1978 carb diaphragms

[bigryamaha]

I’ve got a nice old 1978 650 special that didn’t run for years. I had a knowledge friend help get it running but it’s just not smooth. Starts and idles pretty well and accelerates smooth but sort of skips when you flatten out the throttle. I replaced the petcocks from Mikesxs and replaced the diaphragms with an aftermarket boot. I’m wondering if the diaphragms could be the issue. I went back to Mikesxs but they don’t offer diaphragms for 78-79 years. Any ideas?

 

[Brassneck]

Might be jetting issues (clogged or otherwise)...but could be the slides/diaphragm...have you tried doing a slide drop test?

 

[bigryamaha]

Jets are new and carb was expertly cleaned. I don’t know what a drop test is? Is this something I can do? I notice it flutter most when I ease off the throttle

 

[Brassneck]

You can do a search for it on here, but essentially you manually push the slide up then hold your finger over the air inlet (top of carb opening), the slide should come down very slow. If it falls quickly or without resistance, then your diaphragm is not sealing/torn/has a hole, etc. Here's a shaky video on the concept, but will give you an idea:

 

[YamadudeXS650C]

The Tech section has a great deal in info/manuals for your carbs.

http://www.xs650.com/threads/carbs-carburetors.43/

 

[bigryamaha]

You guys are awesome! This gives me terrific insight on how to track down this issue. Thanks so much

 

[jetmechmarty]

BTW, JBM Industries has replacement diaphragms for the 38 carburetors.

 

[650Skull]

Here is a link to JBM Industries XS/TX 650 Carb diaphragms..... http://jbmindustries.com/Yamaha650.html.

This may not be the problem..........Even though the carbs have been cleaned there could still be a slight blockage............ A caution on the slides falling fast........if the choke plunger seal is worn this can also affect the speed of the slide falling

 

[bigryamaha]

I checked and the new ones on it are from JBM so they may be fine. I’ll try the drop test and check the choke plunger tip

 

[TwoManyXS1Bs]

...the new ones on it are from JBM...

Coupla recent threads of interest.

http://www.xs650.com/threads/carb-slides-slightly-askew.53472/

http://www.xs650.com/threads/xs1b-running-poor.54197/