'81 XS650 (se) carb problem

 Greetings, all:

I recently purchased an '81 xs650 SE from a guy for the princely sum
of $65.  It was listed as not-running but complete.

I connected a battery and gave it a little starter fluid and it
started right up.  As the man had told me, it was only firing on one
side.  As this was a test-start, the air filters were off and the
cleaner box was open, which is a good thing because it let me discover
the problem with the left side: gasoline was pouring out of the carb.
Liquid which may also be gasoline was also coming out of the tail pipe
on that side (because it was only that side I believe it was fuel and
not mere condensation.  Of course, the bike only ran about a minute
and was hastily shut down when I noticed the fire danger).

So, I'd like to know if anyone has seen this kind of thing before
and / or could tell me what's going on with the carburetor on that
side.  Is this a sticking or sinking float situation?  Are these carbs
known to have any kind of chronic problems that I should look out
for?  Where would you look (and don't say "the classifieds, for
another bike")?

I'm new to motorcycle repair, though I do have experience repairing
automobiles and lawnmowers, so I'm pretty sure I can tackle whatever
this thing needs, if someone could point me in the right direction.

Thanks in advance!

 

BigOldCar@gmail.com wrote:


Yes. Disconnect the fuel hose, drain the float bowls and refill them with a
50/50
mixture of Berryman B12 Chemtool Choke and Carburetor Cleaner and let the
mixture sit in the carbs for half an hour to dissolve the gum and varnish
making the float valve stick.

You can buy B12 in a handy pint can for about $3.25 at Wal*Mart.

Then start the engine on the B12/gasoline mixture and put your hand over the
inlet mouth of each carb to get it to suck the mixture up into the tiny idle
mixture ports and passages.

Hopefully you can get the carbs cleaned out without disassembly. If you can't
get them clean enough, you'll need to dismantle them and use B12 in the
aerosol can to spritz out all the interconnected ports and passages.

The idle mixture circuit draws gasoline through the pilot (idle) jet and it
gets air through the pilot air jet.

Fuel and air mix and then the mixture comes out through a single hole an inch
or so downstream of the throttle butterflies. This hole (or port) is
regulated by the idle mixture screw. Turning it clockwise makes the idle
mixture richer.

Amateur mechanics get into trouble because they think that the idle speed
should just keep increasing as they open the idle mixture screw further and
further.

They open the screw so far the mixture becomes far too rich, the exhaust
sound becomes dull and thudding and the idle RPM drops.

So they turn the master idle knob clockwise to increase the idle speed. This
screwup causes the idle speed to increase, but the throttle butterflies are
now open so far the throttle butteflies uncover a pattern of three
acceleration transition ports which are only supposed to come into play when
the rider is trying to accelerate away from a stop.

When the rider blips the throttle, the engine gets such a rich mixture it
won't idle back down for 15 or 20 seconds or maybe won't idle down at all
when it's hot.

No, the way to adjust the idle mixture screws is to think of the carb as
being like some automotive carburetor where you turn the idle mixture screws
clockwise until the idle gets rough from being too lean. Then you back the
screws out about 1/8th of a turn and you're good to go.


Besides easily getting all plugged up with gum and varnish, Constant Vacuum
carbs will occasionally have a diaphragm with a tear or pin hole and the
engine will start and idle but won't run well at larger throttle settings.

Constant vacuum carbs are designed to keep the rider from taking direct
control of the fuel air mixture at large throttle openings and polluting the
atmosphere.

Instead of being raised by a cable, the slide that moves the jet needle up
and down in the needle jet orifice is raised when the engine develops enough
vacuum to pump air out of the diaphragm chambers.

If you find a torn diaphragm, don't pay $100 for a new one, there's a place
in England that makes aftermarket diaphragms.


Google rec.motorcycles.tech for XS650 and carburetor or alternator or
ignition or whatever and you will find that we have discussed most problems
already.

http://houseofmotorcycles.bikebandit.com/parts/Yamaha/XS650H/1981/645996

--
Message posted via MotorcycleKB.com
http://www.motorcyclekb.com/Uwe/Forums.aspx/yamaha/200708/1

 

Some really good information there--thank you.  I'm not really afraid
of disassembling the carb, but I'd really prefer not to.  I will try
this trick you've recommended with the B12 and carb cleaner.


For my part, I don't adjust anything on carburetors but the choke.
The problem, of course, is that the previous owner may have.  I saw on
mine today that there had been some red sealant blocking access to the
mixture screws, but it looks like it has been removed, so either it
rotted and fell out (yeah, right) or (most likely) someone was in
there monkeying around with it.

I hate carburetors, they're so complex.  Why couldn't the guy who
invented fuel injection have been born before the guy who came up with
carbs?


Thanks again; I'll keep an eye out for that.


Trust me: that was never an option!  ;-)

Thanks for the discount link.

 

Some really good information there--thank you.  I'm not really afraid
of disassembling the carb, but I'd really prefer not to.  I will try
this trick you've recommended with the B12 and carb cleaner.


For my part, I don't adjust anything on carburetors but the choke.
The problem, of course, is that the previous owner may have.  I saw
on
mine today that there had been some red sealant blocking access to
the
mixture screws, but it looks like it has been removed, so either it
rotted and fell out (yeah, right) or (most likely) someone was in
there monkeying around with it.

I hate carburetors, they're so complex.  Why couldn't the guy who
invented fuel injection have been born before the guy who came up
with
carbs?


Thanks again; I'll keep an eye out for that.


Trust me: that was never an option!  ;-)

Thanks for the discount link.