91 virago 1100 misfiring

[OldYamaha]

rec.motorcycles.tech, alt.motorcycles.yamaha

My 91 virago 1100 is misfiring at low end.

Actually the only time I can sense that it is missing is at idle.

Background Info

The misfire DOES NOT happen when starting very cool, or overnight
cold. The bike starts and idles good at cold temps for 20-40 seconds
then starts to miss(so far it has started without the choke down to 27
degrees here in Iowa this fall)

When I put an induction timing light on the rear plug wire, there is
an intermittent, but consistant lack of fire... maybe 1 or 2 misses
every 4 seconds, enough that the plug will carbon up, start to pop,
then fail completely within 50-100 miles of in town driving 35- 40
MPH.
I gets bad even quicker with a NKG bpes7, so I have a 5 on the rear.

I bought it this way.

The previous owner took the stock pipes off, and put MAC slashcuts on,
and then "he said" that is when the problem started. He took the carbs
off, and "cleaned" them, and so did I and found the coasting enrichner
diaphram with more than a few brass shavings and cleaned that mess
out,before I tried the timing light.

I have an 81 virago 750, and "borrowed" a coil from that to see if the
"misfire" happened on that coil too... it did. (the coil works fine on
the 750 before and after, so probably not the coil.

On that basis, I bought a used CDI, and I still have the same problem.

Is there any other component on the bike that might cause this,
besides the wiring between the CDI, and the coil?

Harry

[Steve and Karin Gordon]

Post by OldYamaha
rec.motorcycles.tech, alt.motorcycles.yamaha
My 91 virago 1100 is misfiring at low end.
Actually the only time I can sense that it is missing is at idle.
Background Info
The misfire DOES NOT happen when starting very cool, or overnight
cold. The bike starts and idles good at cold temps for 20-40 seconds
then starts to miss(so far it has started without the choke down to 27
degrees here in Iowa this fall)
When I put an induction timing light on the rear plug wire, there is
an intermittent, but consistant lack of fire... maybe 1 or 2 misses
every 4 seconds, enough that the plug will carbon up, start to pop,
then fail completely within 50-100 miles of in town driving 35- 40
MPH.
I gets bad even quicker with a NKG bpes7, so I have a 5 on the rear.
I bought it this way.
The previous owner took the stock pipes off, and put MAC slashcuts on,
and then "he said" that is when the problem started. He took the carbs
off, and "cleaned" them, and so did I and found the coasting enrichner
diaphram with more than a few brass shavings and cleaned that mess
out,before I tried the timing light.
I have an 81 virago 750, and "borrowed" a coil from that to see if the
"misfire" happened on that coil too... it did. (the coil works fine on
the 750 before and after, so probably not the coil.
On that basis, I bought a used CDI, and I still have the same problem.
Is there any other component on the bike that might cause this,
besides the wiring between the CDI, and the coil?
Harry

Sounds like a simpe question but are you sure your wire is not failing?
Other than that I'm at a loss.

Good luck. BTW, I have an '83 Virago 750 that I just put back on the road -
original owner. Love Them Viragos!

[Kim Neubert]

Post by OldYamaha
rec.motorcycles.tech, alt.motorcycles.yamaha
My 91 virago 1100 is misfiring at low end.
Actually the only time I can sense that it is missing is at idle.
Background Info
The misfire DOES NOT happen when starting very cool, or overnight
cold. The bike starts and idles good at cold temps for 20-40 seconds
then starts to miss(so far it has started without the choke down to 27
degrees here in Iowa this fall)
When I put an induction timing light on the rear plug wire, there is
an intermittent, but consistant lack of fire... maybe 1 or 2 misses
every 4 seconds, enough that the plug will carbon up, start to pop,
then fail completely within 50-100 miles of in town driving 35- 40
MPH.
I gets bad even quicker with a NKG bpes7, so I have a 5 on the rear.
I bought it this way.
The previous owner took the stock pipes off, and put MAC slashcuts on,
and then "he said" that is when the problem started. He took the carbs
off, and "cleaned" them, and so did I and found the coasting enrichner
diaphram with more than a few brass shavings and cleaned that mess
out,before I tried the timing light.
I have an 81 virago 750, and "borrowed" a coil from that to see if the
"misfire" happened on that coil too... it did. (the coil works fine on
the 750 before and after, so probably not the coil.
On that basis, I bought a used CDI, and I still have the same problem.
Is there any other component on the bike that might cause this,
besides the wiring between the CDI, and the coil?
Harry

Sounds like the mixture is too rich and the plug is simply fouling.

