So i changed my diaphragms...

[dwaynemcfarlane]

I bought some new diaphragms for my mikuni bs38 from 1977. The old rubber ones were cracky and not tight anymore. I followed this manual :
http://jbmindustries.com/yamaha650.html

But now i feel like the slides arent moving down as smooth(and slow) as they should. Im wondering if its because the rubber is brand new or something is wrong?
I found this vid on youtube and his “15 sec rule” on the slides does not seem to apply to my new ones. Does it matter?
what to do?

yamaha xs650 1977 mikuni bs38

sincerly dwayne

 

[Mailman]

Make sure , when you perform that test , that your choke is in the closed/ normal running position. Otherwise it will draw air in from there and your slide will fall faster.

 

[dwaynemcfarlane]

Make sure , when you perform that test , that your choke is in the closed/ normal running position. Otherwise it will draw air in from there and your slide will fall faster.

Aaahhhhh
Actually i dont know what position the choke was in!!! Thank you!

 

[DogBunny]

First, I did not watch the video, it was too long at 8 minutes.
I have never heard of a 15 second rule. What I think is important is that both slides fall in a matched manner.

Different slides will fall differently when placed in the same carb body. It's partly due to the diaphragms, but it's also due to the slides -- the slides get worn and scored over time.

 

[RustiePyles]

Been working on CV carbs for 20+ years and have never heard of anything like a 15 second rule. A good rule of thumb is as @DogBunny stated, uniformity and smoothness. Covering the draft port and letting the slides drop can be a good test for leaks but 15 seconds is an arbitrary number. Slides that have been drilled for faster throttle response will in turn drop faster as well, that doesn’t mean they’re bad. As long as both slides are falling at similar rates I wouldn’t be to concerned.

 

[5twins]

Although I never timed the drop until recently, 15 seconds does sound like a pretty good "rule of thumb". You want a slow slide drop. This tells you the diaphragm was mounted correctly and is sealing well, has no holes, and is creating good vacuum. I just assembled some BS34's with used, good diaphragms (no holes). Diaphragm rubbers were smeared with red rubber grease, as were the rubber choke plunger tips. Timed slide drop was very slow, about 25 to 30 seconds per carb.

 

[dwaynemcfarlane]

Where exactly did u use grease? I wanna try to do the same
Pictures would be nice


D

 

[5twins]

Sorry, I didn't take any pics. I smeared the entire rubber diaphragm part in hopes it might help soften and preserve it but at the very least, you'd want to put it on the outer edge, that rib that fits into the groove in the carb body. I think it helps it seal better. Red rubber grease is a grease specifically made for use on rubber. It's not petroleum based, it's made from vegetable oil, and it won't harm rubber.

https://www.ebay.com/itm/2-oz-100-P...105549?hash=item3f6c313bcd:g:5Y4AAOSwYIxX8Ev8

 

[Jan_P]

Thinking out loud
Did you put the slides back in the carburetor they came out of .. ???
So the one that was in the left carburetor not now sits in the right and vice versa
Did you clean them on the outside and the inside sliding surface ..
Perhaps with some solvent. Gasoline
Apologize if this is to simple . but sometimes a little non visible wear can affect --
relative movement on machine parts if some oil is there

 

[dwaynemcfarlane]

Would this do the trick?

https://www.henkel-adhesives.com/ee/en/product/petroleum-based-greases/loctite_lb_8106.html

Sorry, I didn't take any pics. I smeared the entire rubber diaphragm part in hopes it might help soften and preserve it but at the very least, you'd want to put it on the outer edge, that rib that fits into the groove in the carb body. I think it helps it seal better. Red rubber grease is a grease specifically made for use on rubber. It's not petroleum based, it's made from vegetable oil, and it won't harm rubber.

https://www.ebay.com/itm/2-oz-100-P...105549?hash=item3f6c313bcd:g:5Y4AAOSwYIxX8Ev8

 

[dwaynemcfarlane]

Thinking out loud
Did you put the slides back in the carburetor they came out of .. ???
So the one that was in the left carburetor not now sits in the right and vice versa
Did you clean them on the outside and the inside sliding surface ..
Perhaps with some solvent. Gasoline
Apologize if this is to simple . but sometimes a little non visible wear can affect --
relative movement on machine parts if some oil is there

Left should be left and right should be right :-/
I really hope its just a little non visible wear

 

[5twins]

No, you want a specific rubber grease, something made for use on rubber that won't harm it. SIL-Glyde would probably work as well .....

https://www.amazon.com/AGS-BK4-Brak...1613509733&s=automotive&sr=1-2&ts_id=15719201

 

[DogBunny]

No, you want a specific rubber grease, something made for use on rubber that won't harm it. SIL-Glyde would probably work as well .....

https://www.amazon.com/AGS-BK4-Brak...1613509733&s=automotive&sr=1-2&ts_id=15719201

No, not Sil-Glyde. It has been demonstrated that Sil-Glyde will swell rubber, despite what the label says.
Red Rubber grease works with rubber, and has the added advantage that it is also compatible with brake and hydraulic fluid. But it is not common in the U.S.

https://www.amazon.com/Trident-acc5...ocphy=9028321&hvtargid=pla-343141348794&psc=1
The other choice is Pure Silicon Grease. Completely inert on rubber. I no longer ever put Sil-Glyde on rubber, but use this instead.