'96 Virago 1100 Special, backfire on deceleration

This occurs mostly when exiting the highway, going down the ramp. First
application of throttle after the backfire causes it to "bog out", tickle
it a little bit and all is well again.

I bought this machine a few years ago and it has always behaved like this.
I see that there are a number of knowledgeable people in this group so
maybe someone can offer an opinion as to whether this just a qiurk of the
bike that has to be lived with, or if there is a solution for it.

On another note, is it my imagination or has anyone else noticed decreased
milage and harder starting with this ethanol crap they're putting in the
gas these days?

 

Zaphod,

How are things out in the Galaxy these days?

There are two kinds of backfiring that can (or will) occur on
deceleration:

There is the putt-putt-putt type. Small backfires but many of them.
These can occur when the mixture is too lean to ignite in the combustion
chamber until after a few intake strokes and then you get a flash of the
raw fuel in the exhaust.

The other type is less frequent, but more severe explosions in the
exhaust. So maybe only one or two explosion per deceleration instead of
putt-putt-putt. These occur when the mixture (on trailing throttle) is
too rich.

Since the problem is happening only on trailing throttle, you can be
fairly certain that the culprit is in the pilot circuit of the carb(s).
Because the pilot circuit is (usually) a fuel passageway, when it gets
gummed up (partially occluded) it generally causes a lean condition. By
the way, this is the most common circuit to get gummed up in a carb due
to the small size. The other symptom you will usually get with this
condition is that the engine will have a "lean stumble" when cruising at
small throttle openings. It will feel like it is surging ever so
slightly and not running quite as smoothly as might be expected. At
larger throttle openings it will run fine because the pilot is a smaller
contributor to the total fuel delivered.

The other possibility is that someone has played around with the pilot
jet adjustment screws (probably to ameliorate the above situation) and
have now over-enriched the setting. The bike will run well when
accelerating and also when cruising when set that way, you just get the
dreaded deceleration backfiring. Also fuel mileage will be sub-optimal
but you may not notice.

The only way to set the pilot screws correctly (assuming the carbs are
not gunked up) is with an exhaust gas analyzer or a colortune. These
tools will both allow you to set the optimum fuel mixture at varying
throttle conditions. The EGA is better but much more $$. A colortune
can be yours for about $50.

Either that or just set the pilots too rich and then progressively back
off until you no longer get the big bang backfires.

 

This is the 21st century, Zaphod, and I haven't seen a tickler on a
Japanese motorcycle since my 1967 250cc roadracer that Japanese Amals
with remote floats.

A tickler worked by pushing the carburetor float down, opening the
float valve and raising the fuel level so high it comes up the
emulsion tube and floods the throat of the carburetor.

That was a very crude and polluting system of starting a motorbike
engine. It had to go.

You may think that the starting enrichener valve is a tickler, though.
If you look at the CARBURETOR diagram
for the XV1100H or XV1100HC Virago on www.partsfish.com or
www.bikebandit.com, you will see that item # 21 is called "starter
set".

"HC" is the model suffix for the California model and it looks like
the "H" model had a California carburetor too.

The "starter set" is a small valve that you open manually to bypass
air around the throttle butterfly. When the valve is open, gasoline is
sucked straight out of the float bowl, and that richens up the fuel/
air mixture for starting.

This system doesn't work worth a damn if the throttle butterfly is
open even a little, so if you want to "choke" your engine after it
backfires, be sure to roll off the throttle all the way.

Back to the CARBURETOR diagram. Item
# 19 "diaphragm set" is a deceleration valve that is supposed to help
eliminate backfiring by closing off an air passage in the idle
circuit.

My Kawasaki KLR has a deceleration valve like that, but it still farts
a little on deceleration.

It's possible that one of the deceleration valves has a torn diaphragm
or is stuck, but V-twins tend to make louder backfires because of the
design of the ignition system.

Motorcycle ignition systems often fire a "waste spark" when one of the
cylinders is on the exhaust stroke, and the waste spark will light off
any residual fuel in the cylinder or in the exhaust system.

Then there is the possibility that your idle jets and idle passages
are getting plugged up with gum and varnish.

I recommend running 3 or 4 ounces of Berryman B-12 Chemtool Choke and
Carburetor Cleaner in a full tank of gasoline through the engine to
clean out all the gum and varnish.

Riding the motorcyle at low speeds until the carbs are clean usually
does the trick, but I have had occasions when I had to remove the
carbs for stripping and thorough cleaning.

Stubborn cases require the EPA anti-tamper plug (item # 26 to be
drilled out so you can remove the idle mixture screws (item # 9),
underneath the plugs.

The plugs will be thick brass and you will have to be careful drilling
a pilot hole in the plug without running your drill into the head of
the idle mixture screw.

