As I'm tearing down the 496 to get #5 sleeved, I'm also trying to eliminate the HORRENDOUS spark scatter. With the EFI timing locked out to set the base timing at the recommended 12°, it randomly jumps anywhere from 8° to 22°. Zero consistency, fully random within that range.

-The oil pump drive/gear is NOS OEM Mopar
-The bushing is brand new
-The distributor is an Edelbrock EFI unit with no advance mechanism, so no slop there.
-Distributor rotor is MSD adjustable and the tip has been aligned to the terminal at 22° BTDC for optimum alignment from 12° - 32°
-Shaft collar on the distributor set to the top of the drive gear to prevent gear climb.

It also did this with the previous MP distributor. The only solid timing I ever got was when I went to a 36-1 wheel with megasquirt. Not an option with the ProFlo4.

With the valley pan off, I can see 2 things happen: I have a few crank degrees of slop in the distributor tang to the drive, and the drive gear to the cam has a fair bit of slop too. I can eliminate the slop in the tang with a quick weld bead and file to fit, but I have easily 5° at the crank due to just the gear. Cam is a CompCams nitrided solid flat tappet unit and has less than 1000 miles on it and shows no visible wear. How much play is normal?

Finally, how much can the timing chain play into this? I can hardly imagine it can bounce much even if the chain had some slack. Lots of drag from the cam should keep it steady, and wear would be expected to be a slight retard from the original set point. I'll post a video of the slop later this morning.

I would be curious to know if it's something to do with the efi system rather than mechanical.

Maybe pull the distributor and spin it with a drill motor to see if the system is capable of steady timing before chasing anything else.

As stated, the problem was present with the MP distributor before that, both when it ran the megasquirt and also carbed with a MSD Digital-6. It was terrible enough that I went to the 36-1 and just used the distributor for distribution.

So is this problem evident with a different timing light? Seems like you have the mechanical part of it covered so it's possibly the tool

Gus

2 timing lights. Old school sears and after suspecting it didn't like the MSD, I bought an MSD light. Same result.

Lash in the gear

Lash at the rotor - gear and tang moving

Lash at the rotor, gear only

Just thoughts, nothing else. First off, is it from one source or many sources? I'd tape a protractor to the rotor and actually measure the contribution of each factor (or mark the rotor and use the rotor diameter and spacing of the marks to determine angle). That would at least give you some hard numbers to look at. I'm struggling with the clearance in the cam gear to the distributor gear being the problem. The oil pump is a constant load on the gearing and I can't see how this would jump. Slop in the distributor tang, measurement with the protractor should tell you what this could be. Wear in the timing chain? Boy, with a flat tappet cam I would assume that the camshaft always has some load on it. Varying timing due to camshaft movement, again with a flat tappet I would assume it would not be moving fore and aft as a roller could.

But I would start out with the measurements to get a reality check on what you are seeing with the timing light.

6,

I agree with you on all those points, which is why it is so baffling to me. I would expect the oil pump to keep the gear hard against the cam, and the decent sized flat tappet cam to keep the cam driven into the block and timing chain taut as well.

Out of morbid curiosity, I tested the lights on my old International TravelAll's 392 and it was so steady is was mesmerizing.

Is it possible the Ignition box is sporadic?

I will get a dial indicator on the rotor tip and math-out each of the 3 possible contributors: tang, gear, and chain. Block is cracked in #5, so I can't run it to check anything that way anymore until I tear it down and get it sleeved.

I'd start with the measurements. If your scatter is more than what is possible mechanically, then I'd start looking at something in the ignition system.

What are you using to trigger the ECM? If it's the distributor pick up, there is a HUGE amount of electronic noise going on under that cap. On my dinosaur Commander 950 I had to shield the cable right from the pickup to the ECM just to get a tach signal that wasn't 3500 RPM wide.

Make sure none of the harness is near any plug wires or the alternator. ANYTHING that can emit RFI can potentially mess with you.

Kevin

Measurements will tell a big part of the story.

The ECM is triggered by Edelbrock's own semi-proprietary distributor. It is a 7 narrow -1 wide window arrangement for both crank and cam sync. I "thought" it was optical, (tooth passes through a slot to trigger) but either way, the wires for it are laying in the valley under the manifold with ALL the rest of the EFI wires, including injectors, and alternator field. What's the recommended shielding material? I'll also try to isolate as much of that sub-harness as possible.

Honestly, the only common element at this point is the digital-6+ box. I had this issue when it was carbureted too. I have a spare and also 2 regular 6AL boxes.

Check the slot on the Dist gear. There are some in the aftermarket that the slot where the Dist shaft engages that are "fishmouthed" from the manufacturer that will do exactly what you described. It should fit snug not loose like they are being made. Had this problem where the Holley dist would keep loosing or fall out of phase. Put a light on it and it would just keep moving around. You can call me with any questions.
Todd

Right, so as stated, it was a completely different distributor and ignition system.

It wouldn't be the first time I heard of a digital 6 box acting weird. I would just hook up a regular 6AL and a carb and see how it runs.

Gus

Excellent Post, I am Going thru and modifing everything you just described. on my stroker
and agree totally' awsome post. You must be a serious machinist/engineer. good luck

Alright... .065 travel at the tip of the rotor for the gear and tang combined. That's 3° of rotor movement, and 6° of crank (timing) movement. That does not account for the timing chain, but there is only a very low chance it could contribute with the loads it is always under by the valvetrain and oil pump drive. I am seeing WAY more than that on the timing light. It would seem that the movement is now limited to the electronic side - either RFI/EMF or the box itself. The Digital 6 is the only common denominator. I cannot say from memory if it was better or worse anywhere along the way, but it was never good. RFI could have always been a factor, since I have always run the distributor lead and the alt charge cable under the manifold near each other, with the exception of the last 6 months, but the problem was still present to the point of searching for a solution and changing out lots of hard parts.

Plan is now to shield the cables from pickup to ECU first and look for an improvement, then swap the box out for my spare digital 6 first, then a regular 6AL. Engine is going to be down until it gets sleeved, so it will be a good moth or two before I can update.

...and yes, cryplydog, I actually am an engineer and machinist. Good call.

Close. Multiple distributors (stock, MP (both with advance and locked out for EFI), Edelbrock), multiple ECU's (Megasquirt-2/ProFlo-4) or direct feed from distributor pickup to ignition box (carb'ed), including reversing the p/u wires, but ALWAYS the same Digital 6.

When you get your engine back together, if it were me I'd be tempted to throw a point style distributor in there and see what the results are. Since the timing "scatter' is occurring at idle you wouldn't have to worry about point float and with points, EMI effects of any kind.

I understand you're thinking that it's stack up tolerances (and may be), just saying I wouldn't rule out the current distributor/ignition system as it seems that's a common issue observed among Pro Flo efi users.

Any time you use a mech dist it will lift and fall as the engine load changes due to the cam moving fore and aft,
try to stop most of the dist lift and fall(thats pretty simple using shims)

Agree, and has been done for a long time now. I have the shaft collar setup on there with zero lash vertically. manually verified with the valley pan off. Fore to aft should be limited since it is a solid flat tappet, so tapered lobes pulling the cam back in the block.