What went wrong? Bike will no longer start.

Hello,
First I would like to say thank you for allowing me to join the forum and become a member of the community. Motor bikes have been a hobby of mine for some time and I’m excited to be here.
Now down to business:
I cannot for the life of me figure out what went wrong on my motor bike. It’s a typical 49cc 2-stroke Chinese motor on a 2010 diamondback Sorrento mountain bike. I installed the kit back in October and the bike has run fine since (except for eating 4 chains, but seeing as it already has 1,082 miles on it I guess that can be expected). I have done regular maintenance on it (cleaning the carb, cleaning the exhaust, oil on everything etc). I was riding it to a price chopper to run some errands when out of nowhere the bike started backfiring like crazy. I started losing speed and the engine started to die so I pulled in the clutch and the engine returned back to its normal idle. As soon as I let go of the clutch it started backfiring again and died. After that it would not start again whatsoever. I originally thought it was the spark plug so I replaced that and that didn’t fix it. I also replaced the cdi box because that had cracked due to cold weather. Now it pops and occasionally jolts when you try to start it but no go. I also noticed a fair amount of fuel is leaking out of the exhaust. Is it possible I blew a gasket? I have a spare carb lying around I might try putting on to see if that will fix it but if it’s an engine problem I won’t bother. My only reason for trying a different carb is that the fuel in the fuel line actually bubbles like a fish tank when you try to start it. I tinkered with the carb currently installed but no amount of screw configurations seem to result in any kind of improvement. I hope I don’t have to replace the motor because it is welded to my frame (that would suck). Any ideas, opinions, comments, or suggestions would be appreciated and are welcome.
Thanks,
-Dlibs

Alright well I checked the magneto. I took it out and examined it and I couldn't find any reason why it would not work. I also ran the bike with the spark plug in my hand and it sparks the way it should (not a pleasant shocky feeling though!). The problem shouldn’t be electrical. I will try changing out the carb tomorrow and let you know how that goes.
Yes I welded the engine to the frame. I work part-time for a company that manufactures helicopter components for Boeing and Sikorsky so I had access to some heavy duty machinery to work on the bike with (a welder being one of them). I could take the engine off with a plasma cutter but the end result would not be pretty. I also used military-grade heat resistant epoxy on the screws so that they would not fall off or become lose while riding. I have to say doing both of these things is probably why this build lasted as long as it did with almost no problems. I previously had an engine kit on a roadmaster mountain bike but it fell apart every couple of miles or so and I snapped the engine mounts twice on both sides of the engine. It was a disaster and I ended up scraping it and the motor bike idea altogether until I was talking to my boss about it and he said he would help me out if I built another one.
The diamondback Sorrento is a great bike I love it with and without the engine. If I have to replace the engine on it I’ll see if I can take the old one off first and just keep the bike as a regular bike (but I will most likely try to stick another engine on it). The bike only had 400 miles on it before I put the engine on. With the engines miles plus the 400 the bike itself has roughly 1,482 miles on it. Despite everything that has been done to it and the stress that the Sorrento has had on it all the gears change fine and everything is still original, even the brakes!

So the mystery continues…
I examined everything everybody suggested so far. Everything seems fine and everything seems to be in relatively good condition, but it still would not start. I went ahead and replaced the carb, and when I did the bike fired up right away. I was happy and I thought my troubles were over. 15 minutes of running the engine later the motor started backfiring and seizing just like before. I turned it off with the kill switch rather than let it die on its own. When I tried to start it again it would not start at all. This has to be the most bizarre thing I have ever experienced it’s like my bike is bi-polar. Has anybody ever heard of a motor ruining carbs? That seems to be what is happening.

yeah I already took the magneto out, checked to make sure all the wires had solid connections, and put it back in and made sure everything was tight. Everybody keeps suggesting that it's the magneto but I am more than sure its doing its job. The magneto looks brand new and every time I test it the spark plug fires the way it should. I don't think its electrical I thinks its a matter of the carb flooding the engine with fuel hence why the exaust leaks fuel everywhere, but how is it possible that this problem could develop with two carbs in such a short period of time? The only answer I could think of to my own question is that there is crud in the fuel tank clogging the carbs. I will check the filter on the gas tank to make sure there is nothing in it, empty the tank, clean it out, and put new fuel in again and see if that makes a difference (again). Ill also take the carb off the motor and clean it out. I'll post again to let you know if any of these things worked.

All fixed!
I cleaned out the gas tank and checked the filter, and both were clean. I took off the carb and ran hot water through it, and while I was doing that this nasty goopy funk came out onto my hand. I was not really sure what it was at first seeing as the gas tank and gas tank filter were clean, but then it hit me. The goop was actually deteriorated rubber from the fuel line. The gas must have eaten away at the rubber tubing while it sat in the tubes when the bike was not used. My suspicions were confirmed when I disconnected the fuel line from the fuel shut off switch and the fuel line actually tore apart. I guess the rubber tube that came with the kit was not a very high quality one (not that anything in the kits are ever of high quality). I was going to order another rubber tube, but I decided that I would not like to go through this mess again at some point in the future. What I did instead was take a pen, cut off both ends of the pen, and use the pen tube as a fuel pipe connecting the fuel line valve and the inline fuel filter. I than used another pen tube to connect the filter to the carb. Not only is the bike now running great, but I actually noticed a slight performance increase (seeing ass the fuel flows right into the carb through the plastic tubes instead of flowing through a maze of rubber tubing). So, for anybody who ever has this problem, REPLACE THE FUEL LINE THAT COMES WITH THE MOTOR BIKE KITS. Thank you everybody for helping me troubleshoot this problem, the help was appreciated.
-Dlibs