how do i set the timing on a 1990 f150

Can you say OOPS!!

Sounds like carbon issues, meaning too much fuel in the combustion chamber. What size engine you have again? I dont recall you ever telling us.

Pull the #1 spark plug, and have someone put their finger over it. DISABLE the fuel pump if electronic. Then hit the key till your friend can feel air push out of the hole, the first time around you are goin to go to far, but bring it back around again, slower this time, and after the air pops out, double check to see that the crankshaft markings are indicating TDC. Put the plug back in the hole, re-enable the fuel pump. Check to see that the Dist. Cap and Rotor line up with the #1 plug.

After you do the above steps, then try to start the truck and turn the distributor till you hit the timing mark on the crank where it should be.

Yes, you just turn the distributor like you would on a 57 Chevrolet. Set your base timing as was already sugguested by pkurtz.

There's a little thingy in the wiring next to the distributor that you unplug - that stops the computer from messing with the distributor. You'll want to unplug that and then set it to the timing indicated on your emissions sticker, using your timing light. Then you can plug it back in.

If the sticker peels off (mine did) it's pretty hard to figure out what it's supposed to be! They like to leave that out of the shop manual so it's one less thing to have wrong.