My latest troubleshooting adventure - won't idle under load, backfiring, etc.

[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.

[RustyPile]

I'd be willing to bet, since the new module didn't fix the problem, your old rotor has a hole burned in it in the center under the end of the contact.. Or the contact button in the cap has burned away.. I've put basically stock HEI distributors in race engines that have 12:1 compression and turn 7,000 plus RPM and never had a failure. The 454 in my '71 C/10 has an HEI.. Never gives me any trouble.. The main problem with today's GM HEIs is most of the modules are Chinese crap...

[tim_mc]

I forgot to mention that I checked for 12V on the ignition (pink) wire, which was OK. I tested the coil with my ohmmeter and was getting different readings across the "BAT" and "TACH" leads (should be 0-1 ohm). Here is an awesome HEI troubleshooting guide I used by WIDESIDE72 on this site:
http://67-72chevytrucks.com/vboard/s...d.php?t=645976

I checked the original HEI rotor and cap and it looks OK. The first coil and module swap got it running well, except for the three times it died instantly soon after the change - like someone had turned off the key. At least the engine sounded good and I was only facing a likely module issue. Later that day it ran fine on a couple of longer drives with no issues, but I figured it was only a matter of time before it would completely cut out again.

I would not suspect that a GM HEI only a year old would have been the issue. When I had module failures in the past they would not fire at all. That's what made this a bit trickier to troubleshoot. I'd rather something break completely, so it was a small blessing when it wouldn't fire at all on Sunday.

A few more tips. First pic is how I marked the cap to a mark on the firewall to get the timing close to where it was. The second pic shows the rotor position pointing directly at the rear carb stud so that I knew where the distributor would need to drop in. I had also manually turned the engine to #1 TDC. Taking pics is a good way to keep track of parts and their positions. Here's a pic intake torque sequence from the attached Edelbrock installation sheet.

[tim_mc]

Update for 6/13/21. Truck has run well all week. Plug checks appeared to be hot/lean. I likely tuned the carb around a weak ignition in the past and now that it has been fixed I thought I should richen it up to be on the safe side. I went one step richer on the Edelbrock 1905 AVS2 metering rods. I was using 1956 rods, but swapped to the original 1449. The carb is now basically a stock AVS2 1905 but with .100 mains instead of the stock .101 jets. This is making a lot of sense to me after-the-fact. I wondered why it was so hard to get the right jet and rod combo to get this thing running decently. Now it's likely that I could have just bolted on the AVS2 out of the box with no issues if I had a great ignition on it to start with.

I re-gapped the copper ACDelco 41-629 spark plugs to 0.055" per DUI's recommendation (0.50-0.55"). Since I can't leave well enough alone, I swapped the cheap Delco plugs used to break in the engine with NGK FR5GP G-Power Platinum plugs (p/n 3248), gapped at 0.054" (that was the wire size on the tool). I'm a fan of NGK plugs from my dirt bike days working behind the parts counter at my local motorcycle shop as a kid.

I thought about going with ACDelco Rapidfire #5 plugs which have the required flat gasket seal and 0.750" reach, but I didn't like their resistance "range" of 6000-15000 ohms. NGK states a fixed 5000 ohm on their spec, and I'm looking for consistent spark on all eight cylinders. I also like NGK's fine electrode. FR5GP plugs are similar to the NGKs listed for my 2002 Z28 LS1 (TR5GP w/Taper fit and .708" reach), but with Flat gasket seal and 0.750" reach needed for the truck's Brodix heads. Finally, the NGKs are cheaper too at $3.69 vs. $5.59 each at my local O'Reilly's.

Test run to the gas station to top off with 93 octane. All is good - quick starts, steady idle, great acceleration.

[tim_mc]

Here's where I'm currently at with my Edelbrock AVS2 on my mild SBC 350. I went from 1-1/2 steps lean on both cruise and power up to #5 on the chart.