Noob

[OldYamaha]

Post by Kim Neubert

Post by OldYamaha
rec.motorcycles.tech, alt.motorcycles.yamaha
My 91 virago 1100 is misfiring at low end.
Actually the only time I can sense that it is missing is at idle.
Background Info
When I put an induction timing light on the rear plug wire, there is
an intermittent, but consistant lack of fire... maybe 1 or 2 misses
every 4 seconds, enough that the plug will carbon up, start to pop,
then fail completely within 50-100 miles of in town driving 35- 40
MPH.
I gets bad even quicker with a NKG bpes7, so I have a 5 on the rear.
I bought it this way.
I have an 81 virago 750, and "borrowed" a coil from that to see if the
"misfire" happened on that coil too... it did. (the coil works fine on
the 750 before and after, so probably not the coil.
On that basis, I bought a used CDI, and I still have the same problem.
Is there any other component on the bike that might cause this,
besides the wiring between the CDI, and the coil?
Harry

Sounds like the mixture is too rich and the plug is simply fouling.
Noob

THe misfire is there on every brand new plug i've tried. That is why I
believe it is electrical. Once I get that figured, then I will
concentrate on the over rich fuel mixture.

I do think that when the other guy put the pipes on, that he set the
idle jet mixture richer to compensate for the pipes, but the idle jets
are stock, 122, 125... what ever the book calls for.

I just need to figure the misfire 1st.

Harry

Harry

[krusty kritter]

This post might be inappropriate. Click to display it.

[OldYamaha]

Post by krusty kritter

Post by OldYamaha

When I put an induction timing light on the rear plug wire, there is an

intermittent, but consistant lack of fire... maybe 1 or 2 misses every 4
seconds, enough that the plug will carbon up, start to pop, then fail
completely within 50-100 miles of in town driving 35- 40 MPH.

SNIP

Post by krusty kritter
If the engine is starving for idle mixture, you'll hear a fart-BANG! out the
exhaust pipe when you roll off the throttle...
If the idle mixture is way too rich, the engine will 8-stroke, firing on every
other compression stroke. The sound of the engine will be toot-toot toot-toot
toot-toot until you open the throttle wide enough to clear out the excess
mixture...

SNIP

Post by krusty kritter
What plug is supposed to be in there, a BPES-8 or a 9?

NKG BPES 7

Post by krusty kritter
You are flirting with a potential hole in your piston crown, Harry. Running a
really hot spark plug like that in order to overcome an overly rich mixture can
result in pinging, and piston crown melting and even detonation that breaks the
upper ring lands...

Oh yeah, I just checked the valves too, they were set a little tight I
could not get a .003 to go on either intake, or even on the exhausts,
so using the go/no method the exhausts now "won't go" a .007, snug at
.006, loose at .005
and that method also for the intakes... I am a machinist for the GOV,
cannons for desert storm 1 amd 2

Post by krusty kritter
Listen for a tinkling sound like a loose valve when the engine heats up. That's
pinging. If you ride your bike with continuous pinging you *will* burn a hole
in your piston. Don't ask me how I know that...

Post by OldYamaha
I do think that when the other guy put the pipes on, that he set the idle jet

mixture richer to compensate for the pipes, but the idle jets are stock, 122,
125... what ever the book calls for.
Those numbers would be the *main jets* not the idle jets...
CV carbs would have idle jets with numbers more like 37.5, 40, or 42.5 on
them...

Uh DUUUurrr, ya caught me there, LOL I ment to say Main, because I
could not find the idle jet screws. Anybody know where I can get a
good book or at least good pictures for my carbs? My Clymer manual is
so generic, that it is almost, almost non helpful????

Post by krusty kritter
You have no way of knowing whether the previous owner has slyly reamed out one
of the jets with some tiny drill, unless you have a set of precision pin gauges
to measure the orifices with...
Find out what you've got in there, and ask the parts guy at the Yamahahaha
$tealer$hip about what is supposed to be in there, buy brand new jets and
install them...
Another thing to do is find the idle mixture screws. They may be on top of the
carbs, or underneath them, downstream of the throttle butterflies...

