You need to remove your timing belt cover, put your engine at TDC, loosen the tentioner pully for the timing belt, mark the belt and pully on both the intake and exhaust cams (so you know where to put the belt back). Partially remove the belt from the cam gears so it is almost all the way off. Carefully remove the belt from the exhaust cam only (the one in front) and with a 17mm wrench turn the pully back 1 tooth (counter clockwise) and put the belt on. if done right then the marks belt will still match up with the mark on the intake pully and be one tooth to the right of the mark on the exhaust pully.
If something there doesnt' make sence, or of you have doubts then I don't recomend you do this mod. If you screw it up, your piston could come up and strike your valves which is a VERY BAD THING. You will be spending quite a bit of money replacing parts. I am not responsible for your actions in doing this mod as I am in no way forcing you to do it so if you screw up don't come crying to me LOL.
I noticed a gain in the mid to upper rpm range while very little loss in lower. This is not the mod to beat all others and it will not give you much but it will give you some. I have also seen this mod give a Rio engine 50 mpg on a highway trip.
There should be no adverse effects if you do this right.
the reason why Kia doesn't do little performance mods is the same exact reason all the other car companies (such as honda, hyundai, ford, toyota etc.) don't. That would be one fo the major issues, Reloability. Let me give you examples of why OEMs (original Equipment manufacture) don't do mods for you such as cold air intake, beefier cams (with advanced intake and retard exaust), high performance exaust and other stuff like that. Its the same reason why stock intake systems are so bad (for almost every car for that matter, well the Honda Civic Hatchback 1.6L SOHC isn't too bad.) The engineers have to follow a code, and thats smooth, quiet, reliable power with the least problems, decient gas milage and smoothest, most quiet ride.
its no secret most people perfer not to hear their car's engine, or feel it for that matter, and thats one of the great sacrifices of power (besides federal emmissions laws, but I am thankful that those are in place so I can atleast breathe easy). The engineers are always making sacrifices b etween best gas milage and most reliable power and most quiet driving. The code is NVH (Noise, Vibration and Harshness). This alone sacrifices a few MPG (Miles per gallon) and HP (horse power). I can prove that. If you get a Short ram Air intake (Link: http://www.2kracing.com/images/maxint.jpg) which uses large and light weight soft aluminum metal tubing with a high flow pleated cotton conical filter, when you first hook it up, you will feel the difference (depends on how bad the stock air intake system as, if it was decient, then you will either just have the advantage of a reusable filter, or better gas milage).
Now you may think "why don't companies do this if it increases Gas milage , increases power and is more reliable then stock intakes?" well this goes back to NVH, The NOISE is a HUGE factor, first off you can hear the air being sucked into the carb when your driving (and yes, its pretty loud, sounds like a vacuum cleaner with out that vacuum's whine). Plus when you run it WOT (Wide Open Throttle), the engine noise is increased, and its quite a bit louder, in fact some people jsut put those intakes on so they can be heard just cause they want to pose a profile that says "my car sounds fast! VROOM!". This would not stand well with customers since its too loud with the air being sucked in and the engine noise is too loud for them.
Now if you were to modify the encaust manifold, put on a high flow cat converter and a performance muffler (mangaflow or flowmaster). That also increases power and gas milage (depending on hwo bad the stock exaust system was), but it makes your car louder. Just like the intake, your car will have a lower pitch loud throaty sound (instead of the higher pitched but mroe quiet sound a bone stock system has. Customers usually don't like this because its too loud for them. Even if it increases your MPG by a few points (if you add both the intake and the exaust mods together), customers would rather have quieter cars.
but we are not typical customers, in fact the first thing I did when I got my hand me down Kia Sephia 2000 LS (with 1.8L DOHC engine) was replace the spark plugs and allt he worn parts since the mechanics did a terrible job of doing it themselves. They left alot of rotting parts in there that were way overdue. After I made the car more reliable I started on weight reduction, the car lost over 100+ pounds and I modified the stereo system. THen I modified the Air intake system (had up to 6 different designs before I stuck with this one. Yes, sometimes you ahve to change something to see how your car likes it before you find the sweet spot). Now I am planning a total exaust work, better spark plug cables (found a way to get 9MMs to fit without problems, just a little long, thats all) and an addon ECU module. All these help alot with power.
Now my intake cost me about 50 bucks for the K&N filter with the metal tubing (that stuff is so easy to cut with a hand saw and is so much lighter then that stock rubber tubing). I could have got it cheaper but I was just learning how to modify my car back then. The first intake mod I made was pathetic, I thought it was so cool, but thats what its like when your first learning. Its fun but it never stops. But be careful, once you put on that first part and feel that "seat in the pants" performance increase (basically where you always feel the performance from startup to engine peak, the kind that you feel proud of every day) you better keep an eye on your wallet, cause it will never be the same again.
also one more thing, the mod that was described by the ones that posted before me are good mods, but if you DONOT know what you are doing, this mod will cost you power, or even a major engine overhaul, since a good amount of the Kia engines are Interference engines, that means if you have incorrect timing on them, a piston will smack into a valve, which could destroy your valve, your piston or both. BE CAREFUL PLEASE! And do your research.
Thanks for the thorough answer Night-Hawk. I'm skilled enough that I could retard the exhaust cam and I don't mind if it's a little noiser since my stereo would be a lot noiser anyway. As far as vibration from mods goes, I don't really have much of a problem with that either. My engine already vibrates (maybe from a loose motor mount) and it also vibrates on the highway. That may be from a tire balance problem. I've heard that Rios are bad about that.