Actually, this occurs because there's gas and too much O2 in the muffler. There's always a small amount of gas going out unburned, and it fires in the muffler because of high temperatures and enough O2.
Some causes are a lean mixture or air entering a damaged muffler because of the vacuum created. If it afterfires with 0% throttle and there are no air leaks anywhere, you can reduce it by richening the pilot screw (counterclockwise).

Also, cold weather is the best for performance, as long as the mixture is rich enough to carry the O2 increase (cold air has more density than when it's hot = more air in each engine intake).
By the way, cleaning the jets with a wire it's not recommended, as you could enlarge its size...

Sounds to me that Jaime has a great deal of knowledge to share. A big bonus for us. :2tup:

cold weather riding
 

does anyone notice a significant decrease in power when riding in coild windy conditions?/ On New Years Day i was riding home from work it had to be at least 20 degrees ..i noticed my engine seemed to be struggling to hold 55 mph its an 07 with very recent oil change ...I let it warm up about 10 mins before i took off i noticed ...the speed kept decreasing on the freeway till i decided to get off and take streets.. i was wondering do ineed to turn that lil idling screw or what

Turning up the idle will not affect the engine speed except at idle. Just a wild a$$ guess here but here goes. Was it real humid weather ?? High humidity and those temps might have caused carb icing.

Hey cleansmoke I just got back from a weekend riding a 200cc 3 wheeler through deep snow -5 C It would not idle without some choke and ran best at 1/2 choke. Try some choke next time maybe 1/8 .

I agree that Jaime is a great asset to the site, but his explanations of backfire and afterfire are incorrect. Just look it up.

The post about cold air density is right on.

Are backfire and afterfire different things? Also, which claim do you think is wrong? Just to know, I'm not angry. :)
In my experience recarbureting the GZ250, a lean mixture causes popping in the muffler, but sure that some gasoline must reach there. That happens when you give no throttle but fuel is still sucked in, for example, going down a hill using the engine brake. The thing is that fuel and fresh air would only need high temperatures to explode.
When the mixture is rich, there's a little or no fresh air remaining in the exhaust fumes (enough fuel to burn everything), so there's not enough O2 for the combustion to take place in the pipes/muffler.

Sorry, I can't explain this better, in English.

Anyway, there are many wrong thoughts about backfire, so maybe I missed something.

Regards.

+1 on the carb. icing.
Also, which direction was the wind blowing, relative to your direction of travel? A head wind (even a slight one) will have a pronounced effect to the GZ.

RE Choke: I think you will find that the carb. on the GZ doesn't have a "real" classic choke but an "enricher circuit" which has NO effect once the throttle is off idle more than a tiny bit.

Jaime,

Your english is fine and I'm really glad you are a part of this forum. You took over carb tuning where I gave up. Also, I wouldn't fare so well en espanol. That's why I don't hang out much on marauderos.org.

Backfire is an explosion that occurs in the reverse direction from the carb back into the air filter housing. It's completely different from afterfire in that it occurs in a very lean carb mixture, in my experience and research. This is the rarest type of "funny" explosion that people deal with.

Afterfire, as I said earlier, is caused by a rich fuel/air mixture and occurs in the muffler. It is also the most common hiccup commonly mistaken as "BACKFIRING". You were right when you said that there is always a trace amount of fuel vapor in the exhaust. The problem, however, in an afterfire situation, is that the amount of unburned fuel is more than trace. When the rich mixture allows enough fuel to enter the exhaust system, the ambient air, which is always in the muffler since it is not a closed system, combined with the heat of the muffler, causes a secondary explosion which may even result in flames coming out of the exhuast. This is called an afterfire.

Looking up the two words in google may give a better explanation than I did.

Sinceremente,
Jonathan

OK, now I see the difference. I thought that backfire and afterfire were the same, sorry.

Well, I've never heard about backfire, but I think it should also had something to do with a too tight intake valves clearance, maybe...

Now that you talk about it, I'm not really sure about which is the real cause for afterfire, as I've seen 2 opposite situations:

1. My GZ250 did some big exhaust popping when it was running too lean, and it didn't when the mixture was too rich.[/*:m:12tnw839]
2. I've seen motorbikes tuned too rich (visible heavy exhaust fumes) to give off flames, exploding with the ambient air just in the muffler exit (as it's supposed to have positive pressure in it (isn't it), the fuel vapor shouldn't explode right inside of the muffler; could ambient air get in at any time?).[/*:m:12tnw839]

So, considering those premises, is it possible that afterfire occurs with a too lean mixture and also with a TOO rich mixture? :??: