cbx500 black smoke

For about 3 weeks after I cleaned the carbs my bike ran great, then ,
while I was riding, the motor started sputtering and I barrely made
it home, I tried cleaning out the carbs but it still runs poorly and
black smoke is coming out of the exhaust.
Any ideas? Thanks in advance
Rick

 

Well, maybe some crud from the gas tank made its way down to the float
valve in the carburetor float bowl and the float valve is stuck, making
the fuel level too high. Or, a tiny bit of crud on the float valve seat
will make the fuel level too high.

When the fuel level is too high, air pressure pushes too much fuel into
the engine and it can't burn the extra fuel, so you see black smoke.
(1)

Also, check your repair manual to see what the float height should be.
It's a dimension (probably around 15mm) from the bottom of the float to
the gasket surface (not the gasket) while the carburetor is upside down
on the bench. So the bottom of the float is actually facing UP while
you're measuring the level, and the bendable adjusting tang on the
float is just resting on the float valve pin.

Or maybe you didn't tighten the idle jet or the main jet enough and it
fell out and when you remove the float bowl, you may find the jet
laying there. (2)

A really nasty possibility to consider is that the rubber diaphragms
(assuming you have constant vacuum carburetors with big round covers on
top) are torn. When the diaphragms tear, the engine will idle OK and
run at low speed, but the vacuum slides don't lift so there is too much
vacuum in the venturi and the carbs suck too much gas out of the float
bowls through the idle mixture bypass and up through the needle jet.

New rubber diaphragms are expen$ive. I'm talking about $90~$100 USD for
a vacuum slide with a new diapragm. That's bad news for somebody on a
motorbike budget.

(1) People talk about "vacuum" as though carburetors somehow *suck*
gasoline into the venturi. Actually, it's just normal outside air
pressure *pushing* the gasoline into a lower pressure area in the
venturi. That lower pressure exists because air has to speed up to pass
through the smaller area in the throat of the carb...

(2) The first time I ever tinkered with a motorcycle carb (I don't
remember why I removed the main jet, maybe it was because I was in a
Sir Edmund Hillary mood, and It Was There) I didn't get the main jet
tight enough and it fell out and was laying in the float bowl. Since
the jet needle restricts flow at all throttle openings below about 3/4
to full open, I never knew that the main jet was taking a swim in the
float bowl until I needed full throttle to pass a big truck. He was
going 55 mph, and I could only go 55 with the engine bogging down on
excess gasoline. I had to give up the attempt to pass and humbly follow
the truck for miles and miles...

 

Turn off the choke, or make sure it is opening up when you move the choke
lever/cable to the non choke position. It is running way too rich.

Ed Cregger

 

Cross your fingers and run a compression check.
Good luck,
MadDog

 

Black smoke is always gasoline. Blue smoke is oil. White smoke is coolant,
if liquid cooled. If water cooled, you mistakenly filled your fuel tank with
kerosene. 8^>

Ed Cregger

 

In the last sentence, substitute air for water. Doh!

Ed Cregger

 

<VVBG>