Reluctor wheel failure, Maybe?
 

Hi fellas, I've built a bunch of these engines...i've diagnosed a fair few of them, enough to know that they CAN be remarkably simple, but when something takes a week and a half to troubleshoot, its either something extremely complex and complicated, or its something more simple than forgetting what color the sky is.
I have a misfire issue with my fresh stroker LS that's driving me mad, cylinders 6 and 8 do not fire properly and backfire like gunshots through the exhaust and doesn't allow it to idle (these aren't pops or burbles, these are 9mm without ear protection loud). Above idle, the fuel flows right out the exhaust and it doesn't backfire but it stumbles and doesn't take throttle obviously because its -2 cylinders. It has fuel, air and good spark.
Heres what I'm working with: 416 LS in my 1969 chevelle, gen 3 iron block, K1 24x forged crank/rods, wiseco pistons 13:1 all said and done, ported GM LS3 heads, stock rockers with trunion, ported LS3 intake, LS7 throttle body, 65lb injectors spitting some E85, EDIT: 240-251@.050 cam, and controlled by a Holley Terminator X Max. The Terminator is wired properly because I ran this exact setup on my 6.0 before dropping valves, hence why its a 416 now. I did not change anything in terms of locations of power and ground circuits (except to the battery, relocated to the trunk with ALL the good grounding from frame/body and leads directly from the engine block), both under the dash and on the engine block.

Base cals are surprisingly good for my dinosaur tuning ability, this thing fires up in 1/2 crank! O2 sensor is placed on the good bank (driver side) and that reads RIGHT at 8:1.
I've done the simple stuff 10x over. Grounds, EFI connections, check for power at injectors/coils, plug wire connections, plug wire order, getting both clicks on the plug wires coil-side, corresponding coil wire color to the correct cylinder, injectors plugged in right, confirming not stuck injectors, getting shocked by the plugs confirming good spark, correct fuel pressure by mechanical fuel pressure gauge as well as the holley trandsucer, changing spark plugs (TR6 @.035), then I've swapped injectors, swapped coils, swapped plug wires, changed cam and crank sensors, pinned the entire wire harness of coil/injector/cam sensor/crank sensor circuits, and found nothing that jumps out at me.
Called holley tech support, and dude said its a very unusual issue, try updating firmware to rule out "ghost" problems. Did that on handheld and the ECU, nothing.
EDIT- COMPRESSION TEST yielded 3% spread between worst and best cylinders, 178-183PSI
Heres what caught my eye: I went into the firing order of the tune, and I swapped 4 and 6. Then I plugged cylinder 4 coils/injectors into 6 and vise versa, essentially using 6's electronics on 4 and vise versa, to rule out a fried driver or extraneous failure in my ECU. And to my surprise it fired right up, my idea worked, AND that issue STAYED WITH ACTUAL #6 in the real firing order. So I started getting the idea that the ECU doesn't know where cylinder 6 and 8 are momentarily, then when it gets signal back it immediately sends spark out which by that time is during the exhaust stroke, which is why it backfires.

So this points me toward the reluctor wheel. I measured my crank thrust--.004.
NOW, I ziptied my phone to the side of the block with the camera pointing down the crank sensor hole, and i DO SEE RUNOUT ON THE WHEEL, probably .030-.040 just by eyeballing it. That's a whole lot more than the out-of-spec crank thrust measurement. BUT. My question is, why would damage ONLY affect 2 cylinders in this case given the fact the crank spins twice in the 4 stroke cycle, meaning 2 different cylinders (1, and 5) share that EXACT SAME LOCATION on the wheel? And 6-8 don't share a location on that wheel, they're 90* off mechanically, and 3/4 turn off in the actual firing order, which makes my head spin.
If I had an oscillascope with good enough resolution, I would read the crank sensor output and compare that to the sample waveform. My Snap-on Modis Edge can read waveforms but doesn't have the resolution when cranking over on the starter (looks like a bunch of pins stacked next to each other), and the engine is difficult enough to crank over by hand that i wouldn't be able to get a consistent turn speed. The car doesn't have wheels/tires or driveshaft so I cant roll it down a driveway.
I've uploaded some supporting videos I took during my troubleshooting.
PARDON MY SWEARING IN THE START-UP VID.
The videos uploaded are slow motion footage of verifying cyls 6 and 8 HAVE SPARK and are not sparking at the same time (I verified its good spark using the touch test), a start-up vid after I switched stuff between 6 and 4 along with a sample of the backfiring, and a vid of my camera down the crank sensor hole-and me speeding through the vid a little finding the runout. (The wheel on the left side edge does move indicating some axial runout).

Am I correct in my suspicion that the reluctor is what is causing my issues?
I have no issue taking the engine out and apart to put a new wheel on there (with the special tool obviously) but if I don't have to do this then I would rather not.
I'm sure other people have had it worse, but theres literally 4 things left to install on this car before I can drive it out of the garage. Big sad.
I could use some extra eyes on this! Thank you all in advance!

Darn no responses, I guess I can chuck that up to having too long a initial post

Since nobody else replied I'll throw you my thoughts.

Yea I didn't have time to read the whole thing but if you are suspecting a reluctor wheel problem I'd get an oscilliscope as you describe and get it working on cranking somehow.

Put it first on the crank sensor and make sure the waveform is solid, then crank the car with CAM and CRANK waveforms on the same screen and it'll be clear if they line up. I can give you a sample known good waveform to compare that to if you need. THEN, you can use a pressure transducer on the scope and see exactly where the valve is opening and the piston is closing and compare that to the reluctor waveforms and you'll know if your reluctor wheels are right. If you have a good scope you can do all 3 at once, I only have a 2 channel scope so I have to do two steps as described.

