I drive an 89' Accord LX. When driving down the road when I depress the clutch and left off the gas to shift gears my car produces a small backfire. It is not a deafening backfire but it is quite annoying. It mostly occurs after the car has sat out in the sun all day. In the morning it never does it, but when I get off work at 3:00 in the afternoon it begins the backfiring. What is causing it to do this and how can it be fixed?