'70-'78 Yamaha 650 Twins with Solex and Mikuni Early Style BS38 Carbs

This is one of the most popular old Yamaha's ever made, the 650cc twins. This lineup includes all the years with early Solex or Mikuni BS38. These carbs can be a monster to build right if you do not ID them correctly or buy most of the horrible kits available. They have many pitfalls and require a very specific way to clean them if you want them you look as close to stock as possible. There are a variety of internal tuning and three basic overall designs. They are also easy to mix-up due to the ID number being acid etched into the upper bell housing and hard, if not impossible, to read. If you become familiar in the differences in the years and do a little detective work you can make these bikes run like the machine they are meant to be!

Note: Starting in '74 Yamaha started year coding bikes in the last letter used in the name. You may see this code on your bikes paperwork or in a parts list or catalog. I leave this off as it is redundant if you are showing the year anyway. There is a chart below to find your year code.

Below is the three basic styles these carbs come in from '70-'77. First there is the first gen 256 used '70-'71. This set used a dual choke system and the only thing that holds the two carbs together is the c shaped choke bracket and the fuel line which is U shaped and hangs below the carbs. The second gen is used from '72-'75 306 and 447 (447 shown below). This carb uses a single choke system and the only thing that holds the two carbs together is the fuel line and air relief line. Notice the fuel line now runs strait from one carb to the other. Finally the third gen '76-'79 584 and 2F0 which went back to a dual choke system and is now held together with a top bracket and main lower bracket like modern carbs. It also uses a choke riser rod to actuate the two chokes. There are many other differences between these carbs, but these are the easiest to see at a glance.

'70 XS1------------------------------------256
'71 XS1B----------------------------------256
'72 XS2------------------------------------306
'73 TX650---------------------------------306
'74 TX650---------------------------------447
'75 XS650---------------------------------447
'76 XS650---------------------------------584
'77 XS650---------------------------------584
'78 XS650---------------------------------2F0
'79 XS650---------------------------------2F0

Yamaha Year Codes