Trouble diagnosing motor miss

[studiog]

Working on my brother in laws' 71. Recent rebuild. Motor has a miss. I have isolated it to the #3 cylinder but can't figure out what is causing it. I pulled one wire at a time from the distributor as it was running and #3 made no difference in motor rpm.
Here is what I've checked so far:
1.) replaced spark plug
2.) replaced plug wire
3.) Compression 145-155 in all 8 cylinders
4.) Cam lift on #3 intake & exhaust is normal


Any comments welcome.
Thanks all.

 

[c84/99y]

i know it may seem like a stupid question but is correct on the dist cap as far as firing order??

 

[studiog]

i know it may seem like a stupid question but is correct on the dist cap as far as firing order??

Good question. I checked the firing order several times at the cap and plugs.

 

[WhiteKnight]

Have you replaced the distributor cap?

 

[studiog]

Have you replaced the distributor cap?

Haven't done that yet as it seems to be getting excellent spark from every tower when I pulled the wires one by one as it was running, but I think I will try a new cap and rotor and see what happens. thanks

 

[vettenick]

Pull the plug wire off the #3 spark plug. Holding the end about an inch from the exhaust manifold, crank the engine. If you get a spark from the wire to the manifold, then you know that everthing from the distributor to the plug is OK. At that point, it's either the plug or a mechanical problem with that cylinder. If you don't get spark, you should back track the wire to the cap.

 

[MMM]

Have you done a manifold vacuum check yet? Need to make sure there are no leaks in the intake manifold gasket. If engine vacuum checks out (>15" if it is a base 350) then double check to see if #5 cylinder is up to par, i.e., the rpm drop on 5 is the same as the rest of the cylinders.
Mike