Supposed to be on top... someone else told me that. But when I had
them off, I did not see anything there... but was not looking for them
there, looking for them underneath like my 81 virago 750.

Post by krusty kritter
If the previous owner knew enough to jigger with other parts of the carbs, he
probably drilled out the EPA anti-tamper plugs and turned the idle mixture
screws out a few turns...
If you can see that he has drilled the plugs out, you can depend on the fact
that he tweaked the idle mixture screws...
When you find those idle mixture screws, turn them all the way clockwise,
counting the turns, until they just lightly seat. Write down the number of
turns and fractions of turns for reference...
Then adjust your idle mixture in relation to idle speed so you get the fastest
idle for the least amount of throttle butterfly opening...
The general rule rule of thumb for idle mixture screws vs. idle jet sizing is
that if the engine runs best (highest idle speed for the least amount of
throttle butterfly opening) with the idle mixture screws turned out less than
one turn, the idle jets are too big...
And, if the engine runs best with the idle screws turned out more than 3 turns,
the idle jets are too small...
After 3 full turns out, the screws aren't doing anything...
I'm not trying to tell you to experiment with swapping a bunch of idle jets in
and out, I'm just telling you what range of adjustment to expect on those idle
mixture screws...
I recently worked on my own Yamaha's carbs (FZR-1000), and found three carbs
had the idle mixture screws set less than half a turn open, and the fourth carb
had the screw set 2.5 turns open...

SNIP

Now that is good info, and I never really understood the principle...
Thanks Very Much...

Harry

Now about that missing fire...?

[krusty kritter]

Oh yeah, I just checked the valves too, they were set a little tight I could

not get a .003 to go on either intake,

I would be very suspicious of my valve guide oil seals if I saw that situation.
Worn out and leaky valve guide oil seals will cause wet carbon buildup on the
valves, preventing them from sealing properly and the engine will spit back
through the carbs...

Uh DUUUurrr, ya caught me there, LOL I ment to say Main, because I could not

find the idle jet screws.

The idle jets themselves will usually be right next to the main jets, but some
carbs have rubber plugs concealing them. Yamaha uses short idle jets that
probably aren't cross-drilled for emulsion air, so there's little need to
actually remove a Yamaha idle jet for cleaning, just shoot carb cleaner through
it...

The EPA plugs concealing the idle mixture screws will be downstream of the
throttle butterflies, on top of the carbs, or underneath, forward of the float
bowl (in relation to the intake tract runner). The EPA plugs on my Suzukis were
made of aluminum, but the plugs on my Yamaha were brass...

Now about that missing fire...?

You should receive the long explanation I e-mailed you...



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[OldYamaha]

Post by OldYamaha

Oh yeah, I just checked the valves too, they were set a little tight I could

not get a .003 to go on either intake,
I would be very suspicious of my valve guide oil seals if I saw that situation.
Worn out and leaky valve guide oil seals will cause wet carbon buildup on the
valves, preventing them from sealing properly and the engine will spit back
through the carbs...

Uh DUUUurrr, ya caught me there, LOL I ment to say Main, because I could not

find the idle jet screws.
The idle jets themselves will usually be right next to the main jets, but some
carbs have rubber plugs concealing them. Yamaha uses short idle jets that
probably aren't cross-drilled for emulsion air, so there's little need to
actually remove a Yamaha idle jet for cleaning, just shoot carb cleaner through
it...
The EPA plugs concealing the idle mixture screws will be downstream of the
throttle butterflies, on top of the carbs, or underneath, forward of the float
bowl (in relation to the intake tract runner). The EPA plugs on my Suzukis were
made of aluminum, but the plugs on my Yamaha were brass...

Now about that missing fire...?

You should receive the long explanation I e-mailed you...
# * 0 * #
^

Thanks for the help Krusty...

A little more info...

1) the carbon fouling is dry powder, half will blow off with air.
2) compression check is low, only about 110 PSI, well below the book
value 156 PSI standard/128 PSI min. (have not tried to see if valves
or rings yet)
3) spark plug caps are around 4900 ohms (5k book
4) coil Primary is 4.0 ohms(4.2 book)
5) coil secondary is 13K ohms (13.2k book)

I did think about a 12 volt car coil the one I looked at is for a
plymoth acclaim Primary is 1.2 ohm, Secondary is 11.8k ohm.

I have a few hours Tomorrow to look deeper at the problem.

