Flat top carbs on '78 R100/7 - Diaphragm replacement

[SCBuckeye]

I am hoping someone out there has had experience doing this.

BMW/Bing recommends replacing diaphragms about every 2 years (too often in my opinion).

My '78 R100/7 has Flat top carbs on it and others I know have the standard pointed top type carbs. It seems that there was no rhyme or reason why BMW used different carbs on the same model and year bikes, but they did!

My understanding is that the diaphragms are the same, however, Bing/BMW has the diaphragms for sale for the flat tops only in a unit with new slides! I talked with Rusty at Max's about this, and he assured me that there are those who have successfully replaced the diaphragms only without having to purchase the slide/diaphragm units. He says that they have used a heat gun to remove the plastic holder that connects the two pieces. I understand, however that this is tricky to do as the plastic pieces get brittle over the years and cannot be purchased separately.

Does anyone have any experience doing this procedure and not having to purchase the expensive slide/diaphragm as a unit? Also, maybe someone can point me to a video or description from someone who has had experience with this?

 

[dennisdarrow]

I guess Bing's only reply to this is buy the whole thing. Are the 2 parts available as part numbers from Bing?..............Any chance to find a pair of "dome" top in the correct vintage?..............How long has it been since the last replacement of the diaphragm. Think you can nurse a couple more years out of them before the "have to" time comes around.................

Hope you get it solved there for you...........The mountain top is always there for you, when you want to take a real test ride.........God bless......Dennis

 

[20774]

No videos that I can think of specifically for the flat top carbs...videos for other types, though. Snowbum mentions using boiling water to soften and expand the ring.

 

[SCBuckeye]

Thanks to both of you for your input. After posting, I checked with Max's and the flat tops apparently were only used on a few of the R100/7's and are not even in the R100 microfischs. Rusty showed me last year that they are listed (as a piston/diaphragm unit) in the R80/7 diagrams. Price: $98.00 each!! OUCH!!

I haven't ever replaced the diaphragms (owned bike for 2-1/2 years) and more-than-likely-neither did previous owners. Sooo...

I checked them last summer, and they seemed to be pin-hole free, but very flimsy and looking like they should be replaced to avoid future problems.

I am going to try to do so in the next few weeks or so.

I now work in a CNC machine shop, and if I had a sample of the nylon (?) "rings" I am going to try to duplicate and mfg. my own. If successful, they could be worth millions!! Think?

We'll see!

 

[milump]

I have a friend that just replaced the diaphagms. Getting the plastic ring off was no big thing but he couldn't get it back on with the new diaphragms at first. I told him to put the slide in the freezer overnight and boil the plastic rings first and they popped right on.

 

[SCBuckeye]

I have a friend that just replaced the diaphagms. Getting the plastic ring off was no big thing but he couldn't get it back on with the new diaphragms at first. I told him to put the slide in the freezer overnight and boil the plastic rings first and they popped right on.

Thanks, gives me some courage to try. I just don't want to mess up, or break one of those rings, and then HAVE to purchase the expensive slide/diaphragm units!

 

[yankeeone]

I did the job on mine, just put the part with the white ring in boiling water, take off the old, put on the new and after boiling just the ring ,reinstall. there is a video on utube , and Bing carb will talk you through.

 

[SCBuckeye]

I did the job on mine, just put the part with the white ring in boiling water, take off the old, put on the new and after boiling just the ring ,reinstall. there is a video on utube , and Bing carb will talk you through.

On the slide/diaphragm, was there a notch or something to help you rotationally orient the diaphragm correctly? When going back into carb, they both as a unit, have to orient to the carbs so that the back of slide is towards the air intake, I think the diaphragm has a notch on it to go into a notch in the carb. Does that make sense?

Thanks!

 

[20774]

Yes, there should be a tab for orienting the diaphragm.

 

[PGlaves]

Yes - there is a tab that goes into a notch in the slide - and another tab at the perimeter that goes into a notch in the carb body.

 

[tattube]

Yes - there is a tab that goes into a notch in the slide - and another tab at the perimeter that goes into a notch in the carb body.

Yup...that's how I remember it.

 

[SCBuckeye]

Ironically, about a year ago, I purchased a set of used, but completely rebuilt, carbs, and had been planning to simply replace both of my carbs as units with the rebuilt ones. However, yesterday, because of the threat of ruining my existing nylon insert on my diaphragms, I simply decided to replace the slide/diaphragms instead of the whole carbs. Worked and fit perfectly. Replacement diaphragms looked and felt new, and certainly felt stronger than the old ones, though I could not find any pinholes or cracks in the old ones.

Took out for a 1 hour spin yesterday, and everything seemed to work fine, except that I think it was running slightly lean just off idle and up to about 1/2 throttle. So...came home and raised the jet needle 1 notch. Will take out and try again - probably next weekend.

So far . . . . so good!

Once running to my satisfaction, I will then take the old slides/diaphragms apart, and see the construction, and then I am going to try to manufacture some of the nylon (better, Delrin) rings to see how that goes. I already have a brand new set of diaphragms so I can re-assemble and have as a back-up set.

Thanks for all of your input!

 

[nilsmenten]

Ironically, about a year ago, I purchased a set of used, but completely rebuilt, carbs, and had been planning to simply replace both of my carbs as units with the rebuilt ones. However, yesterday, because of the threat of ruining my existing nylon insert on my diaphragms, I simply decided to replace the slide/diaphragms instead of the whole carbs. Worked and fit perfectly. Replacement diaphragms looked and felt new, and certainly felt stronger than the old ones, though I could not find any pinholes or cracks in the old ones.

Took out for a 1 hour spin yesterday, and everything seemed to work fine, except that I think it was running slightly lean just off idle and up to about 1/2 throttle. So...came home and raised the jet needle 1 notch. Will take out and try again - probably next weekend.

So far . . . . so good!

Once running to my satisfaction, I will then take the old slides/diaphragms apart, and see the construction, and then I am going to try to manufacture some of the nylon (better, Delrin) rings to see how that goes. I already have a brand new set of diaphragms so I can re-assemble and have as a back-up set.

Thanks for all of your input!

Greetings,

Did you ever make any headway with this project?

 

[SCBuckeye]

Greetings,

Did you ever make any headway with this project?

I did heat up the plastic rings and removed the old diaphragms, and then I took slides into machine shop and then dressed up the tops, so that it will be easier to install new phragms, but haven't done so yet.