Misfire in single cylinder 4 stroke engine

Question

I am having Apache RTR 180.

Tech Specs:-

Displacement: 177.4 cc
Cylinder arrangement: Single
Maximum Power: 12.52 kW (17.03 bhp) @8500 rpm
Maximum Torque: 15.5 Nm @ 6500 rpm
Bore x Stroke: 62.5mm x 57.8 mm
Compression Ratio: 9.5:1
Carburetor: UCAL BS-29
Compression Ratio: 2 Valves, Single overhead cam

I am facing "Misfire". But this occurs at a specific case as follows

Rev the engine to >5K RPM, and now leave the throttle. At this stage I am hearing a "POP" sound from silencer.

However this issue is not occuring at lower Rev Ranges (<5K).

Recently I have changed the spark plug/air filter too. I have given the bike to authorized service center too.

But they told like, "Lets try by adjusting the air/fuel mixture." But that is also a failure.

My biggest question is, If the misfire is occurring, it should occur irrespective of the case right? But why it is occurring at a specific place.?

If the ignition coil is problem, then it will raise the problem irrespective of the case right?

Could someone help me, to identify/solve this issue?

Thx.

Answer 1

In my experience, you are not describing a misfire. A misfire is a lost power cycle. It would typically be evident during steady engine speeds, or especially noticeable under the load of acceleration, which is not what you described.

What you are describing is "Decel Pop". This occurs when the engine returns quickly to idle from high revs. It can be heard in all types of engines.

The sudden closing of the throttle causes lean conditions that resulting in a popping sound as the engine continues thru the 4-cycles returning to idle. If you were to slowly return the engine to idle, you probably won't hear it as much or at all.

Most stock mufflers/baffles block you from hearing this sound, unless the exhaust is damaged, modified or replaced with one that has less baffling. It is especially noticeable on engines equipped with straight pipes.

Sources;
My bike is BACK-FIRING! No, its' not, its Popping exhaust on decel.
Decel Popping Causes & Solutions