just finished 18 month rebuild 1990 f150 4x4 5.0 efi motor.
alumin pistons + .030
tom moss port match lower intake
explorer rebuilt fuel injectors
crower roller cam 15510
new everything, valve springs pushrods valve job
double roller timing chain/sprockets, melling oil pump
new cj pony parts steel gear dist, taylor wires
complete new exhaust, shortys, flowmaster 2 1/2 y pipe single flowmaster cat...3" cat back single exhaust
egr valve
all new front components, water pump, therm housing, alt ps, hoses, smog hoses

left the same:
tpi, coil, accell cap and rotor new from before engine rebuild, all smog system sensors, map, fuel regulator

problem: idles and revs fine, but pulling out from stop and under load, getting single exhaust backfire (gunshot style) sometimes 2 or 3 but still pulls steady between backfires.

have checked timing 10 d (w/o spout connector) tried 12 no diff
vacuum idle, 14-15,

I have been advised
1. vacuum leak
2. timing off by map or ecm
3. knock sensor(new no longer avail. is 1993 + same as 1990? pic looks same

any help would be greatly appreciated
I am retired ....build custom engines mostly chev, some ford this is my first efi rebuild

thnks jay

 

There are 2 firing orders for small blocks, could you have the cap wired for the wrong one? The cam determines which one. I've also seen very experienced mechanics get 2 wires on wrong, and get that symptom. Looks great by the way, I hope you get it straightened out. non HO 5.0 and 302 is 1-5-4-2-6-3-7-8 while 351W and 5.0 HO is 1-3-7-2-6-5-4-8. Notice that only 5-4 and 3-7 are the only changes...

 

thank you for your input. went through the two diff firing orders with my first '94 f150 in 2015. isn't one of fords "better ideas"

the update on my problem is after chasing all of the emission vacuum lines for several hours ( with the help of a previous post here from 2011). I went "back to the basics", and pulled all of the plugs and rechecked the plug wires to match the firing order. seems like the exhaust shop pulled the plug wires installing the new headers and # 4 plug wire was not pushed onto the plug all the way.
it was apparently enough to fire the plug marginally at idle but under load was making the misfire. go figure! running smooth now just needs some break-in mileage.

 

Simple fix. That's always a relief.
Motor looks good.
If you don't have a lot of Ford small block experience, I'd suggest looking up cross firing.
Has to do with the routing of the wires; if you line them up all pretty like they should be you can get a cross fire between the 2 cylinders that are firing one right after the other if those 2 wires are right beside each other.
Later trucks have a sticker right on the rad saddle warning about it.
Just thought it worth a mention in case you didn't know it can be an issue.