Got a 08 Rancher and was out putting a few easy miles on it and noticed that it will backfire sometimes when I close the throttle and the machine is being held back by engine compression. Anyone eles experience this? What could be causing it? Is it running lean?


BeerDude

07 TRX500FM
08 TRX420FM

 

Yep, the idle circuit is lean.
A more complete answer is at the mikuni carb website, and ill paste it here,

4: 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.

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.

 

Well, the new 420's are FI so can't be jetting. Maybe an exhaust leak? Looks like I need to check the header pipe!

 

From what I've read in this and other forums, it's a common problem with the new FI 420s.
Do a search on "420" in this forum and you'll see countless threads on just this subject.
Most say once you've ridden enough it will stop. But the backfiring is definatly a lean condition.

 

i wouldnt say its a problem......it happens......get a power commander to map the fuel injection

i just loaded one of there maps and its been running great.

 

My 420's backfired twice in 6 months, no big deal. Both times I had the throttle closed while engine braking down a long, steep hill.

 

Mine did it a couple of times on the first ride, none yesterday with lots of hills.

I think it's the nature of the beast (FI, emissions settings, etc.).

I have seem some Honda techs say it's just the design and although annoying it's not a big deal. IT will go away with break-in and carbon buildup.

I think if you are in the right gear, not letting off the throttle quickly, etc, it will lessen the chances.

Also, as with some other poster's suggestions, let the FI light on your indicator go out before you start it. In other words, don't turn the key and immediately start it.

Good luck!

 

Mine has done this a few times. It usally happens after i am going for a mile or so and the let off the gas to slow down. It sounds like a shotgun, and scares the !@#$% out of me.

 

<div class="FTQUOTE"><begin quote>Originally posted by: Coursey

Mine has done this a few times. It usally happens after i am going for a mile or so and the let off the gas to slow down. It sounds like a shotgun, and scares the !@#$% out of me.</end quote></div>

lol! I thought somebody was shooting at me the first time I heard it.

 

I would have loved to have seen you guys when you thought someone was shooting at you. No offense, but I bet it was hilarious.