Really need some ignition help

[ahdoman]

Ref; 1988 Paso 750
Hey everybody, first let me say thanks for all your help. I probably wouldn't own my 750 if it wasn't for this board and the resources I've pulled off of it. But, I'm trying to run down an ignition issue and I hope I can get some clarification on some things. I've searched he board here plus Google for anwers but still haven't found what I need.

Here's what's happening; bike starts from cold with choke and will warm up. As it gets warmer it develops a random miss. Once it's up to operating temp it's not too bad. If I shut the bike off and let it cool (not cold, just cool) it's real hard to start again. I've cleaned all of the connections in the wire harness and the grounds. Today I was testing spark with my timing gun. If I put the clamp on the plug wire I can see it's fairly consistent and then every few cycles it appears to miss but it's random. If I put the clamp on the wire leading to the coil (trigger wire from Digiplex) it shows a real strong strobe but it's too fast to see if it's dropping a pulse. It's the same for both sides of the system. Also, if I pull one of the plug wires the idle will drop and it does the same for the other one telling me that both cylinders seem to be running the same.
Here's a couple of questions;

1) Why would the pulses on the trigger wire for the coil seem to be stronger and more consistent and faster than the pulses on the plug wire from the same coil? Am I just seeing things?
2) Does anybody know how to test the crank position wires (2 of them) to the Digiplex? Is one the trigger signal and one the ground? Should there be a measurable resistance value between them?
3) I know the air gap between the crank sensor and the crank is critical but how is it possible to measure it?
4) If I pull a vacuum on the tube leading to the Digiplex should it increase the advance?

Thanks for all your help. I'm hoping I don't have a bad Digiplex unit and that's what I'm trying to determine or maybe it's something else in the system.

[bobst2]

well the single pick up set up has to crank a little to decide what is # 1 cyl as for air gap it has a single bolt and an o ring seal i would not worrie too much about it. unless you hear noice (touching) the pick up is cheep i just bought one for like 20 bucks it is the same as a fiat uno it cured a few wierd things. i am pretty shure high vacume should increase timeing & lo vacome should retard. as for digiplex box they are not hard to get i have a brand new one & a known good sitting in my shop you could buy for a good price.

how does the bike behave when it truley gets warmed up no missfireing or stalling i mean the oil up to temp

i am shure with the help around here and some time you can cure the problem

[JWilliam]

It's cold starting so that is a good sign. I had a top end misfire on my 906 and that was due to very low primary coil impedance over-driving the ignition module. It would get very hot even street riding but on the track it was too much and had a chronic misfire over 7000rpm. I stopped this by swapping the OE coils for Kawasaki GPZ500s units and the ignition was a lot better. Still, the one thing it does is be reluctant to start if I have stopped the engine for 5-10 minutes. Cold start OK. Hot start if I have stopped the engine for 30sec still OK. Your Digiplex and crank sensor either work or they dont at all and they are.

[ahdoman]

Thanks for the input JWilliams. So if my Digiplex or the pickup weren't working then it would be a total failure? It either works or it doesn't?

I've been thinking about trying some new coils and wondering what coils would work from other bikes. I have an extra pair from a Kawi ZX1000 (GPZ1000 engine) I was thinking about trying but I don't know enough about coils to know what is swappable. I know the Digiplex can be a little sensitive so I don't want to upset it.

I spent most of the day yesterday trying to install a new Electrosport regulator only to find out that it won't work. The guy at Electrosport said that it should but upon installation I found a few issues. The first is that their unit generates the 12v signal to your battery light to tell you if there is a problem. The stock VR is a ground switch. It's opposite of how the Electrosport unit works. Also, for some reason, the stock VR pulls 12v from the battery and sends it back down to the stator on the red wire. My guess is that is part of the feedback system to sense if there is a problem. When the bike is running there is AC voltage on the 2 pink wires but nothing on the red from the stator. But, there is DC voltage on the output of the stock VR pin that the red wire is connected to. According to Electrosport their model(s) ESR510 & ESR515 are direct replacments for the Ducati 906 (I have a 1988 750 which is essentially the same Digiplex ignition system). Apparently not.

[jayh]

Electronic ignition modules are notorious for displaying the start - run -misfire - stop - wont start , behaviour that you are describing ,when they are on the way out. (itermittent fault very hard to trace until it is happening)

I have no experience with the digiplex ignition itself,but most work on the same basic principles,only changing in the way that they gather the information needed to fire the coils at the correct instant.
I have had several display similar symptoms over the years,one in particular on an XR600rf would fire
back up again after being slapped hard against a solid object .(I'm not recommending this approach,but it worked until I could get my hands on a xr250 unit as a temporary replacement and to confirm the fault)

I would not assume that their is nothing wrong with the digiplex,until proven otherwise with a substitute.

Of course start with the simple stuff first,plugs,coils,leads.If all else appears to be fine,it might be time for a substitute. J.

[JWilliam]

Thats right, it is an odd system where the Digiplex triggers the spark by dropping the voltage thru the coil to earth. Still, the coil is only a 'output' from the Digiplex and altering the impedance doesn't affect the unit in a negative way. I measured the impedance of the OE coil and got something like 0Ω84 and the GPZ coil 3Ω5 so the OE coils were using more than 4 times the power. Lights are a lot brighter now I can tell you. I also run a cooler spark plug (Nippon Denso X27EPR-U9) with a 3mm spacer to pull the sparking tip out of the combustion chamber (for preventing fouling due to the cooler plug) and that has affected starting when warm. I use an E-Z-Start spray for cold starting so as you can tell I shouldn't be surprised by some reluctance. I'm not saying you should change the plug number by the way.

[ahdoman]

So, let me make sure I understand this; each coil has two wires going to it; one is the power, the other is the trigger wire from the Digiplex. In order to trigger the coil to fire the Digiplex momentarily ties the trigger wire to ground? I assume it gets its trigger activation signal from the crank sensor? Is the Digiplex system a wasted spark system?

[jayh]

I'm assuming that the digiplex works as most ignition units and disconnects the coil from ground,which in turn causes the magnetic field in the primary coil windings to collapse,the energy of which is passed into the secondary windings and onto the spark plug as a high voltage burst.

The Bosch amd Kokusan units with two pickups are both wasted spark as they fire both cylinders every rotation,regardless of power or exhaust stroke,not sure if the digiplex works the same or not,every chance it does though. J.

[JWilliam]

If you want to swap the OE coils then some of the wiring has to be junked and a second DC bus sent to the horizontal cylinder coil. No big deal. The Digiplex works by having a permanent live to each coil and drops this to earth to collapse the magnetic field and trigger the spark etc. The Electrosport reg/rec should work, just remember the Paso alternator is a 2 phase, wire up for negative, positive and over-volt and you should be alright. The charge warning is another problem, I have an idea on that but since I haven't run the engine to test it I'll say no more.

[bobst2]

be shure of one thing the pick up coils are temp sensitive,haveing one start to fail when warm
is what happens. i chased mine for a long time when my brother a certified master technician by several manufactures asked me if i forgot everything he tought me and told me just change the pick up or to shut up and dont ask him any more about it. i did and for 19 dollars from a fiat uno i have had no more igntion issues.
it may not be your problrm but they are hard to diagnose as when they cool (buy the time you limp or push the bike home it will cool enough to test just fine) if anyone would like there bike to backfire and stall at 3200rpm when warm i can send them that problem via the mail called a defective coil that tests just fine when cold.
it dosent cost much to be shure and takes under 10 mins to change be shure to put a fresh o ring as well

[ahdoman]

Thanks bobst. Does anybody know the Marelli part number on the senor? Not the Ducati part number.

[bobst2]

sensor # sen8e there is one on ebay thay want 65.00 for it but i got mine fo 19.00 cant find the name where i got it. it was in florida found it from a fiat uno