WOT is slightly closed at the butterfly. This is only desired airflow.
On CV carbs, the slide controls airflow as the engine runs, trying to maintain Constant Velocity.

 

[Woodstock1976]

Maximum airflow at WOT is slightly closed at the butterfly. This is only desired airflow.
On CV carbs, the slide controls airflow as the engine runs, trying to maintain Constant Velocity.

Oh that’s a good point thanks!

 

[AUTOMAN]

Oh that’s a good point thanks!

Does your SP2 have mags or spokes?
In case you want to change rims.......

cliff

 

[Woodstock1976]

Does your SP2 have mags or spokes?
In case you want to change rims.......

cliff

Is SP2 mean special 2? Is that what I have?
My bike has mag wheels 19” front 18” rear

 

[5twins]

Are you sure the rear is an 18"? I ask because Yamaha never made one for the 650, only 16" mags. If you really do have an 18" then it's been adapted from some other model. And a Special II had spoke wheels from the factory, not mags.

 

[Jim]

Is SP2 mean special 2? Is that what I have?
My bike has mag wheels 19” front 18” rear

Pics?

 

[Woodstock1976]

Are you sure the rear is an 18"? I ask because Yamaha never made one for the 650, only 16" mags. If you really do have an 18" then it's been adapted from some other model. And a Special II had spoke wheels from the factory, not mags.

Yeah I’m sure. I just double checked. The bike is fairly customized also. Someone did a lot of work to it around 2010.

 

[Woodstock1976]

Pics?

 

[Woodstock1976]

I just put new tires on them too. Was weird that the guy who did the bike put tubed tires on them but I liked the look so they are still tubed tires

 

[Woodstock1976]

It’s worth noting that I have all of the guys receipts and before and after pictures for the bike to so if you wanna see any of that stuff maybe I can start a thread for it. The guy was very detailed it seems

 

[46th Georgia]

Very cool!!! I could see it in the 2026 calendar.