I have a 48 inch belt driven walk behind once i get it started it will run but when I engage the blades or put it in gear it runs then stalls out?

Check your fuel filter if you have one.
Sounds like you will need to clean the carburetor or replace your carburetor internal rubber parts like the diaphgram and O rings.
Be sure to use compressed air to blow out all the fuel and air passages.
Be careful when blowing out the passages, because there are sometimes small rubber type seats in the bottom of some of the passages.
Sometimes you can get by with priming the carburetor or using starting fluid and letting it run a few times like that and it will flush the gunk out of the jets,but most of the time you will need to rebuild the carburetor.
Keep in mind that the float for the carburetor must be level when you go to reassemble the carburetor or follow the instrucitons you get with the carburetor kit.
If the mower/weedeater is over a year old, then I also recommend that you buy and install a new carburetor repair kit,because the diaphragm will get hard and that will cause it to be hard to crank.
When you clean your carburetor and remove the jet screws, you will first need to lightly seat the jet screws.
But before you lightly seat the jet screws count the number of turns it takes to seat the jet screws from their original position.
Be sure to mark the turns down on a piece of paper.
That way when you put the jets back in, you know to lightly seat them first and then turn them back out to their orginal position before you started.
This is a FREE answer,Please rate me