Virago Backfire

Q
	I own a new '98 Yamaha Virago 1100.
	When I decided to get something to scoot

around on, the Virago had what I was looking for. The only thing I did not like was that I could not hear the exhaust. I recently installed a set of Cobra slip-on mufflers and now I like the looks and the sound.
    The bike seems to perform exactly as it did with the stock mufflers. However, every now and then, I get a single backfire through the exhaust when shifting gears. This typically happens at lower speeds when the engine is hot.
    It's possible that this happened with the stock exhaust and I just never heard it. The bike runs great otherwise. Could it be die new mufflers, valve clearance, timing or carbs?
    I read your advice to a reader who complained of his bike backfiring when decelerating and you attributed it to lean carburetor settings due to E.P.A. regulations. Could this be a similar problem?

A
	I can't think of one new motorcycle that will
	take an aftcrmarket exhaust without

backfiring. Most already do with the stock system—you just can't hear it. If you think it runs well now, wait till you ride it after a good, sharp tune-up.
Done right, here's how it goes:
    Valve clearance must be set at the loose end of the specified range. When clearance becomes less than minimum spec, valve overlap increases, leading to backfires. The 1100 Virago, like most bike engines, tightens up its valve-stem-to-rocker-arm clearance as miles accrue. Expect a little ticking with the wider adjustment.
    The carbs have to be synchronized to a gnat's eyelash. Then, the vacuum line that drives the air-injection system will need to be pinched off so an accurate reading can be taken with an exhaust gas analyzer. Older 1100s needed to have their low-speed mixture screws backed out to a factory specification of three percent carbon monoxide at idle. The newer '98 model should be adjusted to four percent CO, a little richer. If backfiring persists, that figure can be increased to five percent CO, however fuel mileage may drop at that setting.
    The actual factory mixture-screw setting before adjustment will be one mixture screw completely shut off and die other at a half-turn out! Ballpark will be about 4 to 4½ turns out by the time you're done fixing it. I can always tell a Virago that has not been dialed in on an E.G.A.—it sounds like a big single at idle rather than a V-twin regardless of the exhaust system used.