Running a tad rich

[Leonard]

But a single back fire in the exhaust when you suddenly close the throttle is normal.

I don't have any facts or theories to back me up but I respectfully disagree.   It just doesn't seem right that you would normally get an exhaust backfire when you suddenly close the throttle.  Maybe someone with more knowledge than I can argue my gut feeling.

Leonard

[gapl53]

But a single back fire in the exhaust when you suddenly close the throttle is normal.

I don't have any facts or theories to back me up but I respectfully disagree.   It just doesn't seem right that you would normally get an exhaust backfire when you suddenly close the throttle.  Maybe someone with more knowledge than I can argue my gut feeling.

Leonard

All air cooled engines will do this do to the higher heat factors, (most air cooled engines cylinder heads will normally run between 260 and 340 degrees Fahrenheit, depending on design. For reference paper ignites at 400 degrees Fahrenheit), being transfered to the overly rich mixture  which does not have enough oxygen to ignite in the combustion chamber. This rich mixture will ignite when reaching the exhaust pipe due to the fact that energy wave from the previous combustion episode has collapsed at the open end of the exhaust pipe. When this collapse occurs the energy wave will return back up the exhaust pipe bringing with it additional oxygen in the form of air. This additional oxygen mixes with the rich mixture in the pipe to so to speak lean the mixture enough to cause ignition when the next combustion event spill high heat into the pipe during exhaust.
At the other end of the engine you have the carburetor which works on the venturi principle of air passing over the passages leading to the diffrent mixture circuits and related jets which meter the fuel. This causes a lower pressure or vacuum to occur which pulls fuel into the air stream and down to the engine and into the cylinder where combustion can occur. Air has weight, in fact when they say the proper air fuel ratio is 15 to 1 the proper measurement is 15lbs of air to 1lbs of fuel. The mixture once in motion down the intake channel does not like to turn corners or stop. This is key to the fact that when you shut the throttle quickly the mixture that is between the throttle slide and the combustion chamber keeps moving which cause a vacuum behind it at the carburetor. This in turn draws a larger than normal amount of fuel into the combustion chamber causing the overly rich mixture which cannot be ignited due to the lack of the proper amount of oxygen. We are now back at the beginning where the unburnt mixture enters the exhaust pipe where the returning energy wave brings with it oxygen in the form of air from out side the pipe.
In fact if you read up on RAM tuning it will explain how these pressure waves work.
You always need to keep these in mind when tuning an engine for maximum performance.

[Leonard]

So would the backfire be more likely to occur with an overly rich mixture?  I'm still trying to wrap my mind around the normal condition.  Would a by the book tuned bike with minimal upgrades (less restrictive exhaust and K & N filter, larger main) be likely to backfire when shutting off the throttle?

REgards,
Leonard

[hutch]

On a Suzuki Savage they backfire on deceleration in the pipe due to a lean condition. To make them quit backfiring you richen them up and seal any loose exhuast to head fittings so they don't get extra air in the pipe and cause a backfire. The same goes for Harley and Triumph. My 05 Bullet backfired out the pipe when I put a KN air filter on and free flowing pipe. After putting in larger pilot and main jets the backfire went away, and I still get 80mpg.       Hutch

[gapl53]

So would the backfire be more likely to occur with an overly rich mixture?  I'm still trying to wrap my mind around the normal condition.  Would a by the book tuned bike with minimal upgrades (less restrictive exhaust and K & N filter, larger main) be likely to backfire when shutting off the throttle?

REgards,
Leonard

Well will your air cooled lawn mower pop when you suddenly close the throttle from fast to slow in a hurry after mowing for awhile. It's is caused by the same dynamics and considered perfectly normal. It is always more pronounced in a single cylinder engine, and the larger the engine or the freer flowing the exhaust the larger the pop.

[dewjantim]

My bike pops when I let off the throttle. Usually it seems that my bikes do this with a straight through exhaust or drag pipes. The bike runs really good so Im not messing with the jets anymore, it also gets fantastic gas mileage and the plug looks perfect.  I kinda like the backfiring, hehehe........Dew.

[Leonard]

So would the backfire be more likely to occur with an overly rich mixture?  I'm still trying to wrap my mind around the normal condition.  Would a by the book tuned bike with minimal upgrades (less restrictive exhaust and K & N filter, larger main) be likely to backfire when shutting off the throttle?

REgards,
Leonard

Well will your air cooled lawn mower pop when you suddenly close the throttle from fast to slow in a hurry after mowing for awhile. It's is caused by the same dynamics and considered perfectly normal. It is always more pronounced in a single cylinder engine, and the larger the engine or the freer flowing the exhaust the larger the pop.

  Can't say that it ever has, no.  It does sometime backfire if I turn off the ignition too quickly before it idles down a little bit.  Same thing I suppose.
  I don't want to belabor the subject, my bike doesn't normally backfire when I shut off the throttle and I like it that way.  I was just trying to understand why it would be considered normal when everything I have ever heard claims that a backfire is not a good thing to happen.

Best,
Leonard
--Leonard

[RagMan]

The noise mine makes is not a backfire, exactly  - just a repetitious pop.

[gapl53]

So would the backfire be more likely to occur with an overly rich mixture?  I'm still trying to wrap my mind around the normal condition.  Would a by the book tuned bike with minimal upgrades (less restrictive exhaust and K & N filter, larger main) be likely to backfire when shutting off the throttle?

REgards,
Leonard

Well will your air cooled lawn mower pop when you suddenly close the throttle from fast to slow in a hurry after mowing for awhile. It's is caused by the same dynamics and considered perfectly normal. It is always more pronounced in a single cylinder engine, and the larger the engine or the freer flowing the exhaust the larger the pop.

  Can't say that it ever has, no.  It does sometime backfire if I turn off the ignition too quickly before it idles down a little bit.  Same thing I suppose.
  I don't want to belabor the subject, my bike doesn't normally backfire when I shut off the throttle and I like it that way.  I was just trying to understand why it would be considered normal when everything I have ever heard claims that a backfire is not a good thing to happen.

Best,
Leonard
--Leonard

Trust me, I'm not saying there is anything wrong with the bike not back firing. If it is a stock Enfield, you wouldn't hear it if it did due to the over enthusiastic silencer that is in place. If you put a free flowing exhaust on don't be surprised if you hear an pop through the exhaust pipe. That is normal. But normal is a relative term, every engine is different, as with anything in life moderation is a good thing. If it is continuous, better check your ignition timing (overly retarded, firing way after TDC ), exhaust valve clearance (not enough clearance can hold open the exhaust valve), or your valve timing, which can be change slightly on the Enfield's by adjusting the valve gear backlash (if the valve backlash is improperly adjusted it can cause the exhaust valve to open to soon).
If you watch NASCAR on television, during the short track races (they back off the throttle a lot more than on the super speedways) you will notice flames coming out the exhaust, watch the shots of the right side of the race car, when they enter the corner and back off the throttle. You will see flames coming out the exhaust headers, that is in reality a backfire. If you were standing at that point in the track, you will hear the popping. Which is more like a rumble with multiple cylinders being exhausted so close together