If you just changed the timing belt in your car and the timing marks were lined up now all it does is turn over but not fire and occasionally backfires what is the most likely problem?

If number one piston was not at Top Dead Center (TDC) when the marks were lined up, the timing is probably 180-degrees off. The piston raises twice for each time the spark plug fires: it raises once on the exhaust stroke and then again on the compression stroke (TDC).