305 slider diaphragms

When I replaced my bad piston 2 years ago I installed JBM vacuum slider diaphragms. I almost bought a different kind that are installed using the existing plastic rings that hold the membrane to the collar of the slider, but the JBMs looked much easier because you remove those rings. I used their K-71 diaphragm which is listed as being for my bike and several others. The bike never ran properly and wouldn't go above 45 mph, and since it was blowing a lot of smoke out of the side that had a scored cylinder I assumed it was because of that.

But now I wonder if the lack of power was the sliders. When I took the carbs apart those diaphragms did not fit tightly on the slider. They probably didn't fit snugly when I installed them either, but I probably assumed they would work since they are listed for this bike. I measured the collar and its 1.5mm smaller than the base of the diaphragms, and the diaphragms are supposed to be the smaller of the 2 for a snug fit on the collar of the slider.

JBM site says you can use a small amount of cyanoacrylate to secure their rubbers. That might keep it from spinning but that will not seal it at the collar. They also say NOT to use any kind of silicon sealant because it will damage the rubber, so that's out. I bought a new pair thinking mine may have swelled, which they kindly sent with free shipping, but since discovering the discrepancy in diameter I'm now certain the new pair will have the exact same trouble when they arrive Tuesday.

I'm not sure what to do now. Since I removed the plastic retaining rings, I can no longer use OEM diaphragms and replacing the whole sliders would cost nearly $500 which is out of the question.

 

You could make an offer on these.The sliders are still there. Maybe the diaphragms are usable.

 

yeah used carbs or sliders are an option, just not a great one. I'd almost certainly still have to replace the rubber and then I'm back to where this all started. Then I'd have no choice but buy the other kind of replacement rubber that one seller said "it is not difficult but quite tricky so you should have some technical skills and experience with that type of repairements" ... which I do not have. I've watched their installation tutorial and feel confident I can ruin them, and they cost 50% moe than the kind I bought before.

Would be nice if I could find the type that doesn't need the plastic rings, but in the correct size, but I can't find them anywhere. JBM seems to be the only one that makes them and despite saying they are for the 305, they do not fit properly.  Pretty frustrating that I ruined my sliders for a part that doesn't fit.

 

Well the good news is the brand new diaphragms fit petty well. The bad news is I may have to replace them every year or two.

So, how would I know if my diaphragms are not airtight? What would the symptoms be?

I like to suck air through the carburetors with a shop vac and make sure the slides go up and down smoothly as I open and close the throttle plates.
Compare the operation of the old diaphragms with the new.
If you find the diaphragms are too loose to seal, I would disregard JBM's warning and use sealant.

ckahleer wrote:

I like to suck air through the carburetors with a shop vac and make sure the slides go up and down smoothly as I open and close the throttle plates.
Compare the operation of the old diaphragms with the new.
If you find the diaphragms are too loose to seal, I would disregard JBM's warning and use sealant.


 

Good to know, but what would my symptoms be if they are on the bike? Can I distinguish it from other issues?
 

My kz305 tops out at about 83 mph. If your only getting 45 mph that could indicate leaking diaphragms not lifting the slides, or not lifting them as far as they should.
Imagne if the carbs were cable operated and kinked cables were only allowing you to lift the slides 1/4 of the way up, A top speed of 45 mph would be reasonable.
Idling well, but lack of power as RPMs increase is a sympson of leaking diaphragms, but ignition timing not advancing would also cause that.
I don't know of a way to 100% verify CV slide operation on a running bike. Maybe removing the carb intake boot and using a bore scope to look into the carb, but lack of intake air resistance may affect slide movement.

I think its the diaphragms. I had completely forgotten about the issue I had with the first pair, and have just been riding it this way because I am sick of trying to make it run properly. I can get new slides for 300+ bucks or Mikuni carbs for 400. I think it will be the latter, if I can ever find 400 bucks. I suppose I could pull the slides out and silicon them to finish out the season. I swore last time I put the carbs in that the next time they come out they are going in the trash.

No need to pull the carbs. With the gas tank removed you can access the 4 screws to remove the carburetor tops and remove the slides.

I know that, but there are likely other carb issues that would require removing them. Like you, I also have idle issues and that's not from sliders. That's why just getting Mikunis is attractive. Silicon is likely to be a very temporary fix, and it could end my season early. I might try it in late October just before my riding season ends anyway, as a diagnostic aid.