I have a 2000 Daewoo Matiz

They rebuilt the engine but didn't time it correctly. Now it's running sluggishly and backfiring strangely. How can I fix it properly? Please help...

From what you've said, it's possible the timing is off by a tooth. To check this, you'll need to remove the timing belt and align the camshaft timing marks with the crankshaft timing marks.

And how is that done more or less?

You need to remove the alternator belt, the crankshaft pulley, remove the cover that protects the timing belt and check if the marks on the camshaft and the crankshaft coincide. If they don't, you must loosen the belt tensioner and loosen the belt, remove it and synchronize the marks on the camshaft with those on the crankshaft. Then install the belt and leave it with the same tension it had before removing it.

Hi moto69, a friend of mine already did what you suggested, but he doesn't know the valve clearance, so they think the car is running rough. It starts as soon as you turn the key, but we don't know if that's the problem. If you know the correct clearance, please let me know.