Yamaha XS650 backfire

I started up my 1980 XS650G last night after fitting new mufflers (long long
overdue). By the time I finished it was pretty dark in my garage, so I
couldn't help but notice that the left cylinder was backfiring... I could
see the blue flame inside the left muffler. My old rusty pipes were so
friggin' loud that the backfire may have been present last year, I'm just
not sure. What might cause this? I swapped the plugs between L & R
cylinders, but the backfire stayed on the left (damn!). The bike seems to
rev up okay, could it possibly just be an adjustment of my air mixture
screw? I'll throw a tachometer on the left plug lead tonight just to make
sure I'm getting spark.

Dave

 

Maybe your carbs are getting a little plugged up inside. You might try
adding a few ounces of Berryman B12 Choke and Carburetor Cleaner to a
full tank of gas and going for a slow ride to clean out the idle
circuits.

There are two kinds of backfires related to dirty carbs. One is the
loud *bang!* associated with high fuel levels caused by dirty float
valves.

The other kind of backfire is a wimpy *piffle-piffle-snap!* on
deceleration, That's caused by dirty idle jets and passages.

 

I'll try carb cleaner, I've got a bottle someplace, I recall I ran some
through last year or the year before. The other cylinder runs fine. I
think I'm going to end up pulling these carbs apart anyways at some point.
I would think that a plugged air jet might cause backfiring at idle, but I
really haven't worked on carbs enough to know...

 

If you're fairly sure that the air jet has gotten plugged up, a shot
of aerosol carb cleaner would easily clean it out.

I buy both types, B12 in aerosol form and B12 in the handy liquid form
for adding to the gas tank.

Many Japanese ignition systems will fire both spark plugs at the same
time, using one sensor or one set of points to trigger the ignition.

When you have a waste spark firing, you'll hear rumbles in the exhaust
pipe on deceleration and you'll also get backfires when the carbs are
dirty because unburned mixture will build up in the exhaust pipe over
two or three exhaust strokes.

 

But the XS650 *doesn't*.

 

Incorrect mixture. Incorrect timing (I am not sure if the 1980 model XS
had points or electronic ignition, and I cba to check right now).

 

Well, take the time to *look it up*, you silly ****.

BTW, I'm a cracker. Eat me.

 

Don't need to. I've rebuilt two.

But carry on posting erroneous information if you want to.

 

The XS650G (1980) has electronic ignition, timing cannot be adjusted.

As for incorrect mixture, I'll try re-setting the air mixture screw on the
problem side for best idle although last time I used the "dead cylinder"
method I had quite a time just keeping the bike from stalling. I added some
carb cleaner and went for a spin last night, the bike runs just fine at
higher than idle rpm's.

Thanks.

Dave

 

Yes, and it only has *one* timing sensor and a 360 degree crankshaft,
so
both coils have to fire at the same time. One of the sparks is wasted
because the exhaust valve on that cylinder is open.

If there is a buildup of unburned mixture in the exhaust pipe on that
side, you'll hear a bang! out of that cylinder when the plug fires.

I advise you not to waste a lot of time debating with "The Turd That
Won't Flush". He loves to debate and argue instead of helping riders
find solutions to their problems.

Before you know it, he'll have you apologizing to him and thanking him
for giving you a ration of shit.

OK, just remember that you can adjust the idle mixture screw all day
and it won't change a thing if the idle jets and air jets and passages
are plugged up with gum and varnish from evaporated gasoline.

 

Yeah, I'm just hoping. I don't really want to pull & clean the carbs if I
don't have to.

 

It'll be the carb, then.

 

If they're really gummed, then all the magic snake oil in the world
won't help. Ultrasound cleaning works wonders. I used it on a carb for
an old Honda 125 single last year: brilliant.

Generally, carbs only get bunged up if they've been left for a long,
long time (like well over a year). In a hot climate, obviously, the fuel
inside them will evaporate and gum them up in less time than that.

If the bike hasn't been standing for ages, then it's not going to be
gummed carbs anyway, but just bad adjustment.

 

Deep down I already knew this. I'll try to fit it in on the weekend. It's
just such a pain in the you know where to adjust and balance them. I've got
to take it out for some highway rides and start the painful guess 'n check
process of re-jetting for my new exhaust, so the carbs will be coming off
anyways, one more time won't kill me.

I've got the manual, it's available online for free. Thanks for your
advice.

Dave

 

Listen to this man. He speaks the truth. Lots of people put their faith
in all these magic potions and cures, but ultimately, fixing something
properly is the only sure way.

 

Do NOT listen to The Turd That Won't Flush. He's more interested in
proving himself to be "right" (by his own rules) that he cannot accept
that there are quick fixes to simple problems by using inexpensive
chemicals designed to cure such problems.

I go through this business of plugged up carburetors *every* spring
when I take the motorcycles out of winter storage, and the idle jets
are seldom as plugged up
as Mark Olson suggests, but the engine stalls, won't idle, or
backfires.

 

Steer clear of Dynojet kits. Dynojet is trying to enhance their bank
account by keeping speed secrets *secret*.

I recommend going to www.factorypro.com and studying what they have to
say about jetting CV carburetors. The owner of Factory Pro is *not*
trying to keep secrets, he's telling how to rejet a carburetor using
stock Mikuni or Keihin parts.

Your carbs are probably made by Hitachi, but Mikuni main jets should
fit just fine.

The problem with re-jetting is that most home mechanics do not
understand how the size #'s relate to orifice area.

Since I've explained this concept dozens of times in this NG, I won't
go into it again, but you can google up "round jet" and find the
information.

 

Mikuni, actually.

You really are preaching from a position of ignorance here.

 

Which raises the question: why don't you just drain the carbs before
storing the bike?

 

On Jun 10, 2:04 pm, (The Older
Gentleman)

Inaccessability of drain screws behind fairings, gas tanks, etc.

Easier to add B12 to gasoline and ride the carbs clean.