I just changed the spark plugs and now im getting back firing and i cant go over 50 mph

I agree with above answers. And would add to double check that the leads(if still on correct plugs) are pushed fully home onto the electrode tip of the plug. You could carry out a simple check on your ht leads by making a simple 12v test lamp ( if you dont have a tester) to test for continuity. But only ever ever ever remove one ht lead at any one timeunless you are a million percent sure where they all go back to

Did you change out the spark plug wires are they the same wires you might want to check them just did the wiring on a 99 Toyota 4runner the guy did a spark plug change didn't change wires in It was backfiring so just replace the wires now runs goodsounds like a new musicalcar

Go back and pull each spark plug wire and put it right back on. There is probably a loose one. This is assuming that you didn't pull all the wires off at once when changing the plugs. If you did, the you put the wires back on in the wrong order. Then, you'll need to find a manual online for the correct wiring order for the plug wires.

If they are on in the right order, you need new plug wires. They don't last forever!