Here's the one for a 250 B / C :

www.claussstudios.com/store/p623/Yamaha_..._Air_Boot_Joint.html

And the one for a 250 A :

www.claussstudios.com/store/p617/Yamaha_...ot_Rubber_Joint.html

Main Yamaha page on Clauss:

www.claussstudios.com/store/c49/Yamaha.html

A lot of their stuff is on Ebay as well:

www.ebay.com/str/CSM-Restoration-Parts/Y..._storecat=2547889010

Almost 2 years since I brought it home. Sitting patiently in my shop waiting for me to get back to it. Hoping by summer I can have it going again.

On a whim today I dumped a gallon of fresh gas in the empty tank and decided to give it a kick for the first time in a year or so. Choke on, kick 2x and it hits. Choke off, kick 2-3x and it wanted to run but smothered out but backfired & died. Kicked like crazy and it’s flooded. Dried plug and cylinder, repeat this exact story 3 more times and it starts! Held a steady 4K rpm for a bit, rev it 5-6k sounds pretty good. Almost got it back down to idle then it backfires and dies. I quit on it after that and drained the gas back out. 

I need some help on where to look. Points? Timing? Coil? Gas tank is 100% clean and good flow through petcock. Carburetor clean (float may need slight adjustment as it drips out overflow). New boots on both sides of carb. New plug(s) several times. Ideas please. 

Wow, over a year since I even sat on this! I look at it daily in the garage, but that's as much effort as I've put into it. Really need to get back on this thing soon - deserves to be ridden again!