Charon
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I am sure someone will correct me if I am wrong. You do not have a "backfire". You have an "afterfire". A backfire is when the flame makes its way back through the intake side of the system into the intake manifold and carburetor. However, the "afterfire" is usually, though incorrectly, called a "backfire".
On closed throttle deceleration the engine vacuum goes quite high. The throttle slide is closed, so little air passes (which is why the vacuum is high). The throttle slide being closed, the needle pretty much closes off the main jet. The high vacuum pulls excess fuel into the manifold via the pilot (idle) circuit, and with little air the mixture is probably rich. This rich mixture may or may not burn properly in the engine. If it doesn't, it makes its way into the hot exhaust pipe. Once the engine finally gets a burnable mixture and fires, the flame from the exhaust valve can ignite the fuel in the exhaust, and you get the afterfire. Making things worse, the fuel in the float bowl tends to move slightly forward on deceleration (or lots forward if you brake hard). The passages to the pilot are in the front of the float bowl, so the mixture tends to get richer yet. It seems odd that the mixture will go rich on deceleration, since Suzuki jets the pilot lean to meet EPA regulations, but I think that is what happens. I may be wrong, but I think the simplest remedy is to turn up the idle, which will reduce the engine vacuum on overrun by not allowing the throttle butterfly to close quite as far.
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