Need info-- Mikuni carb Yamaha 250 Blaster

I'm trying to help repair a Mikuni carb from a 250 Blaster by phone. I
need a diagram of the carb to figure out what he is referring to when
we talk. Any online exploded views out there anywhere? I haven't found
one so far.

JohnF

 

I should have added that this has some sort of casting on top of the
carb with electronics attached. One of the things I need to find out
is if these have some kind of internal device that stops fuel flow in
some manner. He gets no fuel to the float bowl and says there is some
sort of tube from the opposite side on the fuel inlet running to a
case someplace and he can see what appears to be a torx screw down
inside the fuel inlet. I'm not familiar with what-ever they have done
to carbs since my newest bike is an '85 and, relatively, simple.

I don't know what year this is either, it's a blue one and he says
there is no tag on the steering head.

TIA

JohnF

 

I looked at the fiche for a 200cc Blaster and it has the throttle
position switch you describe. Apparently the throttle cable operates a
cable sector inside the case and the sector operates a switch that's
part of the emissions control system. Maybe it controls electronic
spark advance?

Tell him to trace out the wiring and see if it goes to an electronic
engine control unit.

The "case" is probably a charcoal canister that's part of the
evaporative control system. The canister stores gasoline fumes when the
engine isn't running. The line your friend saw is probably the canister
purge hose that sucks gasoline fumes out of the canister when the
engine is started.

I recommend that your friend looks for a vacuum hose routing diagram
which may be underneath the seat or under a side cover to see what kind
of emissions control equipment has been installed on his machine.

An aftermarket repair manual would be a good source of info, too...

 

Does the fiche show any type of fuel flow shut-off inside the carb? I
found some photos on EBay and the carb looks stock except for the
black (alum colored) box on the top. He has no apparent fuel to the
bowl and can't blow through the supply tube into the bowl. I ain't
there and he knows just enough to be dangerous.

JohnF

 

You can register to look at the fiche I looked at on
www.partsfish.com...

The fiche I looked at was for a 200cc Blaster, but it was just the
typical Mikuni VM slide valve carburetor that has been around since the
1960's.

I can't imagine Yamaha getting Mikuni to redesign the carb itself for a
fuel shutoff to meet EPA emissions requirements. I can imagine that
throttle position switch working with a power valve on a 2-stroke
engine, or controlling the ignition advance curve though...

The carb I looked at has a float and a float valve. If you let an
engine sit long enough for the fuel to evaporate out of the float bowl,
the gasoline will turn into gum and varnish and the gum will make the
float valve stick. Usually it sticks in the low position and the float
bowl overflows until the gum dissolves. But, I have see the float valve
stick in the up position, blocking fuel flow completely. Some carbs do
have little filters above the float valve seat, so your friend might
have seen something to do with the
filter. That thing that he thinks looks like a torx fastener may have
something to do with the float valve or a filter.

 

Go here: http://www.yamaha-motor.com/sport/parts/home.aspx No need to
register and much more complete. The
device you are referring to is a Yamaha feature called TORS, or throttle
overide system. It kills the engine if the slide does not return
properly when the throttle is released. There is no electric fuel
shutoff on a Blaster.
Jerry

 

I've had him do all the standard stuff to check for blockages. He had
the carb off and the bowl taken down to clean the fuzzzys out of the
jets. I had him spray Berrymans down the fuel line to see if it was
flowing at all but it wasn't. I looked at the Yamaha page and the carb
looks just like any other Mikuni since dirt so I have to assume there
is something in the float needle (small bird or the like) stopping the
flow. I'll be there tomorrow so I'll take a look.

Thanks for the help everybody.

JohnF