From: Rider Magazine (1-800-678-2279) - September 1997
Shop Talk - Tech Q & A - Andrew Macdonald - Page 71
Q:I recently purchased a new '97 Kawasaki Vulcan 750,
which now has a little over 1,000 miles on it. The problem is that it backfires constantly
when closing the throttle. I don't mean popping, I mean loud cracks, like shotgun blasts.
This is both frustrating and very embarrassing.
The dealer has given me several reasons why it does this, but I just
can't believe that it's normal. First they said it was set too rich, and drilled out plugs
and adjusted the carbs. This helped a lot but it still backfires, mainly when it's been
ridden at high speed and I close the throttle to exit a highway. Then the Vulcan will
backfire loudly, three or four times.
Now they tell me this is normal for V-twins. I just can't accept this.
Do I need to re-jet, change exhaust systems or trade motorcycles? Thank you.
A:All Kawasaki Vulcan models are notorious for
backfiring, particularly when exiting the freeway when their engines are crackling hot.
Curing them of this habit is a piece of cake - you simply shut down their air pump system.
But I have to tell you up front that doing so may be illegal in your neck of the woods
because you're tampering with an emissions control device.
In my home state of Arizona we've had annual mandatory vehicle
emissions testing in the Phoenix and Tucson areas since the late '70s. My state couldn't
care less what you do with your motorcycle as long as it's tailpipe emissions don't exceed
the prescribed limits, which are both generous as well as realistic. Should your bike fail
it's yearly test, the Arizona D.O.T. won't issue you a license plate tag until it's
Because of this, motorcycle mechanics in Southern Arizona are super
savvy in the use of exhaust gas analyzer. I was state certified as an emissions inspector
in 1982 and before long I flat couldn't work without an E.G.A., not for pass/fail
information but for diagnosis and fine tuning.
Due to the high degree of training my homeboys receive in the use of
the E.G.A., it's with great satisfaction that I note this column (which gets a ton of
mail) doesn't receive any reports of fuel delivery or driveability problems from riders in
Phoenix or Tucson. None, zero, zip!
Many years back we fingered the air pump on Kawi Vulcans as the culprit
behind those Howitzer-sized backfires. The pump is a simple affair that blows cold air
down into the exhaust tract of each cylinder head via two large rubber hoses running to
reed petal assemblies.
With the engine cold, trace the hoses to each cylinder's reed assembly.
You'll find it somewhere in the area of the exhaust pipe on the cylinder head. Each hose
slides over a spigot that's part of an outer plate held to the cylinder head with two six
millimeter bolts. Unbolt this outer plate, take a small flat-bladed screwdriver and pry
out the reed petal plate from the head. Use a ball peen hammer and gently tap the reeds as
flat as possible. Fill the back of the reed plate flush with high-temperature silicone
sealer. Reassemble everything and do the same to the other cylinder head's reed assembly.
Allow the sealer to set up overnight.
For added insurance, find the small vacuum line that drives the air
pump. Normally the pump is under the gas tank or seat and the vacuum line comes off one of
the intake manifolds. Take a large ball bearing and stuff it into the smaller vacuum line
to plug it.
With the air pump system rendered inoperable and the low-speed mixture
fine-tuned to about 2 1/2 percent CO (carbon monoxide) the Kawi Vulcan is one sweet ride.
Shop Talk - Tech Q & A