91 virago 1100 misfiring

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!

 

<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.

 

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

 

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

 

NKG BPES 7

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

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????

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.

SNIP

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

Harry

Now about that missing fire...?

 

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

 

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

 

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