Are you sure you have high compression pistons ?
The original pistons also have a dome, it`s just a bit lower.
Original vs hicomp piston



I actually read about someone with the same problem in a german forum (the guy had a 89 900SS) and his problem was solved simply by replacing the coils. Had you tried others ?

G.

Thanks G but due to length of diagnosis, have been ultra thorough here - pistons are clean and 'V' shaped dome is sharp and taller than std, I have several stripped engines here. Re: coil swap, no I haven't tried it and I can't understand how it could help or cure problem to be honest, can you? i found best attempt to start and remain running was with a fast churn speed and a 'rising' throttle. Spoke at length to the owner of the other problematic 900SS and so could share all symptoms and things tried and coils was one of them.

Well I think I gave a reason. Properly working coils are crucial. Specially on engines with higher compression that demand more ignition power.
In detail. Typical 4 cycle engines at maximun torque have the lowest consumption and the lowest demand of ignition power (30-40mJ). At lower rmp ranges were the carburation is more difficult and the specific consumption increases the demand of ignition power can increase drastically. Depending on combustion chamber, exhaust and timing it can triple (90-120mJ). At max. power output and during accelleration the fuel demand and the demand for ignition power increases.
Original coils will supply about 40-50mJ. (a DynaCoil ca 110mJ - therefore what most people note first is the improvement when starting the engine).

red: power, green: ignition (spark) energy, blue: torque

The secondary power of a coils is usually not a problem as most range over 16kV. At operating temperature a spark plug requires about 12-13kV to build up a spark and 16-18kV when it`s cold or the spark plug is dirty.

The power output will be reduced if the coils are not supplied with 12-14V (typical voltage loss on most carbed Pasos seems to be at least 2 volts, therefore the wiring mod). Spark plug wires can age. The core can brake (typically the cables with a carbon core) or the isolation can become brittle and the spark may jump to the frame or engine. (I did find both symptoms on a friends Fiat Spider 124)

I just figurred borrowing some coils to test would be worthwile before swapping the complete ignition system because it`s way less work. It may be one of the problems.

The reason why I`m sceptical about the swap of the original Digiplex is that there`s one bike in my neighbourhood with HC pistons and a close friend owns a 906 also with HC pistons and they both run just fine with the Digiplex.

G.

(side note: with starting problems specially with a cold engine the pick-up clearance can be reduced by 0,2-0,3mm. If the problems are in the high revv range you can increase the gap by the same amount. Standard clearance should be 0,7-0,8mm. When cranking the pick-ups should output 0,5-0,6V and at idle 1,6-1,9V)

I'm with G on this, it is time to swap known good parts, not necessarily new parts. Coils,plugs and wires are an easy swap to do. Coils may be shorted,open or have internal insulation issues. Seems to me I remember you stating your 5 ohm coils measure 5.8ohms,that is too much and would to me indicate their need for replacement.
The first thing I would do is run a temporary power wire to the coils directly from the battery. that would eliminate the ignition,kill switch and a voltage drop issue related to any wiring issues. If that does not help,I think the next step would be to ask someone at the DOCGB if they would be willing to help you try some parts from a running bike.
If you can't find some one local let me know and I will send you a working digiplex and coils to try.

I just read on the website of the german Duc tuner and parts dealer KÄMNA that with HC pistons you must use colder spark plugs.
Depending on compression ratio one should use 95, 98 or 100 octane fuel and rejetting may also be required.

G.

in my SSS engine i have 13:1 HC pistons (12.2:1 effective) and an extreme cam diagram (NCR "p" cams) and starting was never an issue; I have it set-up with 5-6 deg base advance.

The only issue I has was a slight difficulty in starting it from cold, specially with "dry" FCRs (I use a vacuum operated fuel pump) but other that that , nothing else.

now with the Dynacoils, the starting is improved massively.

As far as I know with the Digiplex g there is NO base advance (pickup cannot be moved), therefore at the start, the advance is 0 (this is also partially confirmed by the ignition maps you can find on the 906/digiplex workshop manual)

you could eventually trick the Digiplex by disconnecting the vaccum pipe so that it keeps the ignition advance at "zero".

In any case, since yoou already checked and double verified any potential mechanical fault, the only thing that remains is an electric "ghost", therefore if I was you I would ask around to see if you can borrow a working Digiplex, coils & plug leads then I would go to the local Fiat spare parts dealer (or online) and look for the pickup which is a standard Marelli item used in several Fiat models (ducati network will charge you 3 times, but still worth checking)

thought I would add the epitaph (and moral) to this 'storm in a teacup' thread for anyone who might still be interested and to serve as a possible guide to future approach of same :- I fitted the Kokusan system complete retarding static by 2.5 degrees and the engine started and ran perfectly. There wasn't much point in adding this at the time (easily over a year ago) as I was pretty frustrated by some of the feedback at the time. I would like to thank anyone who did help in a positive way but I can't face trawling back through the posts to find you and probably never will.
My general advice? In a desperate attempt to be 'right' all the time, don't over-complicate something that could be simple/logical and possibly staring you right between the eyes.

May all your Pasos run faultlessly and your health not deteriorate

Glad to hear you got it sorted. The Kokusan system is much more durable, and also more adjustable, so its a worthwhile change. Oh, and you can still order parts for it from your dealer. I have a setup sitting on the shelf, waiting for my Digiplex to die some day, but since I just got a brand new replacement one, so it may be another 15 years before I need it.

Did you also go with some higher output coils? I added them to my bike and I can surely feel the difference. Then again, the old one were really showing their age and the leads were as hard as rock.

What brand plugs are you running in the 900 motor? I found the Champions to work pretty well in the 750 motor.

Cheers, Phil

All standard coils at the moment Phil although I might try updating with Dyna at some point plus Ignitech etc. I use NGK D8EA on all my 900 2-valvers partly due to my long term satisfaction with them in Lancia 1600HF

The Kokusan system has two pickups in the case that are separate, and you can advance or retard the timing, even between the cylinders, or together. its obviously not easy, as you need to pull the side cover off, and use a degree wheel and set the screws to hold them in place, all the while not pouring all your oil out. then you bolt it all up, and test ride it. if you don't like it, you have to pull of the side cover again.

The Digiplex has only one pickup, and has no way to adjust it. The only way to change the timing on the Digiplex is to reprogram the unit, which isn't possible as far as I know. It is what it is.

Cheers, Phil