I have a late 60s 440 engine i just completley rebuilt. And we put brand new spark plugs in it, and still cylinders #1 and #7 arent firing as the others are. Its perfectly timed and everything, they are just misfiring. Anyone know why? Can it be the valve seals?

 

Bad plug wires? Bad plugs? Have you swapped wires and plugs around to see if the misfire follows?

Number of issues could cause this.

 

"The basics"

Compression

Spark at the right time

Fuel

It's likely not fuel, unless there's a manifold leak at the 5 and 7 intake ports

So check compression, with both plugs removed or do a leakdown check

Valves? Opening closing? adjustment?

Have you checked spark at the plug end of the plug wires?

(Such things as a defective cap, bad wires, or even bad plugs)

 

Plug wire routing can ???? sometimes causes misfire

 

check your distributor and point and condenser

 

Alright so i did a compression test, and compression in the good cylinders are reading around 100 and in the bad 1st and 7th cylinder it is reading around 25. I also did valve leakdown test and none of the valves are leaking. Is this a sign of bad piston rings? what is the problem?

 

Might be a head gasket, cracked head. Ring or piston problems, it should show lots of oil

I assume with your leakdown you listened for leakage in the intake and exhaust? Did you look in the rad cap for bubbles? Run down someone with a coolant chemical test for combustion leakage.

 

Squirt a little bit of oil in the spark plug hole, and re test. See if compression raises or not.

If yes: bad rings and possibly more.

If no: valve issue, head gasket, cracked head, block, ect as 440roadrunner said.