Most 4 strokes will run if they are a tooth out, but depending on the direction of the error, they will either lose power and overheat, or spit through the intake pipe.
It is normal for the inlet and exhaust valves to be open simultaneously at the end of the exhaust stroke: they are typically both open for about 10 to 15 degrees of crankshaft rotation - 35 degrees for a high performance engine, and 70 degrees for a racing engine that is not expected to idle, even badly. The overlap is roughly symmetrical about top dead center, so you can usually check for a jumped timing belt or incorrectly set timing gear just by seeing whether the position where the two valves are equally open is quite close to TDC. Of course there is no substitute in the end for checking the manufacturer's timing marks.
The problem is much more likely to be a tight inlet valve tappet than incorrect timing, unless the engine has been overhauled.