Ol' Bernie at ATS just did a fresh video showing this concept, different car but you'll get the idea:
The Aha moment starts at around 20:00 in the vid showing reluctor wheel is off (phaser wheel, but similar). You don't need a scope like his, I use a Pico 2204A I think it is with an Ebay transducer and I can get good waveforms while cranking.

Hail Bernie. Dude is a rock star.

I think you can check it fairly easy by making a TDC mark for any cylinder and a fixed bracket connected to the block. Then use a timing light set to zero your mark should line up.
If it's backfiring it has to be off enough to show up checking like this.

I have a marked balancer so it was somewhat easy to check this, cyl 8 should show up at 90 degrees (therabouts) and 6 should show up at "TDC" or near. Well I have 2 cylinders that are shooting 180 out each! 8 was shooting at 270, and 6 was shooting at 180. I'm no longer convinced whatsoever that I have a bad reluctor wheel. Every other cylinder is working correctly so I don't understand how this can affect 2 cylinders that don't have a 180 degree relation in the firing order....

Reluctors don't go bad themselves. They are just a piece of steel. Only if they come loose from the crank (extremely rare) does any weird stuff happen

In this case I meant it would HAVE to have been damaged, bent or installed with too much runout, which I no longer think is the case. Too bad holley doesn't answer phone calls, itd be nice to send my ECU out for testing

You gotta catch them first thing in the morning when they open up

It's so rare I hesitate to mention it, but the reluctor wheel or even whole cam or crank can get magnetized and do goofy things to the sensors. Particularly true in non-factory setups where the grounds are different and accidentally cause electrical flow through these parts magnetizing them. That'd cause strange looking waveforms (which you mentioned I think). I think a bad electrical flow has to happen for a long time for that to happen though. Just thought I'd throw that out there.

I assume a magnetized camshaft or crankshaft would show up on a oscillascope? I just did a waveform analysis on the cam sensor and there was no weirdness at all, it was a perfect square wave. The plot thickens, I'm now not convinced its reluctor wheel, I may be sending the ECU out for a test. I contacted a LS/LT specializing shop near me and they have a standalone management system they use for first fire ups that theyd be willing to chuck on my motor to rule out my holley, I REALLY don't want to take my motor out to go do that though.

Yep, if your waveforms are square and sharp and not inverted and match known-good then magnetism is probably not your problem. I think the waveforms "invert" back and forth and do other odd things when magnetized. Here's another Bernie vid showing the magnetized waveforms, waveform shown near 13:30 I think, fun video to watch:


This car was magnetized from a MISSING ground causing current to go through cam/reluctor, he explains the cause and also shows degaussing, interesting stuff.

I've had some funny issues. if you have a holley you can log crank and cam counts

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Not Entirely True.

On the 24X reluctor wheels, the wheels can separate (come apart) where they are pressed together. I have seen this on a friends turbo charged truck when we tore his new rebuild down after a total failure with only 1500 miles...

Here is a thread where some one else shows it separating where it's pressed together.
https://ls1tech.com/forums/generatio...-question.html

That being said. We had another member here with a corvette that could never get his New 408 engine started. He eventually pulled the engine and found the wheel had been bent/compromised at some point. Either in shipping of the new parts or at the engine builder and they didn't catch it.... He pulled the crank, Purchased the tool and a billet wheel, Then proceeded to fix it him self. He was so happy that he was finally able to enjoy the car again. That was short lived as I don't think it had more than 1000 miles when the engine totally failed... I think He put a new crate GM LS3 So he could enjoy the car again.
Here is were he pulled the engine and discovered the bent wheel.
https://www.performancetrucks.net/fo...4/#post5511613

Thank you for the responses guys, it really helps quite a lot. EVEN if somethings not a smoking gun, theres ALWAYS something to be learned.
So heres my plan. I sent off my ECU to holley for analysis, i'm 50% certain that will fix it, if that doesn't fix it then I have no choices other than taking another look at the harness again with a hawkeye, and if I find nothing from THAT then I pull the motor/pull it apart and install the billet reluctor wheel sitting on my desk right now that I panic impulse bought. I may or may not have a LS specializing shop near me do the engine stuff because I'm sick to death of using my brain, I don't even want to look at this engine anymore. I just want to do mindless assembly and building of the rest of the car at this point lol

I have dealt with a couple different times that a stroker crank is not machined enough and the counterweight triggers the cam sensor. One of the times closing up the airgap between the sensor tip and the wheel cured the trouble. One of the cranks needed the edge of the counterweight cut back so the sensor wouldnt read it. You can take the pan off or look in the crank sensor hole while rolling it over to confirm. I have not done it but many have mounted the reluctor externally because of this. Good luck.

I am VERY interested to see how people mount them externally, do you have any pictures?

There are a few different kits. I think they all mount up front too or replace the balancer. A quick search pulled this up. As far as I know there are a few more options now. https://ls1tech.com/forums/generatio...or-option.html

Holley crank trigger setup if you have an ATI balancer (may have missed that as I know you said your balancer was marked)

https://www.holley.com/products/igni...its/parts/8619

Finally looked at the video and that reluctor is definitely moving around, personally I would pull it and either switch it to a billet 24x reluctor or better yet a 58x/4x setup instead since you can change that in the Holley. Just need to switch cam/crank from a 12V reference to 5V for the crank/cam sensors in the connector and swap the 2 pins in the cam sensor since I think those are different locations but it's easy.

Whoa, I missed the attached videos. Certainly appears to be have a bend in it. I guess I got lucky if that is what you want to call it with my motor in that it just spun as mine actually still ran and drive fine. Currently in the process of removing the crank to have a billet wheel and tacked this time around.