Faulty Muffler?

[tastroman]

 My 2007 Daelim S-2 250 (same engine and muffler as a KYMCO Grandvista 250) developed a backfire this past spring. I never could find the source of the backfire and it has become progressively worse over time.
Now the sound coming from the muffler has changed. It has become louder and sounds different than when the backfiring began. When I take the muffler off it has a rattle in it. I could just get a new muffler but what are the chances the backfire destroyed the muffler and would therefore just ruin the new one. Or could it be the 1st sign of the failing muffler was the backfire.
The scooter has 14,000 miles on it and the muffler has no visible leak I can detect.
Thanks for your help.

[JustWantToRide]

The backfiring wasn't caused by the muffler - but it will destroy it and the engine.  Check the plug to see how bad it is, then check the ignition and fuel to air ratio, fuel filter, fuel lines, valve timing...  Backfiring it an indication of a problem - fix it and replace the muffler before you blow the engine and get stranded (or get tossed).

[axy]

The backfiring wasn't caused by the muffler - but it will destroy it and the engine. 

Why would backfire destroy THE ENGINE?

That is not true.

The first things to be checked are valves and secondary air filter.

[JustWantToRide]

Why would backfire destroy THE ENGINE?

That is not true.

The first things to be checked are valves and secondary air filter.

A backfire is caused by fuel burning without being properly confined by the cylinder and piston.  It can be caused by an exhaust valve not closing properly, unburned fuel igniting in the exhaust port or exhaust pipe, by valve timing that is way off, or even ignition timing that is way off.  Riding around on an engine that that is backfiring is riding around on an engine problem that is only going to get worse until the engine destroys itself.  A backfiring engine is at risk of completely blowing out the exhaust pipe, burning a valve, scoring the cylinder, breaking/bending a valve, or breaking a piston.  Or in short terms - destroying the engine.  There are other thing that can cause it - like an exhaust leak, or a carb that is making the fuel/air mixture too rich, a faulty spark...  But any condition that causes the engine to backfire is a condition that if run uncorrected will eventually leave you stranded.

[tastroman]

 So far I have changed the spark plug, air filter, adjusted the valves and cleaned the carburetor but the backfire persists. The muffler is toast as there is a rattle in it when I remove it. Not sure what to do from here. I have ordered a new muffler hoping that the lack of backpressure is causing the backfire so we will see. I wish I knew how to check the secondary air filter.

[axy]

A backfire is caused by fuel burning without being properly confined by the cylinder and piston.  It can be caused by an exhaust valve not closing properly, unburned fuel igniting in the exhaust port or exhaust pipe, by valve timing that is way off, or even ignition timing that is way off.  Riding around on an engine that that is backfiring is riding around on an engine problem that is only going to get worse until the engine destroys itself.  A backfiring engine is at risk of completely blowing out the exhaust pipe, burning a valve, scoring the cylinder, breaking/bending a valve, or breaking a piston.  Or in short terms - destroying the engine.  There are other thing that can cause it - like an exhaust leak, or a carb that is making the fuel/air mixture too rich, a faulty spark...  But any condition that causes the engine to backfire is a condition that if run uncorrected will eventually leave you stranded.

You have mixed up BACKFIRE with ENGINE KNOCKING. Knocking will do what you described to the engine.

BACKFIRE happens in the muffler and WILL NOT do any harm to the engine.

It does not change the fact that it should be checked and fixed. As I said, on Kymco scooters, if walves are OK, in most cases, the culprit is secondary air filter.

Here is what Kymco suggests for troubleshooting:

Backfiring at deceleration
 Lean mixture in idle system
 Improper air cut-off valve operation

(somewhat related) Misfiring during acceleration
 Faulty ignition system
 Lean mixture
 Faulty accelerating pump

[scuba1065]

How do you enrich the idle system?
 I too have a backfire that probably ruined my 10 year old muffler (or at least contributed to it's demise.) Now that I have a performance muffler, the backfire, which I do believe is exhaust detonation, is much more prominent. The air filters, spark plug... are all good. I am now wanting to turn the pilot screw which I believe will change the low idle mixture??? The question is, Is this correct and if so, which way does anyone think it should be turned?
My guess is right (tighten) to cause a more rich mixture.

[axy]

[MaryK]

AXY
Thanks for the diagram.
I filed it away until I need it.

Scoot safely

[tastroman]

The new muffler and gasket did not fix the backfire. Secondary air filter is on order.

[JustWantToRide]

When does it backfire - when the engine is first warming up, or after it's been running a while?  When decelerating or accelerating?  What did the plug look like when you replaced it - wet or dry, light or dark?  Does it make good power and run smooth except for when it backfires?  How is the seal from the engine to the carb, and the carb to the airbox?

[tastroman]

 It only backfires on deceleration. The plug looked fine when I changed it. I sealed the manifold with  high heat RTV even though the gasket looked fine. I'm thinking the secondary air filter is the culprit.
 One thing I will say is that I'm disappointed with KYMC's parts distribution. When I wanted a new muffler I was told it would take up to 3 months as KYMCO USA did not have one in stock. I'm keeping my fingers crossed that they have the secondary air filter in stock but we will see. I ordered it last Wednesday and have yet to receive a shipping notice yet. I was flirting selling the car and going with my scooter as my only means of transportation but with a 3 month lag on parts that will not be an option.   

[axy]

It only backfires on deceleration. The plug looked fine when I changed it. I sealed the manifold with  high heat RTV even though the gasket looked fine. I'm thinking the secondary air filter is the culprit.

99 % it is.

[tastroman]

 Still waiting on the secondary air filter. KYMCO USA's part distribution really stinks.

[tastroman]

 The secondary air filter finally arrived 1 1/2 months after ordering it, that's the good news. The bad news is that they sent the wrong part. I guess that would be ok if I could get someone at KYMCO USA to answer the phone or return a call but since I can't, I guess I'll just watch my scooter sit for another couple months in the garage.
 I couldn't be more disappointed with KYMCO's parts department and distribution.