On the plus side, the EPA plugs are on top of the carburetors so you
shouldn't have to remove the carbs from the engine to do this routine
job.

Screw a small sheetmetal screw into each pilot hole and then pull the
screw back out with a pair of pliers and the plug will come with it.

The next step is to carefully turn the idle mixture screws CLOCKWISE
until they just bottom out in the holes. Count the number of full
turns and fractions of a turn.

Write the information down so you won't lose it.

Since this carburetor has #40 pilot jets ( item # 15 ), the
idle mixture screws won't be turned very far out, it will probably be
around 1/2 a turn, maybe less.

Once you've written the number down, go ahead and unscrew the idle
mixture screw, saving the tiny spring, washer, and tiny rubber o-ring.
Then you can squirt aerosol carburetor cleaner down the hole the idle
mixture screw came out of.

Then re-install the idle screw with its spring, washer and o-ring and
turn it in until it just stops turning and back it out the same number
of turns that you wrote down.

Then do the other idle mixture screw the same way.

Really gummed up carburetors require removal and thorough cleaning
with an aerosol cleaner. Berryman makes B-12 in an aerosol can, as
does GumOut and STP.

Any fuel system cleaner that contains "petroleum distillate" is for
fuel injectors, it won't do the job you want on carburetors.

If you have the carbs off the engine and the float bowls and
diapharagm covers off and the vacuum slides removed, you can spray
aerosol carburetor cleaner through the idle jet and it should come out
the single hole that is controlled by the idle mixture screw.

The same idle mixture passage connects with the three transition ports
that are covered and uncovered by the throttle butterflies, and the
passage gets air from the pilot air jet which is probably in the
intake mouth of the carburetor.

Sometimes the pilot air jet is underneath the rubber vacuum diaphragm,
though.

Spraying aerosol carb cleaner into the pilot air jet should cause carb
cleaner to come out of the idle jet, the idle mixture port, and the
transition ports.

If you don't see carb cleaner coming out of all the ports, cover the
ports that have good flow with your fingers, and then keep spraying
carb cleaner until you do get flow out of the blocked ports.

Where amateur mechanics get into trouble with adjusting the idle
screws on their carbs is that they think that the idle speed should
keep increasing as they turn the idle screws counterclockwise.

They go too far and the idle speed actually slows down because the
mixture gets too rich. So they adjust the master idle knob to get the
engine to idle at the right speed.

This opens the throttle butterflies slightly, uncovering the three
transition ports.

The transition ports are for acceleration when the motorcycle is
moving with small amounts of throttle.

So the amateur mechanic gets those transition ports open at idle and
the mixture is far too rich, the engine won't idle down.

When the amateur mechanic turns the master idle knob down, the engine
will either run or stall, confusing the heck out of him.

The trick of adjusting idle mixture is to turn the idle mixture screws
back in a fraction of a turn at a time, and when the idle speed
increases, turn the master idle knob down a little.

Then turn the idle mixture screws in a little more, and turn the
master idle knob down again.

Like I said before, you're going to wind up with the idle screws
turned out about 1/2 a turn (or less) from fully closed with those
large # 40 idle jets.

 

Yep, that describes it pretty well.

No "stumble" (yet) so maybe it isn't too badly messed up. I read most of
the postings back thru Dec and noted a number of posts regarding the
addition of solvents to clean the carbs. Might be a thought?

Could be. The bike was repo'd from the previous owner and I have no idea
whether he messed with the carbs or not. I was getting about 50 mpg (same
as my wife's XV 535) with regular unleaded, but it seems to be worse with
this 15% ethanol crap. I first noticed *that* on a bit of a jaunt around
the country a while back- I fueled up in TN, didn't notice that it was the
ethanol mix, and was rather dismayed to find I was in need of fuel
substantially before I had expected to be...and with no idea where or when
I would see another gas station.

When my wife and I are on a trip together I carry four one-liter bottles
of extra fuel because her 535 has a smaller tank, but I didn't have them
on this trip since I was alone and figured the room and weight was better
utilized for other supplies. I wasn't too happy at the thought of pushing
that machine OR leaving it on the side of the road in the TN hill country
while I searched for more fuel. Luck was on my side that time and I topped
out on a hill with a gas station at the bottom just about the time I was
reaching the end of the reserve.

OK, I'll clean the carbs first and if that doesn't cure it I'll spring for
one of the analyzers.

I *was* having the vague notion that perhaps there was a valve
adjustment/timing problem that was dumping excess fuel into the
cylinder(s) and then into the exhaust, or on the edge of running too rich
rather than too lean.

