'81 550L - CV carb diaphragm alternative wanted

[jwegman]

'81 550L -

I'm starting in on my CV carb 'overhaul' and when looking at the diaphragms, they don't seem to have any holes/rips, but they aren't keeping a consistent round shape (at the top) nor do they feel rubbery.

Is it time to replace them?

In looking at the various parts houses, I'm seeing $100+ per diaphragm. Needless to say, I can't pay that for a new set.

Are there any alternate sources for a diaphragm that will fit? Some snowmobile/weedwacker/vw bug or something that uses a compatible diaphragm?

Yeah, I was joking about the weedwacker...

just a note of comparison, my '78 bmw r100 carb rebuild kit (o-rings, diaphragms, and gaskets are $27.95 for the pair of carbs)

Regards,
Jake in SE MN

[Nessism]

The diaphragms are not a serviceable item on these BS Mikuni’s, although an inventive person most likely could figure out some procedure. Finding a diaphragm is an issue though since they are not sold separately. Finding good used slides is not terribly difficult so the slide replacement issue is kind of moot for most of us. My suggestion is to just run what you have or post an ad in the Parts Wanted forum.

BTW, I have two brand new BS32 slides that I could be motivated to part with if someone would properly motivate me. Send me a PM if you want to discuss.

[Steve]

Cycle Parts Nation $59.30
PartShark $104.59 (retail $140.55 )
Babbitt's $63.26
Alpha Sports $143.39

Yep, prices are all over the place, but they are available for less than $100. You just have to know where to look.

Here are some good places to start:
Alpha Sports
Babbittsonline
Bike Bandit
Carolina Cycle
Cycle Parts Nation
CYCLE-RE-CYCLE
Full throttle
Motogrid
MR Cycles
PartShark
Partsnmore
Ron Ayers
Z1 Enterprises

[jwegman]

My suggestion is to just run what you have

Ok. I hadn't taken a good look at how the diaphragm was secured onto the slide. With the Bings on the BMW, they were secured by a collar and four screws (easy to replace).

In the next hour or so while at lunch, I'll swing by the bike and take a real close look at the diaphragms and hopefully I'll be encouraged to run what I have Perhaps I'll remember to bring my camera and snap some pics.

Steve, I appreciate the price comparison shopping!

Regards,
Jake

[Natrik]

As far as replacing the whole diaphragm for another similar one, I don't have any brilliant ideas. But if you have to repair a diaphragm, go get a can of plasti-dip from NAPA auto parts (NAPA part # 765-2527). I use it all the time in my shop to repair torn and leaking carb diaphragms. You may be able to make one out of some sort of thin, strong cloth and coat it in the plast-dip to make it work. Worth a shot, I suppose?

[chef1366]

I have some good slides.

[jwegman]

To follow up; I've finished 'carb' week I had used my existing slides/diaphragms.

When reassembling the carbs, the diaphragms didn't gracefully sit into the top of the bodies, however I positioned them to the best of my ability when installing the carb tops.

...All four carbs have similar slide movments when letting the slides fall under the spring load/gravity with similar swooshing sounds. Hopefully all is well with them...

In fact, last night I pushed and cursed them back into the bike. Ended up using a crow bar to lever them into the intake boots.

Regards,
Jake

[Steve]

The 'trick' I use to get the diaphragms lined up properly is rather simple. Since they usually move around as the slides go down when you put the spring and cap on, hold them up. I push the diaphragm down over the slide so the edge lines up with the carb body, lower the slide into the carb body, but hold it up with a finger in the carb throat so the diaphragm does not fold. Put the spring in place, put the cap over the spring and lower it to the carb body, trapping the edge of the diaphragm in place. Now you can release the slide and let it drop into place. Perfect every time.

.

[jwegman]

Steve, thanks for that tip!

On a related note, I got the bike fired up a few minutes ago. I was expecting it to be rich as I'm 3 turns out on the screws, but I'm only getting a little white smoke out of the left pipe... But it's not too rich to prevent it from quickly reving up to 9000 when I twist the grip!

Man, this thing is a LOT smoother than I remember. Very little vibration when idling and nice smooth revvs. A lot smoother than my '78 BMW r100rs

So, I think I've gotten my carbs cleaned and diaphragms installed properly. We'll see how it'll kick start when cold tomorrow evening when I'm hosting an engine starting party

The kick start is a novelty for me. It's an '81 550L with a '78 550 engine (with the '81 head and cv carbs). I'll post the rebuild story tomorrow in the GS Owners area (with a few pics).

Jake

[doctorgonzo]

Steve, thanks for that tip!

On a related note, I got the bike fired up a few minutes ago. I was expecting it to be rich as I'm 3 turns out on the screws, but I'm only getting a little white smoke out of the left pipe... But it's not too rich to prevent it from quickly reving up to 9000 when I twist the grip!

Man, this thing is a LOT smoother than I remember. Very little vibration when idling and nice smooth revvs. A lot smoother than my '78 BMW r100rs

So, I think I've gotten my carbs cleaned and diaphragms installed properly. We'll see how it'll kick start when cold tomorrow evening when I'm hosting an engine starting party

The kick start is a novelty for me. It's an '81 550L with a '78 550 engine (with the '81 head and cv carbs). I'll post the rebuild story tomorrow in the GS Owners area (with a few pics).

Jake

Just the tiniest smidge of grease in that recess on the top will help hold the diaphragm in place while you put the top on. It's easy to get it pinched.

[chef1366]

Just the tiniest smidge of grease in that recess on the top will help hold the diaphragm in place while you put the top on. It's easy to get it pinched.

Hey, that's my trick.