I’m not sure if rotor phasing can cause a missfire on all 8 cylinders or not.
Maybe at 2500 rpm the rotor is being pulled back a little bit further then what the picture shows when it’s idling and that’s whats causing the missfiring.
The most interesting part of this issue to me is how taking the vacuum advance away seems to clean it up.
RP is the same on all cyls (the spatial distance gap) but if the "gap" is borderline it can be random missing as different cyls have different charateristics but to KISS, the ans is yes. some of these Q's are redundant but you say capping the hose completely elims the issue? What Dave said, shine your light in there while IDLING & see where RP is at in relation to the cap terminal "bulge with (A) the can capped & (B) with it plumbed into manifold & the can will shift it CCW (SB). I'm assuming you have a dialback, get it running at 2500 & note the (total) timing then plug the hose & twist the dist to get the timing degrees back up to where it was with the can operational. NAPA has a rotor (# MO-3000 iirc) that has a .060" longer blade that helps with phasing ($8.xx out the door iirc) cuz it is the total gap that is in question, the circumferential gap (that you see with the light) & the radial gap (longer rotor helps with that) & if the total gap makes the "required" voltage greater than the "available" voltage from your particular ign setup then it will misfire & the required voltage is greatest at WOT at which point the can is non op (but this is a steady cruise from 2500 on up correct?. Short version: circumferential & radial distance is too great/reluctor gap issue (but we covered that potential)/too much timing (way less likely) but do get a number/cracked cap/bad plug wire(s).
Ive checked timing with and without vacuum advance at 2500 .
With vacuum where it starts missfiring is 42-43 degrees.
Without vacuum it’s 32-33 degrees.
The rotor I am running does have a longer blade.
All of the electronic vacuum advance firecore distributors come with a longer blade rotor.
The length of the blade on the firecore rotor is the same length as the napa M03000 rotor.
I have a napa M03000 rotor in my tool box and I have compared it to the rotornthat cane with my firecore distributor they are the same length on the rotor blade.
Awhile back I had done a gear swap on the 8 3/4 that I put in that truck, went from 3.55s to 4.56s and I never got around to re-gearing the front Dana 44 axle to match.
So now the truck is at a shop waiting to have the front Dana 44 axle geared to match the rear axle.
We have over a foot of snow on the ground this morning otherwise I would have ran over and grabbed it today to do a couple more quick tests on the issue and then brought it back.
But without 4wd I’m not sure I could even drive it back to the house from the shop.
