Yamaha XS650 backfire

Additional detail might be helpful.

Intermittent blue flame? How intermittent?

Does the left header pipe heat up at a different rate than the right?

Any chance you could've burned an exhaust valve?

Lots of other possibilities ...

AQ



"The monkey and the baboon was playing 7-up.
The monkey won the money but he scared to pick it up.
The monkey stumbled, mama.
The baboon fell.
The monkey grab the money and he run like hell!"
- from "Dirty Motherfuyer", Roosevelt Sykes, around 1935

 

So just shut off the fuel, drain the tank, whatever. It's what I do on
my old Honda and I *never* have a gummed carb problem, despite sometimes
leaving the bike for six months.

 

The fuel is automatically shut off when the vacuum-operated petcock
closes or the electric fuel pump stops. The float bowls are
inaccessible without removing the gas tank.

Draining the gas tank is impossible without unbolting it, and draining
the tank just invites internal rust.

What I do is add a few ounces of gasoline stabilizer to a full tank of
gas and idle the egine for a while to get it into the carbs.

It's a lot cooler where you live. It gets up to 100 degrees F every
day during the summer around here. My carbs may gum up in a matter of
weeks if I don't have time to ride.

 

That's a problem, sure enough.

That's certainly different weather conditions. I'd do something like fit
an inline manual tap or similar. Some form of shut-off. Then just run
the engine until it's starved of fuel from the carb float bowls.

FI engines are a different kettle of fish of course.

 

Hmmm. I'd say about once every 1-3 seconds. Not regular though. flame..
flame.......flame.flame....flame...........flame.

I don't know but I will check this afternoon.

I guess anything's possible. What actions on my part would lead to a burned
exhaust valve? Would a compression test tell me if I've got a bad valve?

 

Over-tightened (or neglected, which would produce the same effect) valve
clearances.

I still reckon it's mixture, though.

 

The irregularity might be consistent with idle-jet problems.

A truly cheap diagnostic. Just reach down and touch (before they get
too hot).

I likely should have said "warped", 'tho "burned" can be descriptive.
In a nutshell, abuse (i.e. over-revving a hot, hot engine, etc).

Very likely. Got a tester handy?

AQ

"The monkey and the baboon was playing 7-up.
The monkey won the money but he scared to pick it up.
The monkey stumbled, mama.
The baboon fell.
The monkey grab the money and he run like hell!"
- from "Dirty Motherfuyer", Roosevelt Sykes, around 1935

 

Okay, left header pipe heats up WAY slower than right one. Threw a tach on
the lead, it's getting plenty of spark. Removed and sealed off port on
intake manifold where fuel petcock vacuum is taken off in case it's leaking.
No change.

Compression is good ~115lb.

So I bit the bullet and removed, disassembled and cleaned the carbs. Tested
the diaphrams. I put new larger main jets in to compensate for my
aftermarket exhaust, I bought a few different sizes and will have to play
around with them to find the best one. I also was very pleased to note I
have the notched (adjustable) needles. These carbs did not seem in the
least bit dirty to me, and I have worked on some bad ones. On re-assembly I
happened to hold the completed assembly up to the light and huh? What's
this? I could see a fair chunk of light through the right (working) carb,
but the left side looks like a tight seal. Hmmm, that is very well likely
the problem all along. I rough-synched the carbs with drill bits, but by
then it was getting dark so I didn't get it all back together. Will report
back.

 

Looks that way. Where's the light shining through? The carb/mounting
rubber joint or the mounting rubber/cylinder joint, or somewhere else?

If you want a weak mixture, having an ill-fitting carb will provide
it ;-)

 

What I meant was that the idle adjust was set on the left (non-performing)
carb so that the throttle plate was 100% closed, whereas the right (idles
great) carb was adjusted via the synch screw to have a gap maybe 1 or even
1.5mm, allowing light end to end through the carb body. They were terribly
out of synch, with the idle set so one carb worked and one not so much.

I know all about ill-fitting carbs, cracked and/or leaking boots, poorly
sealing air boxes... problems like this seem to seek me out.

Dave

 

Easy fix, though.

 

If the compression ratio is only 8:1, cranking compression should be
about 117 psi, assuming that you live somewhere near sea level.

If the CR is 9:1, you should get about 133 psi after about five
compression cycles.

 

Stock XS650 CR is closer to 9 than 8 to one, IIRC.

Anyway, it looks like he's already traced the problem.

 

Ah, if only it were so easy. So, I put it all back together. New main
jets, new pilot jets. Squirted the be-jesus out of the carb passages with
cleaner. Bench-synched with drill bits. Fired it up last night and....
same symptoms. The bike will start and run with only the right plug
connected but not only the left (yes, I grounded a spare plug in the unused
plug lead to the block). With both plugs connected, right exhaust is warm,
sound is rhythmic bu-bu-bu-bu-bu-bu-bu-bu-bu-bu from right muffler. Left
exhaust is cool, backfires occur when I start it, last only about 30 seconds
or so, at least after 30 seconds I could no longer see blue flame in the
muffler. Sound is kind of a puh------puh-puh------puh----puh-puh-puh from
left muffler. Bike will NOT run on left cylinder only without throttle
opened up some. I can't help thinking that it's just not getting fuel
through the pilot circuit. Maybe I'm going to have to disassemble the left
carb enough to soak the carb body, I dunno.

