Cylinders 5 and 6 not firing

[souptime]

I have a 1989 Ranger that we’re doing a rebuild on. Bought it with a blown up 2.9 v6, swapped the motor out for an 87 2.9. We’ve done a full rebuild on it, when we went to start the truck after putting it in cylinders 5 and 6 aren’t firing. It’s the same cylinders every time.

It has compression. Grounds are fine. It has spark, it has brand new plugs in it and we’ve checked the plug routes. Distributor cap is fine, has a new rotor. Fuel injectors have been cleaned out and swapped around, still the same cylinders dead. The plugs are wet so they’re getting fuel. Tried putting a new computer in it, didn’t change a thing. Vacuum lines and all wires are plugged in.

Any advice? Only guess is that it’s a spark or fuel timing issue but I’ve got no clue how to fix that. Or what would cause these two cylinders specifically to be dead.

 

[Brain75]

If you have Fuel, Air and Fire, you got it all on an old dizzy system.
Possible issues I can think of right off:
1) Bad plug wires, swap wires with the old (blown up motor) as a test.
2) Cracked dizzy cap, swap cap with the one from old motor
3) Other issues with dizzy (internal contact for #5/6 corroded, damaged), swap cap with the one from old motor
4) Valvetrain issues, #5 and #6 exhaust not closing causing no compression... but you said you had compression, care to share those numbers.

When you say it has spark, is that spark on all cylinders - you didn't just test #1 and say "it has spark" you tested #5 & 6 specifically.....
Is it intermittent or does it behave like 5 & 6 have never fired ever?

I had a hairline crack in my dizzy on my '48, you couldn't see it, but a brand new cap fixed it - I figured out way later... not all cracks are visible.

 

[Brain75]

Another question to make you ponder.... Does it miss only at idle, or at all rpms... that gives you some different angles to troubleshoot.
(Haynes)