Help figuring out miss in an HEI 350?

[ihv800]

Hello!

Working on an '82 GMC for a friend trying to get it tuned up and running nice. It was brought to me with a no start condition, an HEI module fixed that up. I used an NOS US made Echlin.

I've got a miss I cannot figure out, though. Cylinders 8 and 5. 8 seems to not be contributing at all. With no load 5 is dead, if you let it load up against the converter pulling the wire on and off 5 does make a detectable difference but it's a weak contribution.

Compression on 5 and 8 is around 130 as are other cylinders that are working fine. Valves are goin up and down so the cam isn't flat, and I have checked and readjusted the rocker arms. I've tried swapping plugs and wires around with good cylinders and the problem persists. I've tried new plugs and wires with no change. I've reclocked the distributor and moved the wires over one post on the cap, 5 and 8 still do the same thing. The spark will jump all over from the cap when you pull those wires off and have good spark at the plug.

I must be missing something but I've tried everything I can think of? I thought the pickup was doing something wonky, but figured reclocking the distributor would move the issue and it didn't. I don't see any ignition issues and it makes effortless compression when testing those cylinders. It is interesting to me that the problem cylinders are 180 apart. Before I pull the rest of my hair out I thought I'd put around the table for ideas.

Thanks!

[eric8]

Well take this story for what it’s worth, I had a no start condition as well that came out of nowhere with an HEI. It made me pull my hair out too. I replaced the module with a U.S. made Standard Motor Company module (apparently the same as Echlin). That immediately fixed the no start condition, but when I revved over 2k rpms it would miss and backfire like crazy.

Sent that module back and ordered the following kit from Proform. They make the distributors that come in the GM performance crate motors. Not shown in the picture, but the kit also came with an assortment springs, and the GM “ZZ series” crate motor center cam and weights. A weak coil can show up good when you ohm it out but cause all kinds of problems frying modules and making them do wonky things. This kit basically replaces everything in the equation except for the pickup coil. Whatever was wrong, it fixed all my issues. Also make sure your distributor has a good ground through a bare distributor hold down clamp to the block. Look for any obvious signs of chafing or wire breakage on the wires going to the pickup coil. The movement of the vacuum advance mechanism back and forth can weaken the wires and cause intermittent connections. Ensure a full 12v at the HEI wire that plugs in to the cap just to rule out a voltage drop.

[eric8]

An additional note, Proform makes the exact kit above under their own name that they said is identical (minus the GM Performance badging), which saves you a little money. $86 for this kit. Really a lot for your money considering what you get, including a new coil, brass terminal cap and rotor, rotor bushing/button, module, new module to cap harness, tunable vac advance and and springs to dial in a nice mechanical advance curve. The weights and springs will let you dial in your all in advance by 3k rpm’s and have a much better curve. Came with a chart that shows what the different combinations of springs will get you.

[Dead Parrot]

Vacuum leak in the plumbing related to the aft intake vacuum port. IIRC - that feeds to both 5 and 8. Too much air, lean mix -> misfires. Also check the brake booster. Those can leak and still provide some boost.