Carb Head Games..arrg help

A friend of mine bought a bike similar to mine a 1992 VT600 Shadow vlx.
It had been sitting for a year and wouldn't start so I pulled the carbs
(Keihin CV), tore them down and cleaned them.

I removed the Welsh plug and pilot screws so I could clean the idle
ciruit well.

I checked the diaphragms on top of the pistons for tears.

Checked the float settings.

Cleaned everything and blew it out with compressed air just like my
mommy taught me.

Then carefully bolted the stuff back on, replaced the plugs, fuel filter.

The bike started right up on the choke and when I took it off the choke
it idle ok, not great but ok. Once it warmed up it got better.

When I attempted to blip the throttle though, the engine would suddenly
die. It would start right up again and again this time more slowly I
opened the thottle up and it would stumble slowly and die again.

I played with different settings on the pilot screw to no avail.

A couple of times when I let it idle for some times it would break out
of idle and let me rev it up.

But something is definetly wrong.

At first I was thinking it was some kind of vacuum leak but I haven't
been able to find one. Everything seems tight (boots).

I took the air filter assembly off and started the bike and again with
the choke out it would run like a scalded dog. When pushed in it would
idle but would die if you even cracked the throttle.

In this phase I did something different. While idling I put my hand over
the intake of the carb (with no filter attached) and it idled much
smoother. In otherwords it liked that. Take my hand it away and it would
falter.

Right now my suspicions are:

Floats may not be set right. I may have had too much of the weight of
the float on the valve.

I might have a weak flow of gas from the petcock. It may be starved for gas.

It might not be able to draw out gas from the main jet for some reason
and goes lean when the butterfly is cracked because of excess air.

I don't know if any of you have ever had this kind of experience before
but man this has me stumped.

I didn't replace any parts when I tore down the carbs because they seem
ok. The bike has around 36K miles and has good compression.

Any insight would be greatly appreciated.

 

You haven't cleaned out the carbs properly. Varnshed fuel is still
partially blocking them - probably the idle jet passages.

It's only the millionth time this sort of problem has been posted.

 

The idle circuit is plugged up. Probably the three transition ports
under the butterfly are plugged.

Air for the idle circuit comes through the pilot air jet. The inlet for
the pilot air jet is probably one of two small holes in the intake bell
of the carb. You may even see a small brass jet right there in the
bell. The other hole will be the starter air inlet. It will be on the
same side as the starter plunger.

If the pilot air jet isn't right there, it may be inside the carb,
underneath the rubber diaphragm.

If the pilot air jet is right there in the bell mouth, you can spray
aerosol carb cleaner very slowly through the jet while the engine is
idling. You don't want to flood the carbs with carb cleaner, it will
run down the intake tracts and maybe dilute your oil. Not good.

The air that comes through the pilot air jet joins up with the gasoline
that comes through the idle jet. Then the fuel/air mix goes to the idle
mixture screw you adjusted.

The fuel/air mixture also goes to three transition ports arranged in a
triangular pattern under the throttle butterfly. As the butterfly
opens, fuel/air mixture flows through the
transition ports, allowing the engine speed to increase.

Another way to clean out the idle passages without removing the carbs
is to squirt a 50/50 mixture of Berryman's B-12 and gasoline directly
into the float bowls through the fuel hose and leave it sit for half
and hour. Then start the engine and run it on B-12/gas mixture.

The pilot screw has only slight effect on the fuel/air mixture. It's
only a trimmer setting, but amateur mechanics are able to screw this
setting up by misunderstanding which way to go with it.

Amateur mechanics turn the pilot screw counterclockwise, expecting the
engine speed to increase. It slows down instead because they make the
mixture too rich. Then they
have to adjust the master idle knob to increase the engine speed.

Later on, when the alcohol additives in their gasoline finally do
manage to clean out the passages, the engine idles really fast and the
RPM hangs up when they blip the throttle.

The correct strategy when adjusting idle mixture is to turn the idle
mixture screw CLOCKWISE until the engine RPM increases. Then turn the
master idle screw down, and turn the pilot screw clockwise some more
until the RPM falters and the engine gets rough. Then turn the idle
mixture screw COUNTERCLOCKWISE a small amount so the engine runs smooth
at the specified RPM.

Crud coming through the idle mixture passages.

Those carburetors don't have a choke. They have a starting enrichener
device which is a small valve in a bypass passage. The bypass passage
is too small to even have a venturi in it. The starting enrichener
sucks gas out of the float bowl just like sucking milk through a straw.
When you open the throttle butterfly, vacuum downstream of the
butterfly drops off, the enrichener can't suck gasoline out of the
float bowls.