On the plugs, if I blast them w/ spark plug cleaner, I can reuse them
3-4 times before I have to pitch them.

Harry

[krusty kritter]

Post by OldYamaha
1) the carbon fouling is dry powder, half will blow off with air.

That would indicate a rich fuel/air mix, then, which would be a carb problem...

Post by OldYamaha
2) compression check is low, only about 110 PSI, well below the book value 156

PSI standard/128 PSI min. (have not tried to see if valves
or rings yet)

Get those valves set to spec before proceeding with the compression test...

I suspect those tight valves that you said only had .003 in. clearance. With
the valves that tight COLD, the clearance will disappear as soon as the engine
comes up to operating temperature, and then the piston will force air backwards
through the carbs, causing spitback and misfiring...

Now, here's a little-known fact about carburetors: they do not know which way
the air is going through them, nor do they know that the air already picked up
a load of fuel out of the float bowl the first time it passed through the
venturi...

So, when the air gets pushed backwards through the carbs, it picks up more
fuel, and when that same air gets sucked back toward the cylinder, it picks up
more fuel, so the mixture becomes exceedingly rich...

If you don't have an airbox or air filters on your bike, you might even notice
a fog of gasoline droplets *outside* the
intake bells of your carbs...

Some of the excess fuel might settle out in your airbox, though, if you have
one. Does the airbox smell like gasoline?

Do your compression check with the engine warmed up to operating temperature,
and hold the throttle wide open so the engine can get air to compress...

Compression testers have quite a bit of volume inside the bourdon tube and the
rubber hose and adapters that connect the tester to the spark plug hole...

Since that volume is fairly large, it takes four or five compression strokes to
build up maximum pressure in the gauge. The pressure should rise smoothly, in
even amounts for each turn...

If you don't get pressure within the specified range, squirt a few drops of oil
into the low cylinder to seal the rings and turn the engine over several times
before you hook up the compression tester...

Too much oil in the cylinder will blow up your compression tester. Don't ask
how I know that...

If the compression rises to specified levels with oil in the cylinder, it's
worn out rings causing the low compression reading. If the pressure doesn't
rise, it's a valve problem...

I had a fuel starvation problem on one bike, and I was running very tight inlet
valve clearances of slightly less than .003, thinking that my valves would
loosen up over time from camshaft and rocker arm wear...

I was wrong, the valves got tighter...

An engine that is starving for fuel doesn't care what it burns, it will suck
oil out of the crankcase past the rings,
up through positive crankcase ventilation system hoses, or down from the top
end through the valve guide oil seals, and it will even burn metal off the hot
valves and the valve seats, so the valves sink into the head, taking up more of
the valve clearance...

When I pulled the valves out of the engine for inspection, I found an
incredible amount of oily black carbon baked onto the heads of the intake
valves, and the valves were burned and pitted badly.

They looked like mice had been chewing on them. I had to buy 16 new valves at
$15 apiece...

My friend who drag raced and road raced motorcycles told me that my valve guide
oil seals were worn out, so I replaced all 16 of them too...




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[OH-]

"OldYamaha" <***@mchsi.com> wrote in news:***@posting.google.com...
<snip>

Post by OldYamaha
The previous owner took the stock pipes off, and put MAC slashcuts on,
and then "he said" that is when the problem started.

<snip>

Makes perfect sense to me. An exhaust system that is not in
tune with the rest of the engine can cause both rich and
lean fuel mixture as well as power surges and flat spots.

Original exhaust systems are designed to give good
rideability and/or power while meeting noise standards.

Unfortunately, aftermarket systems, especially for cruisers,
are mostly made to look good and cause sound sensations
of debatable quality.

Forget about "back pressure", the problem with bad
exhaust systems is that they cause pressure pulses that
wreak havoc with the gas exchange in the cylinders.

[LittleJohn]

Post by OldYamaha
My 91 virago 1100 is misfiring at low end.
Actually the only time I can sense that it is missing is at idle.
<snip> the plug will carbon up, start to pop,
then fail completely within 50-100 miles of in town driving 35- 40
MPH.

It's not the exhaust and not the ignition system. Racers run with straight
tubes and induction analysers only work when there is current flow. When
the plug is carboned up and the system can't fire through it, the analyser
shows nothing.

