Was riding on Saturday and sometimes after an extended run of speeds about 65 - 75 km (45 MPH) the machine would backfire on deceleration.
Any ideas what causes that??
Also had another incident on the same ride.
For no apparent reason the machine suddenly turned left and went off the trail.
Luckily it was into tall grass and not a tree or rock.
When I got it out and looked underneath, the cotter pin and nut holding the steering link into the wheel were gone and it popped out so the wheel was just freewheeling. We had some copper wire so I able to get it together and back on the trail OK.
I've got the feeling that it has been gone for awhile or never there.

 

Mine backfired a couple times like that the last time I was out riding. I don't know what caused it though. We were riding at 6-7000 ft elevation.

 

Mine does that too, they seembe jetted quite rich so when u decel from hi speed the unburned gas in the exhaut is ignited by heat in the exhaust pipe. I moved my metering rod down in the carb but it still does it ocaissionaly. my buddies always say it smells fuel rich to follow me.

 

Mine also backfires occasionally when I let off the throttle. The strange thing is that it seems to do it more often the lower in altitude I go, which is the opposite of what I would expect if it was from a too rich condition. I'm jetted for 9000' and notice the backfiring more often at 6000'-8000' and almost never at 9000'-13000'.

 

Its always caused by lean. You can prove this to yourself, by pulling on the choke, just before throttle let off. the backfire will go away. If it were caused by rich...it would be worse.
Look for air leaks, in the intake or exhaust, and if everything is good, then turn the mixture screw about 1/4 turn richer(out), and itll go away.

 

My brother in laws' Honda does this occasionally and he's tried everything to stop it. His only explanation is that a little unburned fuel gets into a very hot exhaust once in a while and is exaggerated by the fact that one of his baffles is burned through.

 

This is from the Mikuni Carb, website

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.


Copyright (C) 2002, Mikuni American Corporation All Right Reserved.

 

Thanks, that's excellent information. I also notice that if you ease off the throttle instead of suddenly releasing it, it will not backfire. The hard thing about tuning for Colorado riding is that you are constantly changing elevations. Sometimes as much as 5000' difference on a trail.

 

Thanks Folks and thanks hondabuster, great info, puts me at ease.

 

I had a Kodiak 450 for two years and it backfired on deceleration....only when 45 degrees or lower. Never an issue, just an annoyance [img]i/expressions/face-icon-small-smile.gif[/img]
Buck