Issues with BS38 carbs

[drunkaroos]

I am most of the way through my bs34 to bs38 carb swap and have ran into some issues. Firstly, even when the idle speed screw is turned all the way in, there is about half an inch gap between the throttle linkage and the screw, resulting in the bike jumping to 5000rpm as soon as it starts. Stranger still is that if I turn the choke on when this happens, the idle drops to about 1700rpm. I have read online that for tuning I should be turning the mixture screws out until the cylinder starts sputtering, but I have tried that and have had no change in how it runs even when the mixture screw is about to fall out of the carbs. For reference, the carbs were cleaned earlier today in parts cleaner and degreaser, so the cleanliness of the carbs should theoretically not be an issue.

I would appreciate any help or suggestions, as I am trying to get out on the road as soon as possible to enjoy the nice spring weather. Thanks

 

[gggGary]

where is you sync screw set?
Is the throttle cable binding or a farrel not seated on one end
Remove the throttle cable does the butterfly move to the stop then?
This a normal throttle stop idle setting.

 

[5twins]

Sounds like the throttle plates aren't closing all the way for some reason. I suppose the plates could be badly out of sync, one closed more than the other, but they would have to be really bad to show the symptoms you have. Still, it's worth checking. You may need to pull the carbs off so you can look in the engine side of them and see the throttle plates. While off, you can do what's called a "bench sync". This means adjusting the sync while looking at the plates. You can get them pretty close like this.

 

[drunkaroos]

 

[drunkaroos]

This is how they look at the moment.

 

[drunkaroos]

Pulled the carbs back off in order to look at the syncing. Turns out I missed the fact that some brain trust had completely mangled the idle jets on both carbs, and that I am missing the mixture screw springs for both carbs. Time to wait for new parts (again)

 

[gggGary]

There's some tricks to centering the throttle butterflies if they have been off, typically only removed to replace throttle shaft seals.

A late model set of BS38's ready to bolt on and run. Note small even gap at bottom of butterflies.