2 cylinders fail from time to time

[t6nis]

Hi guys.

For starters this hasn't happen a lot, but it still gets me worried. Sometimes when i drive 2 cylinders fail, 1 and 4. I know what u are thinking, that it'd def. the coil, but i have tried it with another coil, the bike was standing tho, and it didn't change anything. The only thing that seemed to make a change in any of this is when i unplugged or plugged my CDI signal cable, or fiddled with the wires.  The wire ends are replaced with new ones.

Are these signs that the CDI caps are going bad, or am i missing something?

[ventYl]

The best way to answer this is to get oscilloscope, wire it to both coils primary circuit (+12VBAT & wire from CDI to coil primary winding) and compare how far voltage drops on each one during recharge. In your case if outer cylinder coil's voltage drop will be significantly lower (which means that the voltage during recharge will be higher!) than the other, your CDI is eligible for final stage transistors replacement. I never saw CDI with blown capacitors so I cannot say how exactly it behaves but most probably the CPU would be unstable thus significant discrepancies in coil charge timing should be observed.

[t6nis]

Is there another way to do this, If i don't have access to an oscilloscope?

[ventYl]

you have no chance using digital and/or analog voltmeter... but if you are able to put together some simple circuit then analog comparator + voltmeter + adjustable voltage source + LED should let you roughly measure the voltage drop.

in extreme case circuit as simple as one LED with two resistors, one pot and voltmeter should allow you to compare voltages across both coils (due to limitations od LEDs you cannot get exact readings using this but simple compare should be enough for you to tell if CDI is OK).

edit: Above won't work. The circuit will effectively form bridge with so low impedance that the circuit either won't be able to light the LED or giving completely wrong output or drawing so much power something will burn.

[greg737]

In your case if outer cylinder coil's voltage drop will be significantly lower (which means that the voltage during recharge will be higher!) than the other, your CDI is eligible for final stage transistors replacement.

If, as you've already pointed out, the coil in question is good and healthy then I'd have to agree with ventYl that the problem might be the CDI.  I feel for you, because that's my old CDI you're running in the bike, right? 

It does sound like the transistor that controls the cylinder 1 & 4 coil may be having trouble grounding the coil properly.

Of course, the actual dynamic of your ignition issue is probably a bit more complicated than just the slow demise of the CDI's Darlington Transistors (one for each of the two coils).  The CDI may have been delivering a weaker-than-normal spark to cylinders 1 & 4 for some time which could result in carbon buildup on the spark plugs, making misfires even more probable.

I guess the capacitors and transistors in these old CDIs weren't intended to last forever.  They do the "hard work" in the CDI - capacitors dealing with 12volt power and the transistors dealing with grounding the primary winding of the coil.  Those activities produce heat as a byproduct which ages the component.

[t6nis]

Could someone tell me what capacitor should i use on this old CDI of mine? I know the diod for the D1 is the same as the D2, but the markings on the capacitor thas just above V1 is burnt. I know that getting this to work is a long shot, but i tought i would give it a try before i start to tear apart my currently working CDI.

[ventYl]

IIRC I've written it somewhere here on forum into different thread. Together with info that from what is available off-the-shelf, TIP150 switching transistor works as replacement of original switching transistors. (at least worked for me).

[t6nis]

Yes, i know that i can use the TIP150 for replacements, but what about the burnt component? Does anyone have any idea what it was?

[ventYl]

i can take a look at my repaired CDI at home and read out values from capacitors.

[t6nis]

That would be great!
Thanks in advance :)

[Gouraami]

Oh no! Good luck with the repair, I hope that replacing the components sorts it out

[ventYl]

The two larger are 47uF / 50V electrolyte each.
The two smaller are 10uF / 63V electrolyte each.

Voltages are not mandatory as long as they will be above 12V. I got the ones which were available off the shelf as long as they were small enough to fit into box.

direction of negative lead can be seen in the picture

I took the time and swapped the unit into motorcycle if it runs and it is (the bike from which this ignitor originates is already dismantled for spares and the other uses Pietcard replacement). Unfortunately I cannot write the ignitor ID code because label is completely washed down (but the rest of bike looks like England-specced 1991 Bandit).

edit: I found out the image of your burned CDI just right now. The positions C30 and C7 are both free (unconnected) on my CDI. I did not trace the circuit but it is possible that C7 capacitor is part of RPM limiter on some newer Bandit CDIs (post-1992 Bandits should be limited to 170km/h).

[t6nis]

Okay, but whats that black thing? The thing that's just below C7?

[ventYl]

Dunno :S

PCB label is V1 and text printed on part says: [M] 7k270 s06.

Datasheets say it is varistor which nicely correlates with V1 label printed on PCB.

[t6nis]

This just sucks, so i think there is nothing to do with this CDI, i should have another broken one lying around somewhere, but i can't seem to find it. I guess there is nothing to do but to change the transistors and caps just in case. I checked the spark also, it was kinda purple and weakish. I had new spark plugs in and i found out that just after 40 km ride or so, one of the brand new spark plugs was miss fireing, or the spark was not between the electrodes. This bike is giving me a headache...
But i shall overcome this! :)