I have a 1987 f150 it getting gas and fire it wont start what could be wrong with it?

get the help from service and repair manual it will help you to locate and fix the problem by your own and to get the manual go to the given link http://toolsnyou.com/

make sure that you replaced all wiring correctly and that the solenoid kicks in. Also if you left a ground off it wont start either

did you look at the ignition control module located on the distribitor this will prevent you from having any spark if this is shot.

Have you checked anything? Distributor, cam sensor, rotor turning?

If you are getting fuel and spark at the cylinder then check the timing. It's not uncommon for worn timing chains to jump at shut off or start up. To check turn the engine over until the timingmarks on the crankshaft line up at 0. Trace the wire back from #1 piston, which is the front one on the passenger's side. On the cap it should be just to the right of the tab that locks the cap in the proper position on the distributor. Lift the cap and check the position of the rotor button. It should be directly under the tower for the wire or it should be 180 degrees off. If so turn the engine 1 turn until the timing marks line up at 0 again and look under the cap. If it's not lined up with the #1 wire, turn the distributor until it does. Tighten the distributor down and try the truck. If it fires you are looking at replacing the timing chain. If it jumped once it will jump again. Hope this helps.

The F100 ended in 1983, so there is some confusion on your 1987 Ford F100. The F150 started in 1983 and was available in 1987, but not the F100. You have this listed under the 1981 Ford F100 category.
The 300 CID = 4.9L In-Line (Straight) 6-Cylinder.
In order to best answer your question I have included both the 1981 F100 4.9L and 1987 F150 4.9L below. The only difference is with the Distributor Cap Configuration and Connections.

For the 1981 F100 300cid=4.9L:
Firing order:
1-5-3-6-2-4
Cylinders:
(back)
...6
...5
...4
...3
...2
...1
(front)
Distributor:
...(back)
.........4..[.]-clip
2
...................1
6
................5
[.]....3

For the 1987 F150 300cid=4.9L:
Firing Order:
1-5-3-6-2-4
Cylinders:
(back)
..6
..5
..4
..3
..2
..1
(front)
Distributor:
.......(back)
.................4
.....2
[.]
.......................1
..6
.........................[.]-clip
....................5
.......3
.........(front)

Let me know if this helped, or if you have additional information or questions. Feel Free to contact me at FixYa.com!

try changing the igniton module on the distribuitor in most cases it's the problem if you dont want to buy it you can take the ignition module to a autozone and they will check it for free

Your timing is off. Did you have the distributor out when you changed gaskets or maybe took the plug wires off?

Is it flooded? Have you tried holding the accelerator to the floor? You have gas spraying out of the injectors when cranking? Does it even try?

check your fusebox in the truck. and under the hood for a blowen fuse. if there none. then you have a sencer prolbem. it not getting fire.