Multiple recent ignition issues on an '87 911 (36-2 wheel/Fo

[Tippy]

Background: 3.4L 911 engine (air-cooled 2 valve) boosted to 21psi of boost (BW S366) using MSII (v3.57) and Ford EDIS-6. Using an aftermarket 36-2 crank trigger wheel (narrow/thin wheel with long teeth) with GM VR sensor.

I've spent many years tuning this setup, and have gotten pretty much everything dialed in to where I want it. Cold starts perfect, no dead pedal (lean tip-in) at any time, great acceleration enrichment for quick spool and super low TPS threshold (dialed out TPS/MAP noise), and nice idle.

Since I've gotten all of this dialed in, I am not sure if some of these issues have been always there, but had so many tuning spots needing to be addressed, I simply may have not known or noticed them. There are a couple that I know where not there before.

Here's what I have that I cannot figure out:

1. Backfiring taking off in the 1000-1500 RPM area (AFR's fine). The backfiring will not clear up until I am out of this range. It usually does not do this when the engine is cold to cool. Once really warm/hot, the backfiring starts.
2. Very jerky spot in the 2300-2600 RPM range, mainly 2500 - perfectly cyclic that will not stop until out of this range or in the 4th/5th gears. 1st and 2nd gears exhibit the most cyclic jerkiness. Engine temp seems to not play a role. Appears to be about a 200-300 RPM cyclic jerk about 2-3 back and forth a second. Mind you, this is a light car, with super light rotating mass/clutch, and super stiff road racing suspension, so it's very noticeable.
3. Intermittent cut-outs at RPM's over 4,000 usually, on or off boost. Boost levels appear to not make a difference. Again, engine temp seems to not matter.
4. Super quickly fluctuating RPM's being displayed in TunerStudio. Tach (VDO) seems to display RPM smoothly.
5. Jittery RPM lines in TunerStudio. Indicator?

#3 was the first issue I noticed quite awhile ago. #1 and #3 have been pretty recent within a few months.

What I've done to combat above.

1. New NGK plugs (resistor type)
2. Ensured gap for plugs were 0.020" for high boost "flame-out" mitigation
2. Fattened up AFR's during take-off RPM's/loads (in the 13's AFR)

The biggest thing I did before all of this I believe was playing with R52 and R56. I believe I turned the "voltage" pot a full turn to dial out an extra square wave I was seeing on the oscilloscope that was causing lots of sync errors during cold starts and revving down to idle at stoplights flooding engine. This stopped the sync error leading to stalls.

Between old datalogs and new ones since the R52/R56 adjustments, I believe is when the RPM jitter started? Not 100%.

I am wondering if I need to play with R52/56 again? If so, what needs to happen?

Are my spark plugs too closed gap wise?

[cmonref]

Tippy, re your #2:

My 1980 911SC has had a similar jerkiness all of its life (33 years for me). It had it when it was CIS and still there with MS3X. I call it the "parking lot cruise" because that is the RPM and gear scenario where it occurs. When in a parking lot, I just deal with it. It is more noticeable in 1st or 2nd because of the torque multiplication sensed by my butt dyno, but is still noticeable in 3rd. I would never be in 4th or 5th under those conditions.

With MS3X, I found that I could move the objectionable RPM/speed combo around by varying the ignition timing map. A steep curve of the degrees advanced vs RPM made it more prevalent and noticeable, and lowering the slope would lessen the effect. It is still there, just less objectionable. The effect on acceleration is not noticeable since I left the curves alone above about 3500 RPM.

It does not occur above 4000 RPM or with higher power. But that would be during acceleration and would not last that long even if it did occur.

You have boost, mine is normally aspirated. My ignition curve is likely steeper than yours, but might NOT be.

[grom_e30]

Ford EDIS-6. Using an aftermarket 36-2 crank trigger wheel

is that a typo edis uses a 36-1 wheel

[Matt Cramer]

Not seeing an RPM sync issue, so it's not the trim pots. The VE table looks like it needs more tuning as I'm seeing points where it is too rich or too lean, though.

[Tippy]

Tippy, re your #2:

My 1980 911SC has had a similar jerkiness all of its life (33 years for me). It had it when it was CIS and still there with MS3X. I call it the "parking lot cruise" because that is the RPM and gear scenario where it occurs. When in a parking lot, I just deal with it. It is more noticeable in 1st or 2nd because of the torque multiplication sensed by my butt dyno, but is still noticeable in 3rd. I would never be in 4th or 5th under those conditions.

With MS3X, I found that I could move the objectionable RPM/speed combo around by varying the ignition timing map. A steep curve of the degrees advanced vs RPM made it more prevalent and noticeable, and lowering the slope would lessen the effect. It is still there, just less objectionable. The effect on acceleration is not noticeable since I left the curves alone above about 3500 RPM.

It does not occur above 4000 RPM or with higher power. But that would be during acceleration and would not last that long even if it did occur.

You have boost, mine is normally aspirated. My ignition curve is likely steeper than yours, but might NOT be.

Yeah, I have the "neighborhood creep" too, but put a big drop in timing in the low load/2kRPM area, and it's virtually gone.

What I'm having is severe jerking at ~2500RPM. You have to let off, or it rattles the car.

[Tippy]

Not seeing an RPM sync issue, so it's not the trim pots. The VE table looks like it needs more tuning as I'm seeing points where it is too rich or too lean, though.

Hello Matt, can you elaborate on the map where you think I'm too rich?

To combat latent AE, I added a lot of fuel in the high load/idle RPM bins to combat lean tip-in that plagued this engine.

It's a very large intake with only 3.4L's.