[tim_mc]

Posting my recent experience in case it helps others. The set up: New GM 350 SBC crate (p/n 12681429), Brodix IK-180 heads, COMP XR258HR roller cam & lifters, Edelbrock EPS intake and AVS2 carb, new GM HEI installed in May 2020. Truck has always started easily with no choke, good idle.

The symptoms: A few weeks ago the truck had an occasional backfire or load up when driving. It progressively got worse to where it would start OK, but would die when put into gear. It would idle and rev OK, but ran badly under a load, or when stopping at a stoplight. I had to put it in neutral a few times to keep it from dying at intersections. Later it would lean out and backfire after it warmed up and eventually would not restart when warm. Disconnected vacuum advance - no change. Pumped carb and saw fuel spray from the annular primaries in a fine mist as the AVS should. Good 4.5-5psi fuel pressure at gauge just before the 40mu brass fuel filter. Removed valve covers. No broken rockers or other obvious valvetrain issues.

A closer look under the hood a week or so ago showed oil pooling on the intake near #8 cylinder, and some coolant on the intake at #1. Intake bolts weren't loose, but did take a little tightening. I figured blown intake gaskets may have been causing the lean condition/rough running. Note: I did not re-torque the intake bolts properly during break-in, and may have taken a few bolts loose and retightened while swapping throttle & TV cable brackets. Regardless if this was the cause or not, I was going to fix the intake leaks.

5/26/21: Ordered Fel-Pro 1204 gaskets per Brodix instruction sheet - they have the block-off for the heat cross-over passage on the intake. Removed intake and cleaned heads, intake and bolts. No breaks on the gaskets. No coolant on dipstick and just a few drops in the lifter valley, maybe from removing the intake. Also took a look at the carb and fuel filter. Took off the carb air horn and cleaned out some fine gray sediment, maybe carbon from backfiring? No clogs or other obstructions. Cleaned and reassembled carb.

5/28/21: Replaced gaskets. Used Permatex Ultra Black silicone on the lifter valley (China) walls. Used Permatex Aviation on bolt threads (instead of the Permatex Teflon thread sealer I used before). Torqued bolts in proper sequence in three stages to 25 ft/lbs. Let silicone set for over 24 hours.

5/30/21: Tried to start truck. Crank, crank...nothing. Had wife crank while I checked for spark on #1 to the header with a screwdriver. Didn't see a spark and was too chicken to grab it by hand. After church, got a Harbor Freight 90° In-Line Spark Checker ($3.99 - smokin' deal), Advance Auto Parts/Carquest HEI ignition coil EBC1483, and 4-prong HEI module CBE1004 (Made in USA!). Installed module and coil. Fired up and ran great with good light on the spark checker. Took for short drive and it died instantly, seemed electrical in nature. Let it sit for a few minutes, fired it up and drove it home. Truck died again on a short drive, so I disconnected the tack lead, which is also wired to a kill switch. Truck died unexpectedly again. Reconnected tach lead since it wasn't the problem. Let it cool, fired up and made it home. Drove it a few times later that day with no issues. Starts and idles OK. No problem with idle dropping at stop lights in gear. Concerned that the new module may be flaky.

5/31/21: Ordered a Davis Unified Ignition DUI-240001BK 50KV cap/coil kit and DUI-000222 module from Summit. (Edit: Reviews of the DUI parts were mostly excellent with claims of improved starts, power and idle. According to DUI, their Dyna-Modules have more electronic dwell than a stock module, so the coil saturates longer and produces a more intense spark. I figured while I was at it I'd bump up the coil output with the DUI cap & coil kit that includes a burn-through resistant brass terminal rotor.)

6/3/21: Distributor upgrade kit and module arrived and installed. Fired up.
WOW. I thought buying a stock GM HEI would be adequate, but this was a great upgrade. Truck starts super fast, idles great, and seems to have a cleaner exhaust. DUI recommends opening the spark plug gaps to .050-.055", so I may do that soon. You can see in the pic below that coolant was getting around both bolts near #1 cylinder (upper right corner). The second pic is everything back together as it should be. I'll report back after a few more drives, but so far very happy and wish I had upgraded the ignition sooner.