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Backfire Causes and Cures

This is a discussion on Backfire Causes and Cures within the ATV Tech Tips forums, part of the Can-Am Tech Area category; Backfires in Exhaust. Note : It is normal for many high performance exhaust systems to moderately backfire or pop when the throttle is closed from ...


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Old 06-27-2005, 06:10 PM
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Backfires in Exhaust.

Note:
It is normal for many high performance exhaust systems to moderately backfire or pop when the throttle is closed from mid-to-high rpm. In fact, one should expect a well-tuned high performance engine to "pop" and "crackle" when the throttle is closed at high rpm. The popping is a result of the air/fuel mixture becoming very lean when the throttle is closed and the engine is rotating well above idle speed. It is also necessary that the exhaust system have rather open mufflers for this to occur.

Why This (normally) Happens:

1) When the throttle valve is in the idle position, fuel does not flow out of the main system (needle, needle jet, main jet). Fuel is only delivered to the engine by the pilot (idle) system.

2) The combined effect of the closed throttle and elevated engine rpm is to create a fairly strong vacuum in the intake manifold. This vacuum, in turn, causes a high air flow rate through the small gap formed by the throttle valve and carburetor throat.

3) Under these conditions the pilot (idle) system cannot deliver enough fuel to create a normal, combustible air/fuel ratio. The mixture becomes too lean to burn reliably in the combustion chamber. It gets sent into the exhaust system unburned and collects there.

4) When the odd firing of the lean mixture does occur, it is sent, still burning, into the exhaust system where it sometimes ignites the raw mixture that has collected ---- the exhaust then pops or backfires

Other possible causes:

Air Leaks:

Any source of fresh air into the exhaust system can create or worsen the conditions that bring about exhaust popping. The most common entry point is the junction of the header pipes and mufflers. Even a small air leak can dramatically increase the intensity or likelihood of exhaust system popping. A high temperature silicone sealant, as can be found in many auto parts stores, may be used to seal the pipe/muffler junction.

Lean Carburetion:

While exhaust system popping may be considered normal, it is certainly made worse by an overly lean idle circuit. Be sure that your carburetor's pilot jet is the correct size and that the idle air mixture screw is correctly adjusted before looking for other causes of popping. The procedure for adjusting the pilot circuit is covered in the Tuning Manual.

Ignition: If exhaust system popping is very loud, irregular and accompanied by loss of power, then you should suspect that the ignition system is not performing as it should. If, for some reason, the ignition sometimes fires at the wrong time, then exhaust popping can become very energetic (loud). Look for failing high tension leads (plug wires), failing ignition coil(s) and especially switches or connectors as possible causes.
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  #2  
Old 07-23-2007, 12:08 PM
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On my stock OL 800 when i let off the throttle, hot or cold motor it will very slightly pop every once and a while. The valves were adjusted about 20hrs ago, maybe i should check them again,is it running to lean? what should I do?
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Old 06-22-2011, 07:17 PM
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muzzy exhaust

I have a 2011 renegade 800. just put a muzzy exhaust on it..it backfires and at real slow speeds feels like it is hesitating. Is this normal?
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Old 06-22-2011, 07:40 PM
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Quote: Originally Posted by bbdram4u View Post
I have a 2011 renegade 800. just put a muzzy exhaust on it..it backfires and at real slow speeds feels like it is hesitating. Is this normal?
Sounds like it needs a fuel tune with a PC III or PC V. That would clear it up.
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Old 06-23-2011, 03:46 AM
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muzzy exhaust

easy 2 do?
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Old 06-23-2011, 10:26 AM
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