So, I'm thinking to completely disassemble the carbs from each other and the
chunk of angle iron holding them together, switch them side for side, and
see if the problem moves. I did find out my fuel petcock leaks in the ON
position, so tonight I'll seal off the vacuum take-off on the left manifold
and see if that helps... I could test the compression in the right cylinder
too, but I'll wager that's about the same as the left... let's face it guys
it's a 29 year old bike with the original rings, I might not get 9:1 any
more.

Thanks for all the insight so far.

 

If the compression ratio is only 8:1, cranking compression should be
about 117 psi, assuming that you live somewhere near sea level.

If the CR is 9:1, you should get about 133 psi after about five
compression cycles.

I live at 1,100 feet, so 9:1 shouldn't be too far off of my 115.

 

Have you drilled out the EPA antitamper plugs in the sprues on top of
the carburetors, just forward of the diaphragm cap?

Refer to this drawing. Item #47 is a different part, but the line
points near the sprue I'm talking about. The anti-tamper plug is
probably made of brass and it is fairly thick.

Take a small drill and carefully drill a pilot hole in the anti-
tamper, then screw a small sheet metal screw into the pilot hole and
pull the screw and anti-tamper plug out with a pair of pliers.

Then, before removing the idle mixture screw, turn it in gently until
it just stops.

This may be as little as 1/4 to 1/2 a turn with the large pilot 42.5
pilot jets.

Write down the total number of turns or fractions of turns for each
idle mixture screw.

Then remove the idle mixture screws, being sure to save the small
spring, tiny washer, and tiny rubber o-ring.

Segregate the idle mixture screws, springs, washers, and o-rings as a
set, dedicated to each carburetor.

Then you can squirt Berryman B12 or any clear aerosol carb cleaner
handy into the hole the idle mixture screws came out of.

When you squirt carb cleaner into the #44 pilot air jet in the intake
of each carb, the cleaner should squirt freely out of the hole that
the idle screw came out of.

It should also squirt freely out of the #16 pilot jet and also squirt
freely out of a triangular pattern of three idle transition ports just
downstream of the throttle butterflies.

I find it necessary to reinstall the idle mixture screws and hold my
finger over the pilot jet orifice to get carb cleaner to flow freely
out of the 3 transition ports
when squirting it through the pilot air jets.

When you're ready to readjust the idle mixture screws back to the
factory settings, turn them clockwise gently, until they just stop,
then turn them back counterclockwise to the same number of turns that
you counted when you removed them.

http://webservices.motorsportdealers.com/parts/partImages/YAM/2/02/0758/0028.Gif

16 JET, PILOT (#42.5)

17 PLUG (rubber? It keeps fuel near the pilot jet orifice)

44 JET, AIR PILOT (#135.0)

 

The geometric compression ratio of 9:1 does not change with altitude,
but
the ambient air pressure does change, it's about 3.6% lower on a
Standard Day which is 59 degrees F, sea level, dry air.

If you're curious about what I'm talking about, google for "ICAO
Standard Atmosphere Table".

Air density at 1100 feet and 59 degrees is about 2.9% lower than at
sea level.

Air density at higher temperature than the standard 59 degrees F has
even more effect than altitude change.

If you're doing your compression tests in 90 degree ambient air
temperature you can expect the readings to be lower than what's
specified in a factory manual that assumes you're doing the test in
Tokyo on a nice cool day.

 

I doubt it's a compression problem. As you appear to have surmised,
engine wear is usually consistent on both/all pots unless something
fairly nasty has happened.

Fuel tap? Doubt it, but you could try running the thing with the tap
on the Prime setting, just to be sure.

The best thing to do is probably to have the carbs ultrasonically
cleaned. It's not expensive and it works - I've had it done.
Ultrasound dislodges all the little bits of crap and varnish that
soaking won't get (nor will the proprietary snake oil). Or get a
decent set of used carbs, clag those on and see what happens. I had to
do that too, a few years ago, when I was restoring an old Honda CB500T
whose carbs (as it turned out) were really beyond hope.

By the way, the 500T wasn't worth restoring, but I just felt perverse.
I wound up with a fine example of an utterly dreadful motorcycle.

 

I went to a car show once and there was a restored Pacer.
Your statement made me laugh and remember that.

Clay

 

I've seen pictures of the Pacer. Even thought it looked quite good, in
a distinctive sort of way. What was so bad about it?

<Thinks>

Hang on, while I switch off my binary filter.