Thanks for your input. I've made a note of your suggestions and as soon as
I get a break in the weather I'll check it out. It's so cold the machine
won't start, too cold to be working on it outside anyway and I don't have
enough free floor-space to bring it in the house...lol.

 

Ethanol is an oxygen bearing fuel. It should make your engine run a
little cleaner during the summer.

Running cleaner with ethanol would be the same as leaning up your
jetting slightly.

Are you talking about carrying gasoline in plastic 1-liter bottles?
You're a rolling fire bomb.

I recommend obtaining some kind of auxillary gas tank made of steel,
with a shut off valve. Then you could attach it securely to a luggage
rack or sissy bar and run a fuel hose to a T-fitting in the fuel line
going to the carburetor.

There is something of truth in that speculation.

Valves that are too tight won't seal right and you won't get the
compression you need.

Tight intake valves allow reverse airflow through the carburetors as
the piston rises on the compression stroke.

A carburetor is a dumb device, it doesn't know which way the air is
flowing, backwards or forwards.

The carburetor picks up a shot of gasoline when the air is flowing
backwards, then gets another shot if gas when the air is flowing the
right way.

When you read your spark plugs, you might see the indication that the
engine is running very rich, but you have no power because there's no
compression.

OTOH, your intake valves may fail to seat properly because the valve
guide oil seals are worn out. The engine sucks oil down the valve
stems and it burns onto the intake valves because they are a lot
cooler than the exhaust valves.

The valve heads will have a lot of hard black carbon burned onto them.
Chevron's Techron additive is supposed to keep carbon from building up
in the intake tract and on the valves.

You can do a "shock treatment" to remove carbon by adding about 8
ounces of Techron Concentrate to a full tank of gasoline

A real biker would put his extra furniture out in the barn and move
his motorsickle into the house...

The last Japanese motorcycle manufacturer to use a choke on their
carburetors was Kawasaki. I think they gave up on chokes in the late
1970's.

Now the "choke" knob is connected to a bypass starting enrichener
valve in the side of each carburetor. It's like a tiny carburetor
built into the side of the big carburetor.

It provides a very rich fuel air mixture for starting. It may be as
rich as 1 part fuel for every part of air going through the bypass.

Riders have a lot of trouble starting their engines because they don't
understand how the bypass starting enrichener works. It's a bypass
around the throttle that sucks gas right out of the float bowl.

If you twist the throttle grip while you're trying to start the
engine. the butterflies open and vacuum decreases and the starting
enricheners don't work.

If you're having problems starting your engine, *don't twist the
throttle grip*, find the master idle knob that adjusts idle speed and
turn it all the way counterclockwise so the butterflies are completely
closed.

Pull the "choke" knob out and start the engine. If you have to play
with the throttle once the engine fires up, that's fine, you can nurse
the throttle until the engine warms up and then re-adjust the master
idle knob so the engine idles at the correct RPM.

If the engine just won't idle with the "choke" knob pushed all the way
in, your idle jets are plugged up.

Trying to ride the motorcycle with the choke knob pulled out and the
idle jets plugged up is dangerous, you can't control the engine speed
under those conditions, the engine will idle too fast or stall as you
change the throttle position.

 

Eek. I've got a couple of one-litre Sigg metal flasks (google for them)
that I've used for fuel. Well, they're designed for it.

The OP makes no mention of plastic so I trust he's used something
decent.

 

sigg.ch? Their amazingly shitty website doesn't indicate any gas
carrying ability. They don't seem to be vented. Of course I could be
misreading their amazingly shitty website.

And did I mention their website is amazingly shitty? Steered me right
away from their product, it did.

On the other hand, I have carried gas in a 2-liter Pepsi bottle when the
gas station had no fuel cans. It was only from the station to the bike
though, and even by that time it had pressurised pretty well. That's
when I discovered just how incredibly short-legged my NT650 was.

-gc

--

Yes, and after 12 weeks of rehab, you might even stay off the stuff.

 

Yes, their website is rather uninformative. No, their bottles are not
vented. This is, however an advantage if the fuel bottle is stored
_inside_ your backpack, which is the intended/original use: backpacking
fuel. It's similarly an advantage in many motorcycle applications.

MSR makes similar bottles (more common in the 'States than Sigg):
http://mountainsafetyresearch.com/stoves/bottles.asp

The two biggest advantages that the Sigg/MSR-style fuel bottles have
over something like a soda-bottle are:
-they are approved fuel containers (while most here don't care, it can
make the difference between the station allowing you to fill or not, if
you have to re-fill on the road)
-the cap design allows you to open them against the pressure built up,
while the o-ring (designed to hold fuel!) will seal it well, which may
not have as reliable a seal.

