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 Post subject: Re: Technical (non Paso) 1990 900ss
PostPosted: Thu Oct 28, 2010 8:45 pm 
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Joined: Sat Sep 18, 2010 8:00 pm
Posts: 136
Location: UK
model: 900 SS
year: 1990
paso750 wrote:
But, according to giocast that´s all ok.
I really wonder what the real problem is.

G.


Hi all
Just to 99% put this one to bed - After over three weeks of trawling through various Forums, I stumbled on a thread describing a 1991 900SS with identical symptoms to mine. His problems started immediately he fitted new high compression pistons, and he eventually cured them by changing original Digiplex system to Kokusan (with adjustable pick-ups) I immediately went to peer through spark plug hole at TDC and Voila! - that unmistakable 'V' wedge clearly atop of piston. I am incapable of fitting Kokusan pick-ups right now, despite having all necessary bits, but will post as soon as it's running. :beer:


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 Post subject: Re: Technical (non Paso) 1990 900ss
PostPosted: Thu Oct 28, 2010 9:17 pm 
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paso grand pooh-bah
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Joined: Sun Jul 18, 2004 12:00 am
Posts: 5272
Location: southern Germany
model: 750 Paso
year: 1987
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
Image
Image

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.


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 Post subject: Re: Technical (non Paso) 1990 900ss
PostPosted: Fri Oct 29, 2010 9:41 am 
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Joined: Sat Sep 18, 2010 8:00 pm
Posts: 136
Location: UK
model: 900 SS
year: 1990
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. :thumbup:


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 Post subject: Re: Technical (non Paso) 1990 900ss
PostPosted: Mon Nov 01, 2010 4:58 pm 
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paso grand pooh-bah
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Joined: Sun Jul 18, 2004 12:00 am
Posts: 5272
Location: southern Germany
model: 750 Paso
year: 1987
Quote:
and I can't understand how it could help or cure problem to be honest, can you?

Yes I can, but I don`t why swapping a Digiplex ignition system with a Kokusan would help. (I hope I understood that correctly)
Considering perfect conditions (timing set correctly, pick up distances correct, ignition working perfectly, no electrical issues etc) why would a different ignition module that basically only provides the ignition curve work any better ? Although it might, which I doubt, I rather think the real issue is cured by the swap itself meaning that it`s not the Kokusan making the difference but ie the changed wiring, pick-ups, coils or whatever other change comes along in such a swap.

Simplyfied, engines with more cubic inches or higher compression require two things. More starting power (starter motor, battery etc) and a stronger ignition. The latter you will only get by ensuring there`s no voltage drop at the coils and if that isn`t enough by stronger coils and good ignition cables.

I do hope you get your bike running again soon ! Keep us posted.

G.


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 Post subject: Re: Technical (non Paso) 1990 900ss
PostPosted: Tue Nov 02, 2010 11:59 pm 
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Joined: Sat Sep 18, 2010 8:00 pm
Posts: 136
Location: UK
model: 900 SS
year: 1990
paso750 wrote:
Quote:
and I can't understand how it could help or cure problem to be honest, can you?

Yes I can, but I don`t why swapping a Digiplex ignition system with a Kokusan would help. (I hope I understood that correctly)
Considering perfect conditions (timing set correctly, pick up distances correct, ignition working perfectly, no electrical issues etc) why would a different ignition module that basically only provides the ignition curve work any better ?


At the risk of repeating... You say you can understand why a coil swap would help but don't then go on to give your reasoning ?? - I added the 'can you?' as a way of being open in the discussion, but obviously need to re-word now :- there is nothing logical/there is no sense, in saying that a stronger spark with a quicker response time (once triggered) would help my condition in any way whatsoever... it's retardation that I am looking for - have you read all my notes? :wink: and on the contrary, (also in my notes) as the kokusan system differs from my standard set-up in that it does offer a retardation facility, it is a totally logical solution and the only direction to go in now.
Just to recap, and I ain't doin' it again.. :- I have all the symptoms of an engine that is firing too early/ I can confirm that I have aftermarket high compression pistons/ I have read 3 owners accounts with similar set-up and symptoms one of which were identical and having spoken to him at great length, am more than happy that this is the solution. I have had a great deal of help but mainly encouragement from this thread, and will certainly post again when the swap is completed. Best Regards :beer:


