I have a 2001 Arctic Cat 500 4X4 that has backfired since the day I brought it home from the showroom. The backfiring happens after it is at full throttle, and you let off. I have tried adjusting the needle in the carb, but no real improvement was obtained. I recently bought a new jet and K&N filter kit (which did improve the trottle response and HP) but it still backfires about 50% of the time when I let off the throttle. Do I need to adjust the mixture screw more? any other ideas?

 

Normal for the lean condition created by high rpm and a closed throttle.

 

Youre right about the mixture screw..go about 1/4 turn richer (ccw or unscrewing), and the poping should stop. Itll probably start easier, too.

 

Another thing you can try besides richening the idle mix screw is to set you exhaust valves on the loose side of the spec.- I believe its .007-.009 so set it to .009 and that should help it out a bit too if the valves were a bit tight from the factory.

 

Cool thanks I will try that!

 

Bear is right. Mine has done it from day one. I dont think it will hurt anything, will it? I guess this is pointed at Bear[img]i/expressions/face-icon-small-smile.gif[/img]

 

make sure the little things like your breather hose out of your gas tank is not pluged or pinced? if thats good valves.

 

This comes from the Mukuni website, and explains why it pops on sudden decell.



4: Backfires in Exhaust


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.

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 backfiring. 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 backfiring.

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.