Hanging Throttle BS38's

[nj1639]

My '83 is my 'go to' bike for daily riding. It's done and needs no more, mechanically speaking. The '77 was done mechanically too but became a parts tester when I acquired the beaten and abused '78, and in the process of getting the '78 road worthy the '77 wound up with the '78's carbs.
I had to disassemble and clean those carbs out as the mud daubers had found a nice home there. Once cleaned, reassembled and mounted they ran but not great. The bike would start fine and accelerate but as she warmed up she would hang with a slow return to idle. So today I figured I'd get into them again and try to figure this out. First thing I did before pulling them off was to check the diaphragms. Left carb first, raise the slide, cover the top crescent hole, release slide.......excellent, slow drop. OK, right carb. Lift the slide......wtf? This don't feel right.
It seems that Fredintoons Mr. Bodger character was into this carb and put the slide in backwards.....looks like he visits here too from time to time........ lol
Alright, took the top of the carbs cover off in situ and turned the slide, bevel side now facing towards the air filter. Buttoned her up and road tested. Could it be this easy? Yep. Bike runs sooo much better, and the sound of the stock headers with the Commandos is almost heavenly.
Next up is to count the teeth in that rear sprocket. I'm thinking it's a 32 I've put in there and if it is I'm going back to a 34, fun factor.

 

[5twins]

I like to see a 33 tooth grin when I look at an 18" rear wheel, lol.

The '78-'79 BS38s are very nice, smoother than the '76-'77 set because of the spring-loaded needle.

 

[nj1639]

^ 33 I've yet to try. 32 is just too off, there are times with the 34 that I'm shifting up when I'm already in fifth gear.......

 

[5twins]

33 is what Yamaha put on the Euro 18" wheels. I think it suits the 650's power output perfectly. If you try it, I think you'll like it.

 

[retiredgentleman]

I use 17/33 on my 1978 Special. Goldilocks gearing........................not to high, not to low.....................ah, its just right!

 

[5twins]

RG, you could use a 32 on your 16" wheel. It's only slightly higher than a 33 on an 18" wheel. The 33 you have now isn't much different than the original 34 on an 18" wheel .....

 

[nj1639]

Took her out for a ride late yesterday after cleaning up some wiring and no more than two miles out she was hanging throttle again.
Pulled the carbs off this morning and I see that the slide has moved around to where the bevel is towards the engine side.....again. So, good to see that I most likely HAD set it correctly the first time but am quite dismayed...lol...that it's turned. Not sure what's going on here or how to stop it. These diaphragm turn freely on these slides, with little resistance.
Carbs are off a '78, diaphragms are marked BS 38 77 M on the lip.

Epoxy the SOB's to the slide?

 

[5twins]

I've never seen one that moved. They're usually pinched in real tight. Are these factory diaphragms or JBM replacements? I have heard of the JBM's turning. The "fix" is a dot of Super glue between the slide and the diaphragm lip.

 

[nj1639]

There's no other markings on the diaphragms to tell me either original or jbm's.
The slide has about a 6mm space for the diaphragm to sit in but the thickness of the diaphragm there is about 4mm, leaving a 2mm gap. Hmmm.
Well, I've got an idea to try and if that fails then the super glue comes out........

 

[weekendrider]

The JBM's do not have the tab/dog ear to orient/index them like the oem?

 

[5twins]

No, no location tab on the JBM's. It's up to you to install the slide pointing correctly, then hope it doesn't spin while in use, lol.

NJ, got any pics of your slide? It sounds like it's been modified or tampered with. There's usually a white plastic ring under the diaphragm and tight up against it, holding it in. I have no idea how to remove an original diaphragm intact or if it's even possible. When you install the JBM's, you cut that white plastic ring off and remove it. That releases the old diaphragm and it slides right off. The JBM has a thick lip around the slide, like 1/4" thick, and that's supposed to hold it in place and from spinning. Sometimes it does, sometimes it doesn't.

 

[weekendrider]

Since he included the super glue tip on the website I'm thinking the spinning happens more often than not.

 

[nj1639]

Must be jbm's as there's no alignment tab.
The thickness of the diaphragm where it wraps around the slide misses a 1/4 of an inch by 2mm's. So, I need a filler of 2mm to help snug this up....or the glue.
I went to the mom and pop auto parts store in town and went through an assortment of O rings with the slide in hand for measure. I picked up a couple pair to try as the space filler. These are 3mm thick so it's tight now and not turning, If they stay in place......... 2mm flat ones would be the ticket.
Got everything put back together and now I'll have to test it out in the next couple days.
Of course, if this don't work there's the glue....or little tie wraps or......

 

[TwoManyXS1Bs]

...diaphragms are marked BS 38 77 M on the lip.

Epoxy the SOB's to the slide?

Yep, them's JBM markings.
I installed with warm soapy water, grips great.

http://www.xs650.com/threads/jbm-industry.45107/

The JBM site recommends super glue, if needed...

 

[nj1639]

I went through a collection of o rings that I remembered I had and found some that we're 2mm in thickness. I believe these are for the cam nut. Thanks to richardthemotorcycle1 for these.. Popped the 3mm ones off from the mom and pop store and fitted these on the slide diaphragm gap. Perfect. While the carbs were in hand I poked and probed all orifices carefully. Found one on the inlet side that must have had a bit of blockage, not sure how that would affect the function but there was a bit of resistance. With everything probed and o rings in place I put these carbs back on the '78 and test ran. Perfect. Choke, startup, throttle response, acceleration.....these are nice carbs.
I threw the bs 34's that were on the '78 onto the '77.
Hanging throttle. Throttle cable is good, carbs idle stop spring is strong and snaps the butterfly's closed. I've got a couple more things to check yet........