2000 legend 900, new t o me. Backfires flame out of the exhaust. It looks like aftermarket pipes. Should I rejet or go back to original exhaust?

believe carb adjustment bad & carb needs to be rejetted to exhaust, believe you need to add a higher flow gas valve & possibly new plugs & wires

Backfire when throttle down can be;
1. dirty carbs that are running lean and need cleaning. Also the rubber grommets that hold the carbs can be leaking at the backside or even dry rotted.
2. no air filter
3. broken or open vacuum lines
4. an aftermarket exhaust that needs rejetting or an open exhaust system

If the mechanic did not replace the exhaust pipe gaskets in the head or on any connections between the head pipes and the mufflers , the air leaks could be igniting unburned fuel in the exhaust , causing the popping and back firing. Try replacing any gaskets in the exhaust system.
If you have a flat spot or hesitation when accelerating than you probably also need to rejet the carburetors. Bigger main jets should fix that condition. A jet kit may also contain differently tapered needles for the slides to help compensate for the change in the acceleration curve.

Ok, due to different terminology used by different people, I'm going to assume that by "backfire", you mean it pops through the exhaust system when you back off the throttle. On stock exhaust systems which use the "crossover" connector to connect the front and rear pipes, the backfire is usually caused by an air leak somewhere in the exhaust system.

When you back off on the throttle, the mixture is usually too rich to burn in the exhaust system. But, if you have an air leak allowing the scavenging effect of the system to **** air in, the mixture becomes combustable and is ignited by subsequent exhaust pulses.

Check your exhaust system gaskets and replace them if necessary. This should stop your problem. Rejetting the carb may be necessary as well.

hi,might be able to help,i am hoping that the backfire is on the off throttle,if it is then is quite easy,check the exhaust system for air leaks,air gets sucked into the system through leaks,mixes with the hot exhaust gasses,leans out the mix inside the pipe then bang..1 backfire,the other cause could be particularly if you are running an aftermarket exhaust is that the system is running to lean and needs to be rejetted most times though it is air getting into the exhaust through badly fitting seals and gaskets..hope this helps...cheers ian

Purchase a EFI controller. I suggest a Fi2000R. It is easy to hook up and fairly easy to set by the instructions.

your carbs are dirty. and probably need to be balanced also... bring the bike bto a garage and have them cleaned by a profesional.... also after market pipes don't give your bike the proper back pressure .. that can mess up your carbs timing

Backfires are when fuel travels through the cylinders without combusting then explodes in the exhaust pipe. Fuel supply/combustion issue. It seems likely there's a problem either with the mix going into the cylinders (carbs fuel lines or air intake), or with timing. These can affect a bike at particular speeds and throttle positions. If using the choke has helped at some point that's a further indication (the choke changes the gas/air mix). ,,,