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*.


--
BMW K1100LT  Ducati 750SS  Honda CB400F & XBR500  Triumph Street Triple
Suzuki TS250ER
If you don't know what you're doing, don't do it. And RTFM.
chateau dot murray at idnet dot com