What could cause 2 dead cyls

[ALS85GT]

Hey guys, I have a friend of mine who owns an 85GT, 302, Elderbrock manifold, holley, jacobs ignition, mech fuel pump, shorty headers, stock heads.... and he lost 2 cyls the other day under normal driving. ---He and his "ace" mech uncle haven't been able to figure out what's the problem. Here's the list of changes after this happened:
Rebuild dist, new cap, rotor, wires, new intake manifold gasket. Tried the stock ignition. Nothing. GOOD compression all cylinders. No oil mixed w/water and visa versa. The motor was rebuild last year and was running strong. Good vacume, no apparent leaks.
I gave him another intake manifold to try and another carb.

PS- So we have spark to all sparkies, compression, not sure about fuel yet.

Anything missed? Thanks............................

[Phantom5oh]

Well with a carb intake unless the runners are clogged then the fuel should make it to the heads. Has he popped the valve covers? Could be that his intake valves are not opening. If thats the problem then it could be a bad lifter, not enough oil getting to the lifters to pump them up so they are not working right. Check the valve lash, it might be off.

What cylinder #'s are dead? Is it #7 and #8? If so then maybe he should reroute the plug wires because if wire #7 and #8 are touching or running close to eachother the spark can jump to the other wire giving symptoms of dead cylinders.

There is mor but this is a start. Hope this helps.

Steve

[UNICORN]

Did you do a compression test on ALL of the cylinders...
If the compression test shows two dead cylinders, then the engine has probably partially dropped a couple of valves.
I've seen this happen before with stock heads., of course you'll have to pull the heads to fix this problem..:(

[ALS85GT]

Phantom5oh, the valves sticking could be something to look at, bad lifter or maybe even push rods too. I'm not sure if they were #7 & 8. I will advise him to remove the valve covers and check this.

UNICORN, He did a compression test but not sure if they were all cylinders.

The compression test showed good pressure at the two cylinders.

When removing the sparkplug wires while running, the car will run the same, as if the wires were still attached. This and the fact that the car runs lousy gave him the diag of the two dead cylinders.

Thanks guys, much appreciated......

[PKRWUD]

Tell him to start it for 10 seconds, then shut it off. Pull the plugs from the suspected cylinders. If they're wet, it's probably an ignition problem. If they're dry, the intake valve isn't opening.

Take care,
-Chris

[Unit 5302]

I've heard some bad feedback on Jacobs Ignition over the years. It wouldn't suprise me if the ignition has a problem.

[ALS85GT]

OK, thanks for the advise. I will post the results............

[ALS85GT]

My buddy found the problem, while removing the valve covers for the second time he started pushing dn on the springs and found two broken ones. This makes sense but what's really strange is that the broken spings don't correspond to the dead cylinders??

He thinks it may have to do w/the manifold somehow connecting to the dead cylinders. He had these heads rebuilt and the guy who built them added "chevy" valves? My buddy wasn't too excited about this but the car ran ok.

Thanks for the input.....:)