carb diaphragms

[jwh]

hi all, just wondering if the carb diaphragms are the same in 1100's and 1200's?, anyone know.

[FJmonkey]

Yes.

[56 CHEVY]

The diaphrams may be the same, but I thought the slides were different.

[racerrad8]

No...

The 84 & some 85's use a flat bottom diaphragm slide. After that they are all the same with a curve cut into the bottom.

They are not interchangeable without emulsion tube & needle replacement as well.

The rubber diaphragms are not a Yamaha replacement item. There are some aftermarket rubber diaphragms you can purchase, but they do not allow for the full & complete stroke of the slide. This effects the mixture of the mid-range needle setting.

Re: Replacement BS36 carb diaphragms

I would not recommend them as the restricted stoke is going to cause your bike to run lean.

Randy - RPM

[FJ_Hooligan]

My old '85 had a shrouded needle jet and cut away slides just like the later years.  There was probably a change over sometime during model year.  I also remember that '85 used 110 main jets whereas the other early models ('84 to '87) used 112.5s.

Hooligan

[FJmonkey]

What Randy said...

[fintip]

Alas.

Pure conjecture, but would it be at all reasonable to consider just shortening the needle?

[Pat Conlon]

No, that's not the answer. You first need to understand how the jet needle and needle jet work, and why there is a taper to the jet needle.

Then there is a issue of the slide blocking airflow..

Kyle, I pulled your post over from the Files Section for discussion here.

[fintip]

Any chance someone has a picture of the early diaphragms? I'm have a hard time visualizing the difference.

My statement was made (elsewhere, was moved) in reference to the fact that the aftermarket diaphragms don't allow the last 4-6mm of slide travel. I guess it wouldn't work because then it would be 4-6mm off during the rest of travel--so then one would have to change the main jet size, and the whole curve would be affected--and yes, the airflow velocity wouldn't be able to be compensated for, quite.

I'm curious, though, if in actual use the slide ever actually fully opens? Or at what point it does? Would be interesting to see a 3-d model showing a CV carb in action during use, clearly showing the slide action and also showing the fluid and air dynamics in motion.

[fj11.5]

Have a look on randy,s web site, may have pics of the early diaphrams, but just imagine the 1200 ones,  but instead of having the curved bottom, its flat instead

[jwh]

the ones i have are the flat bottomed ones, they came as part of an 1100 engine in bits that i was given it is pretty much scrap as it has been in bits in a cellar for 10+ years, quite good for bolts and valve shims but not much else but the carbs that came with it have these perfect diaphragms, so if they are not suitable for my 1200 (curved bottoms) is it possible to just swap the rubber bits?, just wondering as carbs on bike fine at moment but handy to have spares.

[brdprey]

  thanks to randy, he not only had the diaphrams at a decent price but the exact replacement.  trust his wording.