This is part 1 of 3 installments of the Yamaha XS650 Model ID. Here we will cover the carburetors so you can distinguish the specific differences between models and their components.
Yamaha XS650 Carb ID
(North American Market)
All 1970 – 1979 models use the Mikuni BS38 carburetors
All 1980 – 1984 models use the Mikuni BS34 carburetors
These OEM carburetors are emission-type and, generally, they are not very good performers. They were made for smooth acceleration and optimal fuel economy.
Models that were fitted with BS34’s came jetted very lean from the factory. They were imported into California and, therefore, had to meet the strict standards of the California EPA. Factory setting of the fuel enrichener screw was 1.5 turns out from the bottom but enriching to 2.5 – 3 turns usually improves idle.
Both BS34’s and later BS38’s have the fuel mixture screws covered by a small metal plug (or limit positioner) and have a non-adjustable, clip-positioned slide needle.
Low-speed fuel mixture screws can be accessed by inserting a small sheet-metal screw in the cover plug & pulling it out. Note: a small rubber washer is located at the bottom of the mixture screw hole under a small metal washer, make sure it is intact.
Original float valves have a small screen on their back-end that clogs easily with sediment. Discard it if damaged & blow fuel line clear. The graph below shows the optimal float level setting for the appropriate models.
The pilot jet is accessed from the float bowl chamber in a passage covered by a rubber plug. This plug must seal the passage so check it’s condition.
BS38 70-75 Pilot jet types differ from 76-79 types, as shown below.
Main Jets (large round style) are located at the bottom of the float bowl and can actually be accessed and adjusted without removing the carbs from the motorcycle. BS38 Carbs have the main jets located INSIDE the float bowl body.
A hovering idle that is slow to fall indicates a lean condition, so check for restricted fuel supply, clogged jets, or air leaks. A stumble on acceleration to redline usually means a too lean condition, so increase main jet size and decrease air jet size until bike pulls cleanly through range.
REMEMBER: Any changes from stock exhaust, air box or filters will require jetting/ mixture changes.
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