Ok all I’m in need of help. I have a 1974 L82 which I wanted to update to a little more efficient and workable (yes I see have achieved neither at this point so it was not likely the best of ideas) ignition system. I replaced the points distributor and coil with an HEI 45k Volt unit and replaced wires and even rewired the ignition system correctly. After getting it running with the new HEI in accordance with Lars timing I have a pretty severe knock and subsequent light backfire when I let off high RPMs. Basically I set up at 18 BTDC on idle and 36 all in just like it instructs. Since it was so bad at that point I had to retard it a little but still keep getting terrible knocks. All cylinders fire and from my testing I don’t have a jumped chain. I’m so miffed right now because nothing has been moved around and I even removed the whole system and checked rotor placement and alignment with the terminal so I’m pretty sure there’s not an issue with a misfire nor is there an issue with the plugs not getting Spark because I checked them all. It’s so frustrating because it was fine before and now I can’t get rid of this knock. Any help or suggestions are greatly appreciated. Thank you.

 

WAG
? perhaps your timing mark on your damper has slipped ... damper slipped since the Last time you set timing with Old dist ?


֎ begin by Verifying #1 piston truly @ TDC when timing mark shows Zero

 

I’m not sure how to check if it has slipped but I would not count it out until I can check. Any suggestions on a good check method?

 

acquire or make a spark plug hole "piston stop" aka TDC checker ... you can buy em for under $20

https://www.summitracing.com/compare

 

If that’s the case @jackson then it hasn’t slipped any. At TDC it’s def on the mark

 

What happened to the idle speed when you retarded the timing?

A change up or down will give you an indication if the timing was too advanced or insufficiently advanced.

 

When I retarded it the idle dropped about 100rpm

 

you sure you have firing order correct? it is easy to swap 5 and 7.

 

definitely in the correct order

 

All good points from the guys above and if your sure thats all ok.... then you have to go back to what you changed.

Lets focus on this a bit "I have a pretty severe knock and subsequent light backfire when I let off high RPMs" "I had to retard it a little but still keep getting terrible knocks"
When is it knocking?

If your centrifugal advance / vacuum advance and your physical mechanical timing are good and working properly then it almost sounds like cross spark contamination from somewhere.

I only ask these cause your distributor is new and hey some stuff comes broke or wrong even if new:
You said you rewired the ignition switch 12v line for the HEI so i am guess you have a dedicated 12v line for the HEI NOT the resistor wire from the harness for the old coil?
Is it possible you have crossfire going on in your cap or wires?
Or you have a plug wire crossed up possibly?
Your not using one of those correct-a-cap distributor caps are you? (seen these cause all kinds of misfires)
Is this a reputable brand distributor and Ignition module? (seen cheap Chinese modules do a lot of weird crap)
Your springs and weights are correctly setup on the centrifugal advance, not just flopping around with NO springs or stuck?
You vacuum advance diaphram arm is connected mechanically to the advance plate and is working not just flopping around and is returning .. not staying applied when it shouldnt be?

Run the car in complete darkness and look for arching going on at the wires, cap and where they route. You can even mist the wires with a spray bottle full of water to see if you have arching going on.

I know some guys wont like this but you could stick a straight pin down each of the 8 plug wires at the cap an run a jumper lead to the carb stud over to the pins one at a time and if the knock goes away when its on one of them you found the trick cylinder. You dont want to just remove plug wires on an HEI.. you want to short it to ground so the voltage goes somewhere and doesnt try to make a new ground through the cap or wires on its own. (Use a heavy insulated pair of pliers if you go this route when you switch to different pins or you'll get a good whack)

I'm sure you know this diagram back and front by now... just for confirm.

 

What unit did you buy?

What wires did you buy?

 

Ok guys I spoke to Lars and he made a suggestion. He said that the L82 is too low of a compression to cause any real knocking so it made me ask what else makes that sound. You can all get a good laugh at my expense here because when I went to check the right side of the engine I just decided to try putting my hand down near the wires to check for loosening. I almost singed the hairs off my hand due to the apparent lack of a donut gasket on that side. Turns out I blew it to hell when it backfired on the first startup. Found all the shrapnel in many locations around the car.

Thank you all for your help. I will update tomorrow once I finish the install and get it up and running. @Big2Bird As to answer a few questions you all asked, I bought the HEI and Wires as a kit from Summit and I believe it was ACPulse branded.

@mysixtynine I really appreciate your in depth answer and I actually printed it out and added it to my Haynes manual because it’s a great resource to have around when troubleshooting. I really appreciated your response.

 

I'm glad you (likely) found your problem.

I'll leave an anecdote here for a friend who doesn't want to be humiliated.

When a friend investigated his backfire at throttle let-off (afterfire, really), amplified greatly by the sidepipes, he found that he had accidentally hooked up the vacuum advance to ported vacuum. The car was otherwise tuned to @lars' spec. Once the vacuum port was corrected, the car idled much better, and the afterfire disappeared. There is now a vacuum gauge always left in that car.