Remove the old tire, tube, rim strip. Use a quik stik, screw driver, monkey wrench, whatever tool works for you. With a complete change of tire & tube, we DO NOT have to examine them to find out WHY the tire went flat. Discard them both. If you want to save the old tire or tube, follow the additional instructions.
Carefully examine your rim with the rim strip removed. Scrutinize it thoroughly for any imperfection. As a wheel ages, the joint of the rim may separate slightly, resulting in a rough spot. A less costly wheel may have a weld that's rough. The rim may have taken a "curb shot," leaving a rough exposed area. It's amazing how a small rim imperfection can eventually "rub" a small hole into a tube.
If you find an imperfection, file it smooth. A piece of emery cloth also works well. You don't have to do a massive reconstruction project of the rim. Just make sure everything is smooth. Make sure no spoke ends are sharp enough to pierce the rim strip.
Now we’re ready to replace rim strip, tire and tube. Begin with the rim strip. Make sure it completely covers the flat portion inside the rim. NO TWISTS. Do not use a double rim strip. Next, place one side of the tire over the rim. From this point forward, you do not use any TOOLS, SPOONS, SCREWDRIVERS, QUIK STIK, MONKEY WRENCH, NOTHING!! NADA, a favorite BikemanforU expression.
Inflate the tube just enough to give it shape. Insert the slightly inflated tube between the rim and open end of the tire. Start by placing the valve through the rim strip hole. Now begin to force the tire over the rim working equally on both sides away from the valve. You should finish opposite the valve. If it begins to get difficult, say half-way around, you may want to let a little air out of the tube. Watch what you're doing. If the tube looks like it's "peeking out" in-between the rim and tire, back up a little and try again. No matter how hard it gets, only use your hands to return the tire to the rim. Some tires are harder to get back on the rim than others. If you run into a tough one, use the palms of your hands and kind of roll it over the rim. The skinny 700c tires may be the most difficult, and a little soap solution may help. The more tires you work with, the easier it gets.
Now, it’s time for air. Your attention to detail is not relaxed at this point. Start with a small amount of air, say 5 to 7 pounds of pressure. Examine how the tire is "seating" on the rim. Check both sides. Particularly in hot, humid weather the tire may "stick" to the rim in one spot and not around the whole rim. If everything looks good, continue inflating to the maximum inflation psi for your tire.
Caution: If you continue to inflate when the tire is not properly "seated," you may experience a big bang, destroying the tube and maybe the tire. The resulting destruction of the tube was not the sign of a defective tube, but rather the inability of the tire to contain the tube as it slipped off the rim.
Additional Instructions: If you’re just replacing a tube, go through all the steps above + one more. You must find out why your tire went flat. Most common reasons are these foreign objects: nail, tack, wire, thorns. If any foreign objects are still in your tire and you don't successfully remove them, a new tube will quickly succumb to the same flat state.
Thorns are tough. If you remove one and figure you're home free...BEEP WRONG! Whenever you find one thorn, rest assured there are probably more, maybe many, many more, especially if you're cycling through wooded areas. Sometimes a tire may be so infested with thorns that your only hope is to discard the tire. Thorns are not always apparent with eye or feel. They may be embedded and don't pop their ugly head until you inflate your tire. Then, with a new tube, your thorny tire goes flat again.
One other common cause of a flat tire is a "Vampire Bite." If your tires are not properly inflated (not enough air) and you experience a pothole episode, the tire and tube compress against the rim. This causes 2 side slits, resembling a vampire bite. This scenario is most experienced by the Young Rough Riders.
"Finding Out Why Your Tire Went Flat"
Sometimes you just can't, but that is a rare case. The quality of tires and tubes today is extremely high. The likelihood of a new tube or new tire being defective is EXTREMELY rare. There is a reason why a tire goes flat, and as you replace more and more tires and tubes, I challenge you to always find that reason. So in closing, remember...