BS34 Bench Sync

I have my rack of BS34s on the bench to clean and replace pilot jets. I'm going to check the fuel levels doing the clear tube test. Last time I had them out, I changed the butterflies to be open by letting a little sliver of light in. I'm not sure this was a good change. I have a couple questions:

1.) Can someone give me the steps needed to bench sync these CV carbs?

2.) Should the butterflies be set to be slightly open or closed at rest.

Thanks
Nick

Hi, Nick; I've been staring at the same thing this week! While I know that the BS34's on the XS11 may be slightly different than the ones on a KZ, (Google will lead one to Jamaha forums, as well as other places muahahaha) I'm thinking the baseline bench sync may be similar enough. in the carb outlet, up over the butterfly there are three little holes; position the butterflies with the set screws so that half of the innermost hole is visible. This will leave the b-flies open a crack, yes.

Another bench sync procedure I've seen in the FSM and on this forum is to use a particular something small (drill bit? bread tie?) as a feeler gauge to measure the gap when setting the b-flies, but I'm sorry to say I can't recall what size measuring stick you might use. The idle circuit holes idea just seems more logical for some reason.

You then do the vacuum sync on the bike, and it seems it is best to baseline the idle mixuture, sync, check the idle mixture again, check the sync again, repeat 3rd time if still sober. Bench sync is just for a solid starting point for vacuum sync, it can't replace it..

hope this helps,
Rick

The way I have done mine is: I set the adjustment screws to allow the butterflies to appear shut at rest but the key is that when you turn the throttle rod, they all open the same amount at the same time.
So, you sync as usual by syncing 3 to 4, then 1 to 2, then 3-4 to 1-2.
This method usually gets me pretty close.

All good info provided above...

After a good vacuum sync on a healthy engine, the butterfly's will be exposing about half the first pilot mixture jet hole at the top of barrels.

Sync is exactly in the order Grant has described.

You dont need to use a drill bit/wire/etc IF YOU ARE GOING TO VACUUM SYNC. If you arent going to vacuum sync, then 1: Your nuts, cause cv carbs really require proper vacuum sync, and 2: Do as accurate as possible a bench sync, and although it will still be fairly off, it will work OK.

When Vacuum Syncing with manometer, remember that low vacuum means the butterfly is open more, higher vacuum means the butterfly is closed more. If one or more cylinders has very low vac and requires alot of adjustment to get up to vac of others, then it most likely has an air leak.
Get the cylinders vac synced, then turn the idle down as low as it can go. Wait a minute idling, then check the pipes by spraying water mist on them and make sure they are all still vaporizing/steaming water (still firing correctly), check sync again. If one isnt firing, then there is likely an air leak, and the butterfly is closed completely to achieve equal vacuum.

Good Luck!

I put the carbs back in after installing the remainder of my carb rebuild kits (main, pilot, and needle). Of course she cranks but won't start. I'm going to pull the rack again and put the old needles and pilots back in and see if I can't get her running again. One of these days I'd really like to go riding...