BS38 Carb questions

[AdamX]

Hi All - I recently picked up a 77 standard and am excited to get this on the road. Starting with the fuel system, I'm seeing odd things with these carbs that I have not seen on other bikes. In this instance I have two different mixture screws. I've overhauled carbs on 4 cylinder bikes and their internals were the same which brings by question. Is this normal for this model?
2nd question is how do I identify the model of carb? I've verified the opening to be 38mm but there appears to be several versions. There are several carb kits for this year, each with different needles and jets. Thanks in advance.
-Adam

 

[queenslegs]

www.amckayltd.com/carbguide.pdf

 

[5twins]

No, it's not normal to have different mix screws. The screw on the left looks like an original '76-'77 mix screw. The screw on the right with the stepped tip is a later E.P.A. style but it doesn't look exactly like any that came in later 650 carbs. Also notice the finer thread pitch it has. Someone either adapted it or forced it into the carb. Or you could have a whole different carb swapped onto the set to replace a bad one. If you show us pics of the carb set, we can help I.D. it. The '76-'77 carb set was a linked set mounted on brackets as a "bank" and using one throttle cable.

 

[AdamX]

I figured it may have been the wrong screw. You'll see in the pics that the vaccuum screw is a substitute (althought easier to get to!). Let me know if you need more photos. Thanks!

 

[TwoManyXS1Bs]

We have a tentative "carb ID" chart, to help narrow down the carb model years.

BS38 carbs, with 38mm throats, single cable linked throttles, dual chokes with connecting shaft, and 3-hole intake horn.

As 5twins surmised, you have a 76-77 set...

 

[AdamX]

Thanks everyone. Regarding the mix screw threads. The stock screw are coarser but about the same diameter as the other screw. Am I needing another carb body or can I retap those threads? Any thoughts on that?

 

[TwoManyXS1Bs]

I can only assume that your 76-77 mix screws have the same threads as my earlier 71 carbs' screws. If so, there's some pics and dims in this thread:

http://www.xs650.com/threads/revers...bs38-carburetor-pilot-circuit-idle-jet.42002/

You'd need to acquire a tap, grind it down to be a bottoming tap. Then, the real challenge would be to synchronize the thread tapping to pick up what's left of the original threads. A machinist's task, if you're uncertain...

 

[weekendrider]

imo it would be quicker/cheaper to acquire another carb set.

 

[5twins]

Those carbs have been seriously messed with, lol. Don't know if the mix screw hole can be salvaged. The butterfly plates are in upside down too.

 

[xjwmx]

See if the good screw will screw into the hole the fine thread screw came out of. My gut feeling is the fine tooth thread screw was never in far enough to make a difference. The slot would have stripped before then. If it will screw in then all you need is another good screw.

 

[AdamX]

See if the good screw will screw into the hole the fine thread screw came out of. My gut feeling is the fine tooth thread screw was never in far enough to make a difference. The slot would have stripped before then. If it will screw in then all you need is another good screw.

I did try to re tap with the stock screw but only went half way. What a shame.

 

[xjwmx]

^Depending on what replacements are like -- you could take it and both screws to a machine shop and explain the situation. They can do some pretty miraculous stuff. If the cost isn't justified, then before getting new carbs and kind of for fun I would get a tap of the right size and try to get it to hook up and run it in there -- probably grinding the nose off of if first. Think of a good way to block shavings.

 

[AdamX]

Its been a while since the last post but someone told me I left you all hanging out there so here is the update. I was able to get a new mix screw in place without too much trouble. Twisting slowly at the start to find any existing threads was key. Had to back it out a couple times to start at different spots but eventually found them. The screw went in smoothly once that happened. Tightens and doesnt just spin. Counted myself lucky and moved on to the next task. Sorry for the late update. -Adam

 

[AdamX]

The butterfly plates are in upside down too.

Hi, Do I need to flip them back or does it matter?
Thanks
Adam

 

[5twins]

Yes, it matters so yes, you need to flip them back around. The number should face out and be right side up, like so .....

If you look at the butterfly plate when it's closed or nearly closed, you'll notice it isn't quite vertical, it's angled just a little bit. The top and bottom edges are angled as well so they will seal or conform to the bore when the plate is at that slight angle. It's a critical thing in order for the carbs to operate properly.

 

[TwoManyXS1Bs]

... Do I need to flip them back or does it matter?

Yes, otherwise idle and off-idle transition won't work right...

 

[AdamX]

Thank you. I'll add that to the list when I run through the carbs AGAIN this weekend.