Whats wrong with my 2000 Mitsubishi Montero sports

I changed the val cover gaset and intake val and replaced spark plugs and alternator because oil was leaking from the val cover gastet and getting into the alternator causing it to short out. While doing this I also replaced the timing belt. After doing all this the car will idol but with a lopeing sound very rough with very raw gas fumes, If I press the acelarator it only spits backfire and and no acelration you cannot drive it cause it just dies you can turn the car off and restart and it idols perfect and you can acelarate it for about halg a minute then on its own just just starts lopeing again. It did not do this til I changed everything. What could be the problem. Thanks in advance for anyway you could help me on this.

Gary