If the idle passages are plugged, the engine can't get gas from
anywhere, it dies.

Sure. You increased the vacuum, made the carb suck harder, maybe it got
the gas it need past the jet needle. That's a very small variable
orifice when the diaphragm can't lift the needle out of the needle jet,
it's maybe several thousandths of a square inch. The idle jet orifice
is much larger than that, but if the passages are all plugged up, very
little gas gets to the engine.

Well, go ahead and check the dimension if you want. The tang of the
float should just be resting on the float valve pin. The float level is
from the bottom of the float to the aluminum gasket surface on the carb
body. If the adjustment specified is, say 14 millimeters, plus or minus
1 millimeter, if you set the float at 15 millimeters, that will shut
the fuel off earlier. If you set the float at 13 mm, the fuel will be
shut off later, the engine may run a little "soggy" because it's too
rich. With a 13mm float setting, it's easier for the engine to suck gas
out of the float bowl, it doesn't have to lift it so high.

It's easy enough to pull the fuel hose off the carb and let it run into
a container to see if you have good flow.

Bzzzt! The main jet could be completely plugged, or you could be
standing there with the main jet in your hand and the engine wouldn't
exhibit the symptoms you describe because the engine doesn't run off
the main jets at the small throttle openings you describe. Once the
diaphragm does lift the needle out of the needle jet, flow through the
main jet just begins, but 75% of the fuel the engine needs still has to
come through the idle circuit.

It's just inescapable. The idle circuit is the heart and soul of a CV
carburetor. Get to know and love your idle circuit. Keep it as clean as
possible by running carb cleaner through the gasoline several times a
year.

Oh, gawsh. No, none of us have *ever* had that kind of problem with our
CV carbs before. ;-)

Clogged idle jets and passages are the most common problem we talk
about, except for dead batteries. And clogged idle jets and passages
inevitably lead to dead batteries,
because newbies can't get their motorbike started.

 

Ck bless your heart for your detailed post. The carb 101 refresher
course was appreciated.

I was aware of the significance of the idle circuit and it has one of
the smallest jets in the carb. The interior galleries in the carb body
are also very small.

What I usually do is use Berryman's B-12 spray with the red tube
attached to the nozzle and pressure feed solvent through the opening
where the idle jet goes and then chase it with compressed air. While
doing this I observe (with eye protection becuase that crap stings) the
transition ports you mentioned, to see if the expell solvent (which it
did). Then I did the same thing through the idle screw port, solvent
air. I guess I didn't "rinse and repeat enough" because I believe you
are correct that there may be some debris still in there.

I have been thinking of separating the carbs and putting them in an
ultrasonic cleaner with some simple green. A car mechanic that I know
does this with old British car carbs that he works on and he uses it in
conjuntion with solvent cleaning. He says the ultrasound really cleans
the interior cavities.

I don't mind pulling them again as long as I can make them stick. I just
don't want to get stuck in some "Ground Hog day" where I do this over
and over again.

Thanks again

CK

 

<snip>

Ultrasound cleaning is becoming something of a cottage industry here in
the UK, because the carb clogging problem is so serious. One of the
classic magazines actually ran a feature on it a year or so ago.

 

Make/jury rig a way to blow compressed air (& a little B-12) back from
the throttle plate area thru the idle circuit & back into the float
chamber. It's a lot easier than taking everything apart again. There's
a good chance you can flush the crap out of the idle circuit & get it
into the car bowl where it can be drained out. I fixed my Yamaha
Virago that way.

 

man, i know why its like taking the carbs of that bike! i must have
done it only about 1000 times on my 1100 virago when i was in korea. I
ended up putting the Vance and Hines pipes on, they had a really great
sound without being over the top. At idle they're quite mellow. I
went with the dial-a-jet system to add a bit more fuel and changed the
stock bug eye air pods for a free flowing unit that looked a bit like a
supercharger. You can get the carbs perfect on this bike if you fiddle
with them enough, so that it starts on the first revolution every time
and idles really smooth.

Getting the carbs off never seemed that hard, but putting them back in
is a bugger. to do it without removing knuckle skin, spray a little
wd-40 or whatever into the boots, then put the first carb in. this is
usually pretty easy. to get the second one in all you need is a mini
tie down ratchet. put it around the cylinder and over the carb...
ratchet it until carb pops in. done