This thread has _lots_ of info about people fuel-carrying setups:
http://www.advrider.com/forums/showthread.php?t=187146&highlight=extra+fu
el

<end thread hijack>

--
Later,
John



'indiana' is a 'nolnn' and 'hoosier' is a 'solkk'. Indiana doesn't solkk.

 

Ah, OK. I didn't see a fuel bottle at all on the Sigg site, despite
spending nearly 20 minutes looking.

Now these can be quite useful, as you say, and I plan to get some. All
the small plastic fuel cans around here are such odd shapes that they
don't fit well in my Givi bags.

Thanks!

-gc

 

An interesting idea. But if that were the case wouldn't the engine run
just as rich at open throttle as on trailing? So his back firing and
poor engine running would be at other than trailing throttle, no?

It just seems far more likely that the problem is the carbs are out of
whack.

 

My best guess is that the idle circuits are all gummed up, but I
wanted to answer the OP's theory about valve clearances or valve
timing.

 

Which one? The former or the latter. If the former (lean), then by all
means try to clean out the pilot jets (as well as the rest of the carb).
If the latter (rich) try cranking down (CW) a very small amount on the
pilot screw(s). You can turn them in gently fully CW until they seat
and count the turns to see where they are to begin and then adjust them
in approximately 1/16th turns from the starting position to see if you
can find a happy place. Make your testing when the engine is fully
warmed up and actually ride the bike around a while in each position
before deciding if it's better or worse.

Or no stumble because it may be the "too rich" condition which doesn't
have that symptom. If so, you may be a tweak of the pilot jet screws
from nirvana.

 

Yeah, I found them...well pictures without descriptions of them... but I
suspect that was because I knew what I was looking for. It's a shame,
because Sigg does make some really high-quality stuff. I've been
impressed with what I've seen.

Be sure to check out the advrider thread, and look at the pics of the
guy who straps 4 of those things (the largest size, I think) to the
outside of his cases (front and bottom of each case)! That's a whole lot
of extra fuel!

There's also a couple of other bottles/containers mentioned there, some
of which comes with pour spouts (a problem with the MSR-style bottle).

--
Later,
John



'indiana' is a 'nolnn' and 'hoosier' is a 'solkk'. Indiana doesn't solkk.

 

The latter, when I was printing out your previous notes, I noticed one
paragraph that I had read too quickly, which seems pretty dead on as a
misadjustment of the pilot to too rich. I'll probably find that the plug
has already been drilled out.

I'm thinking that this is most probably correct.

 

No, I may do the occasional dumb thing on a bike (not as many as when
I was younger and thought I was immortal), but that isn't one of them. I
use lightweight aluminum bottles designed for backpackers to carry the
white gas for Coleman stoves.

It's not so much the furniture as the fermenters and brewing equipment. I
suppose I could do without beer for the winter...oops, I'm sorry, I had a
temporary bout of insanity there.

 

I meant "tickling" the throttle- just cracking it open two or three times.
Usually, after the backfire, a "normal" application of throttle makes it
act like it's flooded, cracking it open a couple of times straightens it
out.

Good info on the carbs, more detailed than the service manual. I've saved
it for future reference. Thanks.

 

Have you ever stopped to smell the exhaust pipe for raw gasoline
fumes? An engine that is running really rich will 8-stroke. The
cylinders will fire on every other compression stroke, making a sort
of "br-r-r-ing" or stuttering sound.

8-stroking could be caused by a sticky float valve that raises the
fuel level too high, by jet needles that have been raised up too high
by a previous owner, or idle screws that have been tweaked too far
open. or by oversized mainjets.

If you look for the EPA anti-tamper plugs that I mentioned, they would
be on top of the carburetors above the butterflies, and find they are
missing, a previous owner probably opened the idle mixture screws too
far and did the other things that cause an engine to run too rich.

But the usual problem is that the idle jets and passages get plugged
up with gum and varnish over a period of time and the owner discovers
his engine will start and idle but won't accept any throttle and dies.

That problem sneaks up on the owner over a period of time. He first
starts noticing the backfiring on deceleration or stalling as he comes
to a stop.

There are two types of backfires commonly experienced when the idle
jets and idle passages get clogged up. One is a "piffle-piffle-SNAP!"
on deceleration, when the throttle butterflies close, and the engine
can't get enough fuel to run smoothly.

The other backfire is a "fart-BANG!" when a lot of fuel accumulates in
the exhaust system and is lit off by the waste spark that fires with
the exhaust valve open during the overlap period of the cylinder
that's not producing power at the moment.

Riders also notice the sound of backfiring through the carburetors
when the engine is cold. This is caused by weak valve springs or
carbon deposits on the valves resulting from leaky valve guide oil
seals.