Hi, sir Ferman. I'm a fan of Top Gear, and I really like your column. My car is a 1996 Toyota Corolla XL. I have been experiencing what appears to be the sound of backfire coming from my muffler. My mechanic told me it must be the carburetor, which needs cleaning. He told me to purchase a carburetor cleaner, but unfortunately the sound didn't go away. What could be the reason? Thanks a lot in advance.
Hi, Caesar. What you're encountering is popularly known as an "afterfire," meaning that combustion is occurring in the exhaust system instead of the combustion chamber. A backfire is when combustion occurs in the intake system. However, nowadays the term is usually used for both instances.
In your case, there are a few things that may be causing this. If the carburetor is improperly set or out of tune, and your engine is either running lean or too rich, then either one will cause an exhaust system backfiring. Too lean and the air-fuel mixture doesn't ignite, dumping the unburnt mixture into the exhaust system, where it will ignite there due to the high temperatures. Too rich and there will be an excess of unburnt mixture, which again will get ignited in the exhaust system.
You might also want to check your ignition system's timing, because if it’s too advance or too retarded, it will also induce a backfire in the exhaust when combined with the above factors.
Another thing to look into are your spark plug cables or high tension wires. If there is an electrical leak in one or more of them, and they are cross-firing (the energy from one cable jumps to another), then a spark plug could be lighting up when it isn't supposed to. If this occurs when an exhaust valve is open and there is enough unburnt gases in the exhaust, it can cause an exhaust backfire as well.
I'm actually more interested in the circumstances before you started encountering the exhaust backfire. Did anything change in the car that may have affected the carburetor's tuning, such as an exhaust system change or some other modification? Knowing that will help track down the cause much more quickly.
By the way, your mechanic wasn't wrong in recommending a carburetor cleaner, as clogged pathways in the carburetor can lead to tuning changes in the air-fuel mixture. It’s also a quick and cheap way to determine if the problem is there or elsewhere.
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