Kim is correct. The carbs are mucked up and it's running too rich. A
correctly tuned Virago needs full choke to start anytime the engine temp
drops below 40 F. Find someone with an exhaust analyser to nail down the
fact that it is running rich and then go get a book that explains
asssembly/disassembly of those carbs. Better yet, just take it to someone
who knows Viragos and tell him that the carbs were taken apart by newbees.
It's an all to common problem. He'll know what to do.

LittleJohn
Madison, AL

[OldYamaha]

Post by OldYamaha
rec.motorcycles.tech, alt.motorcycles.yamaha
My 91 virago 1100 is misfiring at low end.
Actually the only time I can sense that it is missing is at idle.
Background Info
The misfire DOES NOT happen when starting very cool, or overnight
cold. The bike starts and idles good at cold temps for 20-40 seconds
then starts to miss(so far it has started without the choke down to 27
degrees here in Iowa this fall)
When I put an induction timing light on the rear plug wire, there is
an intermittent, but consistant lack of fire... maybe 1 or 2 misses
every 4 seconds,

SNIP>

Post by OldYamaha
Harry

But not when starting cold?????!!!!!!

CLICK!!!!!!!!
AAAAAAAHHHHHHHHHHaaaaaaaaaaa!!!
as light turns on inside thick skull.

carburation!!

Will post more as soon as discovery week continues. LOL

Harry

[krusty kritter]

Post by OldYamaha
CLICK!!!!!!!!
AAAAAAAHHHHHHHHHHaaaaaaaaaaa!!!
as light turns on inside thick skull.
carburation!!

Carburation *and* excessively tight intake valves, working against you...

# * 0 * #
^

[OldYamaha]

Post by krusty kritter

Post by OldYamaha
CLICK!!!!!!!!
AAAAAAAHHHHHHHHHHaaaaaaaaaaa!!!
as light turns on inside thick skull.
carburation!!

Carburation *and* excessively tight intake valves, working against you...
# * 0 * #
^

WELL, WELL, WEll.
Krusty you are a god @#*) genius, or a GOD blessed genius, and I don't
know you but "WITH HEARLDS SHOUTING" you were correct. The front carb
had a setting of ... 1/4 turn, while the rear carb had a setting of
... 4 1/2 turns.

I set the front to 1 full, and the back to 1 1/2 turns, and the
misfire disappeared. BUT the screw for the rear is very tight to turn.
At first I thought it was bottomed out(closed), but with some light
screwdriver work, opening and closing it 1/4 turn it freed up some. I
will look closer this winter. It was almost 80 here in IA, sooo, I
went for a ride.

THANK-You.

Harry

[krusty kritter]

I set the front to 1 full, and the back to 1 1/2 turns, and the misfire

disappeared.

Glad to hear that you are making progress...

BUT the screw for the rear is very tight to turn.
At first I thought it was bottomed out(closed), but with some light

screwdriver work, opening and closing it 1/4 turn it freed up some. I will look
closer this winter.

When water gets mixed with gasoline containing alcohol,
calcium carbonate forms, and that plugs up jets and jams up screws. If you find
a white, powdery substance in the hole where the idle screw came out of, that's
what it is...

Alternatively, if you find some brown stuff that looks like old gummy shellac,
that's just the residue from old gasoline that has evaporated...





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[Bub Bubble]

Hi guys. I am a viragoer from South America. This thread is amazing. So much carburation knowledge, so finely explained. I wander what happened to that bike that started running better. The owner didn't mention if he also changed the plugs looking to use the standard for the bike bp7es. I guess then the pilot jet was too big. I'm high up the Andes using a 40 idle in my mikuni carbs 125 front and 122.5 back such as the manual states. Recently I tried to swap to a 122.5 front and a 120 back and the bike didn't rev all the way up to red. So I changed them back to stock. But my main jets in 40 are fouling my spark plugs even using a bp6es. I am glad I read all the good advise here. Now I know what to do and what to look and listen for when troubleshooting. 16 Years later but it's still good info for my 92 virago with 18000 miles and getting stronger. Thanks.

[g***@gmail.com]

Hi..I have a 98 virago xv1100.I had the carbs cleaned and synced.drives like a dream now..or so I thought..then it misfires again..soon as I change the plugs bpr7s..the misfire goes away.I dnt understand why it does this..before I had the carbs sorted..it would foul the plugs after a ride and when I remove them.they smell of petrol.now after the carbs have been sorted..plugs dnt smell of fuel.