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 Post subject: Re: Technical (non Paso) 1990 900ss
PostPosted: Wed Nov 03, 2010 9:05 am 
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paso grand pooh-bah
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Joined: Sun Jul 18, 2004 12:00 am
Posts: 5272
Location: southern Germany
model: 750 Paso
year: 1987
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).
Image
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)


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 Post subject: Re: Technical (non Paso) 1990 900ss
PostPosted: Wed Nov 03, 2010 10:14 am 
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Joined: Sat Sep 18, 2010 8:00 pm
Posts: 136
Location: UK
model: 900 SS
year: 1990
paso750 wrote:
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.

I understand your reasoning here G, and thanks for reply - there is quite a choice out there in aftermarket H/C pistons, and maybe mine are impractical/too high for a road bike. Best regards.


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 Post subject: Re: Technical (non Paso) 1990 900ss
PostPosted: Wed Nov 03, 2010 11:10 am 
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paso grand pooh-bah
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Joined: Tue Jul 29, 2008 9:50 pm
Posts: 3189
Location: Hilltown,Pennsylvania
model: 906 Paso
year: 1990
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.

_________________
Ducati,making mechanics out of riders since 1946
There's no problem so bad that a little fixing can't make it worse! : )
If it ain't broke keep fixin it till it is
88 750
90 906
92 907ie


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 Post subject: Re: Technical (non Paso) 1990 900ss
PostPosted: Wed Nov 03, 2010 12:23 pm 
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paso grand pooh-bah
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Joined: Sun Jul 18, 2004 12:00 am
Posts: 5272
Location: southern Germany
model: 750 Paso
year: 1987
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.


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 Post subject: Re: Technical (non Paso) 1990 900ss
PostPosted: Wed Nov 03, 2010 3:28 pm 
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Joined: Thu Nov 29, 2007 2:38 am
Posts: 293
Location: UK
model: 907 I.E.
year: 1990
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)


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 Post subject: Re: Technical (non Paso) 1990 900ss
PostPosted: Mon Oct 07, 2013 8:48 am 
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Joined: Sat Sep 18, 2010 8:00 pm
Posts: 136
Location: UK
model: 900 SS
year: 1990
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 :thumbup:


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 Post subject: Re: Technical (non Paso) 1990 900ss
PostPosted: Thu Oct 10, 2013 4:23 am 
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paso grand pooh-bah
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Joined: Sat Sep 03, 2005 12:00 am
Posts: 1318
Location: SF Bay Area
model: 750 Sport
year: 1990
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

_________________
Duc in the Bay
1990 750 Sport x2-Rosso Blanko (900ss copy) & Nuovo Nudo (Scrambler project)
1991 907 -mostly stock
2002 ST4s - Lots of mods.


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 Post subject: Re: Technical (non Paso) 1990 900ss
PostPosted: Thu Oct 10, 2013 11:49 am 
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Joined: Sat Sep 18, 2010 8:00 pm
Posts: 136
Location: UK
model: 900 SS
year: 1990
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 :)


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 Post subject: Re: Technical (non Paso) 1990 900ss
PostPosted: Thu Oct 10, 2013 2:58 pm 
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paso grand pooh-bah
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Joined: Sun Jul 18, 2004 12:00 am
Posts: 5272
Location: southern Germany
model: 750 Paso
year: 1987
Quote:
The Kokusan system is ... also more adjustable


:?: what do you mean


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 Post subject: Re: Technical (non Paso) 1990 900ss
PostPosted: Fri Oct 11, 2013 4:04 am 
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paso grand pooh-bah
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Joined: Sat Sep 03, 2005 12:00 am
Posts: 1318
Location: SF Bay Area
model: 750 Sport
year: 1990
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

_________________
Duc in the Bay
1990 750 Sport x2-Rosso Blanko (900ss copy) & Nuovo Nudo (Scrambler project)
1991 907 -mostly stock
2002 ST4s - Lots of mods.


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