no spark on 2 cylinders

[colektm364]

This is my first post here..
Going down the road the other day headed to work and my truck starts running like crap. It is a 1987 silverado with a carb set up with the normal performance goodies. This a new crate motor. It will run so I dripped water on the header tubes one at a time and found that the #2 and #8 cylinders are not firing. The plugs are about 2 years old so I changed them and it still has a bad miss so I pulled the 2 plug wire off at the cap and the spark looked weak and yellow, the same on #8. So i thought the coil, at this point while im in there i wanted to up grade anyway so I put on a Accel super coil, cap and rotor and new 8mm wires. After all that it still is not firing on #2 and #8 What am I missing? It has ran great up to this point!

 

[crazy4offroad]

Cap & rotor is what I would have replace before the coil. You may also want to check the pick-up coil. It has contact points and like a reluctor ring, down deep in the distributor, under the ignition module. If only 2 or 3 cylinders are dead those are the 2 things I would think are having a problem.

 

[colektm364]

But wouldnt the pick up coil make it a random miss fire and not the same two cylinders? I am waiting for it to cool down so I can take a compression test and also check for a loose rocker arm nut on that bank

 

[crazy4offroad]

The coil would be random, and so would the ignition module, but things that would be more consistent would be plug, wire, cap/rotor, and pick-up coil. Good luck and post back what you find.

 

[87sierra]

check your continutiy thourgh the cap and the wires

 

[Jims86]

check your continutiy thourgh the cap and the wires

Take the cap off and check for play in the shaft, and do a compression check on those 2 cylinders.
Spark looks different out in open air, than it does under compression.