Two of my quads was jetted to rich from the dealer because the dealers would rather be on the safe side. After dropping the main jets down a couple of sizes, the backfiring has almost completly gone away.
The constant backfiring on my quads was caused by a rich condition.

 

With my 2001 250 with manual choke, if you leave the lever half way on it will cause all kinds off backfires.

 

Im not sure how to respond.
If you dont believe the information directly from the experts who design, manufacture and test and support the product...what could i say to convince you?
If you do a search in these forums on exhaust popping, or back fires...youll see in the last year, ive answered about 80 of these, always with the same advice..the idle circuit is too lean. There are various causes, but thats the reason.
Ive been working on carbs for 35 years, and so far, ive never seen it other wise. And so far no one has ever come back and said that didnt fix it.
Too bad i dont live closer...youve got a very unusual 250, that backfires , and the only reason is because its rich, due to the choke being on. Id like to see it, because itd be the first one, ive ever heard of where rich mixtures have caused backfire..

 

Well, I will admit it makes it run like crap to, when the choke gets let on. But yeah it back fires, it was worse when there was an exhust leak at the doughnuts, but it still does it to this day. Part of my problem with this lean stuff is that I was a firefighter for a while and learn a lot about how fires and explosions work. Too lean and no burn or fire, it's just plain old physics, either have the right combo of elememnts or it's not gonna happen. Used to be they taught the "Fire Triangle"; fuel, oxygen and heat in the correct ratio and you get fire or explosion. They've added a fourth now but it doesn't change anything. Have you ever heard of a "flash over" or "backdraft"? In both cases you have a ton of heat and fuel, but a lack of oxygen, aka too lean. Add air (open the door, break a window or cut a vent) and boom, explosion and fire. So I guess to lean means, it's not gonna happen. BTW, I have seen both conditions in real life many times.

 

The way i see it, the discription of the conditions you described..." In both cases you have a ton of heat and fuel, but a lack of oxygen, aka too lean. "is actually a too rich condition. Adding air lets it burn. Think of being lean as having too much air, and too rich is too much fuel.
The same thing happens in the combustion chamber...if the mixtures are too rich, then theres not enough oxy to let it burn...or pop...unless an exhaust leak adds air. Note that one of the things to look for when having the popping in the exhaust is leaks, both intake and exhaust.
You are correct that its fuel in the exhaust pipe, which causes the popping. If there were no fuel, there would be no exhaust popping or backfiring. The only thing we disagree on, is how the fuel gets there. The discription provided by mikuni, is what their research has come up with...hard to believe, but lean is the cause.
Think about it this way, ...the richer the fuel mixture, the higher amounts of CO, CO2, nitrates of oxide and raw hydocarbons in the exhaust, and the levels of oxygen are lower as the mixtures get richer. Cant have an exposion in the exhaust pipe with out air.
One simple test, i usually tell people to try, when they have exhaust popping...is to pull on the choke just before the pop happens. This makes the idle mixture too rich, and the popping will stop. Also, if scooter1967 pulled the choke on, while shutting down, his popping would stop too. But if rich were the reason for the popping, pulling the choke on would make it worse.

 

Bear, I'll apologize if I step into the wrong arena.... but keep in mind that if the fuel mixture is too rich when being dumped into the exhaust, due to it being below it's UEL (Upper explosive limit), it's still not going to explode, because of the muffler is full of CO, yet another rich environment (lack of O2 to bring the fuel down into it's explosive limits).
With it running like crap with the choke on.... Is because you’re starving it!

Just my two cents! With all respect!

 

Don't apologize for a different view, that's what keeps life interesting. But since you threw you hat in the ring,

The choke isn't an old style choke with a butterfly valve, but an enrichening system with an adjustable valve to dump extra fuel in to the engine when the lever is pulled to open said valve, the air is not restricted, except by the throttle valve and the slide. So not starving it but way to much fuel (over feeding it actually).


If the fuel/air mixture is in a too rich condition then it is above it's UEL. You can't look at the air as all it's component, just the oxygen, as that's the part that is burned with the fuel. The trouble with this conversation is that we are not talking apples and apples. Fuel and air mixture need to be around 13 (air) to 1 (gasoline) to burn in a gasoline engine. You have to use that as a baseline. Normally, not enough fuel is too lean and not enough air is to rich. But if you think about it, you can call too much air(way above the 13 to 1, say 20 to 1) to have a lean condition and not enough fuel for too lean also(again say 20 to one but the fuel is shy this time). The cylinder holds so much air and you must match the fuel to that volume (in our case the exhaust is just another cylinder). Not enough fuel, lean; too much fuel, rich. So from hondabuster example the cylinder is too lean but the exhaust system or at least the outlet of the exhaust is rich enough to ignite the fuel/air load once extra air is present. I can buy that, except that when the throttle is dumped, the fuel (the heavier of the components involved) will continue to flow after the throttle valve or slide are closed (don't blame me, physics again, oh yeah and the vacuum controlled slide don't close very fast when compared to the mechanically controlled throttle valve) and the high vacuum will also encourage the fuel to continue to flow at first anyway, so to rich. When the fuel quits flowing then we end up to lean.

Also don't be blaming the alchohol as it is a motor fuel also, just that it needs to be a bunch richer than gasoline, but then again once the gas ignites the alcohol burns fine. When running alcohol in our race boats we generally just removed the main jets to get the mixture down to the 6 to 1(more fuel this time) or so need to make the alcohol ignite properly. Oh yeah, alcohol burns very slowly when compared to gasoline (timing bumped as high as 50 to 60 degrees BTC compared to 28 to 38 degrees for gas). The alcohol is an octane booster and an emissions component to gas.

Hondabuster, I also wish we were closer, this would be a great discussion/debate and a good learning expeience all around.