Chinese atv! Poor design! Need help!

[OutlawVJV]

So a friend of mind recently purchased a TaoTao asian atv. Its a low quality bike but it does the trick for him. Its his first time riding and he wanted to start out small so he learns how to drive them. I knew they werent the best quality, but i never knew they were so poorly designed. Every time he rides in the rain or a little water. They airfilter takes in water and bogs the engine. The design was a little cup with a sponge in it, aimed down towards the tires all open. So it was horrible. So we desided to make a custome aluminum airbox and use some hoseing to attach a K&N air filter to the carb. Now considering its a 300cc loncin motor i assumed that i could use a K&N filter from a honda 300ex and just customize the tubing. So we did all that and everything looks great. Problem is we started it up and its not running the best. Engine runs well, air filter is taking in air. However, the exhaust is just back fireing and pushing out the air very hard. Now the exhaust is horrible as well. Its a small 3/4 inch piping wich leaves the back with only a small 1/2 inch pipeing. Could the exhaust itself be the issue. It seems that its having a hard time letting out all the exhaust fumes, and is holding the motor back. Would buying an after market slip on such as fmf or hmf resolve the issue. Were trying to do what ever we can to get this atv to run at its best. Not so its fast or powerful. But so it at least runs nice and is able to handle the terrain a real atv should. Any suggestions or addvice would be greatly appreciated. Thanks.

[Delbert]

You're on the right path, but you also need to open up the exhaust now that you got the intake breathing better. ONLY PROBLEM. You will also need to rejet the carburator. If not you have the motor running so lean that it'll self distruct and then you'll not only have a cheap ATV, but a cheap ATV that you can't sell because the engine needs rebuilt and you can't readily get parts for. If fuel injected, you'll need to have the ECU reprogrammed because it'll be too lean also, and that will be by Dealer only(if they have proper mapping). You need to know what size pilot and main jet is installed from factory. Then get a pilot jet at least 2 sizes larger(idle to midrange jet) and get a main jet at least 1 size larger(midrange to wide open). Take carburator apart and swap the jets. After installation of jets, you'll need to readjust the air fuel mixture screw for better carburator response and running ability. NOW, if jets aren't available, you need to find somebody that has a drill press(and knows exactly what they're doing) and have them redrill your stock jets a couple millemeters larger to the size required. Hope all this makes sense? In other words, this ATV already is running lean(to pass emissions laws) and with opening up intake and(or) exhaust, without changing out to larger jets, you'll make it run even leaner.
Delbert