'81 XS650 (se) carb problem

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BigOldCar@gmail.com - 25 Aug 2007 17:01 GMT

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!

Reply to this Message

Albrecht - 25 Aug 2007 19:25 GMT

>Is this a sticking or sinking float situation?

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.

>Are these carbs known to have any kind of chronic problems that I should
>look out for?

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.

>Where would you look (and don't say "the classifieds, for
>another bike")?

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

Reply to this Message

S_Wear@Hotmail.com - 26 Aug 2007 02:27 GMT

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.

> 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.

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?

> 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.

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

> If you find a torn diaphragm, don't pay $100 for a new one

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

Thanks for the discount link.

Reply to this Message

Albrecht - 26 Aug 2007 03:07 GMT

>I will try this trick you've recommended with the B12 and carb cleaner.

If you can get the bike running reliably enough to ride it, 4 or 5 ounce of
B12 in a full tank of gas will clean the carbs out further.

>For my part, I don't adjust anything on carburetors but the choke.

CV carbs don't have a choke plate like a car carburetor. They have a bypass
enrichment valve that allows air to go around the CLOSED throttle butterfly.
This airflow sucks gas straight out of the float bowl for cold starting.

So, leave the throttle grip alone when starting the engine.

>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.

OK, you found the idle mixture screws. How hard can it be to start with one
full turn open and keep turning the screws clockwise 1/4 of a turn at a time,
while
adjusting the idle speed lower everytime it speeds up as you lean the carbs
out?

>I hate carburetors, they're so complex.

I hate fuel injection. It's so complex, with all the switches and sensors and
the ECU that needs an aftermarket chip or reprogramming the EPROM by a laptop
loaded with a program downloaded from the internet.

And after all that sh.t, the FI system may still cause jerky throttle
response at small throttle settings...

>Why couldn't the guy who
>invented fuel injection have been born before the guy who came up with
>carbs?

Maybe his mother said "No, no! Not without a condom!"

Frederick William Lanchester joined the Forward Gas Engine Company Birmingham,
England in 1889. He carried out what were possibly the earliest experiments
with fuel injection.

Hungarian János Csonka was the co-inventor of the carburetor with Donát Bánki,
patented on February 13, 1893.

But fuel injection systems are more complicated and expensive than most car
owners would be willing to pay for if they weren't forced to buy cars so
equipped.

The first FI systems I ever heard of had a mechanical pump that pressured a
fuel rail and recirculated excess fuel back to the gas tank. The amount of
fuel injected was controlled by a simple fixed orifice. The first FI systems
were for racing or for aircraft.

EFI was only added to cars because of the demands of the various air quality
agencies.

Reply to this Message

S_Wear@Hotmail.com - 27 Aug 2007 13:44 GMT

>the ECU that needs an aftermarket chip or reprogramming the EPROM by a laptop
>loaded with a program downloaded from the internet.

If you're into performance-tuning, then yeah, it's a bit of a hassle.
I agree with you on that point. I don't even own a laptop or a scan-
tool! But I like economy and reliability (I know, boring). I also
have a good understanding of how the sensors and whatnot work together
to feed data so the ECU knows what to do, so I generally know where to
look when problems do arise.

Would this be a bad time to mention that I don't like condensor-points
ignitions either?

I had a Torino with points. I yanked and adapted the electronic
ignition system from a later junkyard Comet. An older guy I knew at
the time swore I was stupid for doing it, because, he argued, with
points, if you have problems, you can fix it yourself on the side of
the road with a small piece of emory cloth and a screwdriver. My
argument was that the electronic system *wouldn't* give problems, most
likely, and it would require no maintenance, to boot.

I guess it's all about what you're used to and comfortable with. Me,
I'm baffled by that funky box with all its protuberances and tiny
brass pieces.

Reply to this Message

S_Wear@Hotmail.com - 01 Sep 2007 03:45 GMT

That B-12 did the trick nicely. The engine is now running on both
sides and is no longer spewing fuel. It is not running properly;
however, that's what tuning is for!

Reply to this Message

Albrecht - 01 Sep 2007 14:44 GMT

>That B-12 did the trick nicely. The engine is now running on both
>sides and is no longer spewing fuel. It is not running properly;
>however, that's what tuning is for!

Yabbot, you said that you didn't like to mess with anything on carbonators
except choke adjustment. :-)

Reply to this Message

S_Wear@Hotmail.com - 26 Aug 2007 02:32 GMT

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.

> 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.

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?

> 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.

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

> If you find a torn diaphragm, don't pay $100 for a new one

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

Thanks for the discount link.

Reply to this Message

Motorcycle Forum / General / Yamaha / September 2007

'81 XS650 (se) carb problem