My bike runs on one cylinder

[maxmar]

I have a 74 ducati 750 gt
It runs on one cylinder but it is intermittently.
Since the last 200 miles. The bikes makes strong burns in the exausts
I checked valve clearance on the incriminated cylinder , seems little low on admission .
I cleaned the carb , fuel filters , put new spark plugs , doesn t do anything !!!
it seems the spark is not always blue , but also yellow ? Spark not enough power ?
I have an electronic ignition and coils from modern ducati ...
I have to change battery every 7 to 8 month , but after charging battery again have the same problem , i know those electronic stuff need more voltage to turn properly can it comes from battery? I tried with the power charger on the battery but same problem .
Anybody has an idea ?
I plan to cross the coils to see what happens .

[Spagjet]

you mean a roundcase GT I suppose being from '74, so maybe by modern Ducati you mean electronic ignition like that found in the squarecases. if that's the case it could very well be the high and low speed pick ups. I have had my share of nightmares with my GTS pickups and had to change them a few times over the 25 years I have been riding it. they can be a bugger to diognose because of the intermitent thing but if it really is the pick ups they pretty much either work or they don't. Some times you can think it's intermittent because of it cuttin gin and out and the revs change. One thing to try is to start it up, if it's running on one pot slowly increase the revs till it goes over 2200 rpm, if it's one of your low speed pick ups all of a sudden it will run on two pots and then cut back to one as you drop the revs back under 2200 (and vice-versa for dead high speed pick up). If it is the pickups you can check the wires and the plugs for continuity but if you can take them out of the equation there's nothing you can do to fix the actual pick ups except get an exchange re-conditioned unit. This can be a bugger sometimes, I did it one time years ago and installed the one I got back and it wouldn't work on one of the pick ups either just a different one to the one on my own faulty unit, so I sent it back and got them to send me another one only to have a similar problem, third time lucky, nup, the third one didn't even have the backplate sitting square as it was skew-iff when they poured the shellac in. Fouth one worked.

Nowadays it's cheaper to get one of those digital ign set ups, I haven't gone there yet but will the next time my pick ups decide to retire from active service without notice.

[wdietz186]

You don't say what electronic ign. you are using. The only two that I have any experience with are the Dyna [excellent system!] and the Lucas-Rita. If you are using the Dyna system the recommended coils for street use are 5 ohm although the 3 ohm coils have been used with good success but they run a bit hotter [draw more current]
The Lucas-Rita system uses two 6 volt coils wired in series and works pretty well with the supplied coils or with at least a 3 ohm dual output coil. You may want to check the resistance of the primary side of the coils you have if your system a Dyna or Lucas. Too low a resistance can cause a misfire after running a while.

Now to the real [most likely] cause of your problem, Both of those ign. systems and most all the modern Ducatis won't run when the battery voltage is less than approx. 10 volts. There just isn't enough energy to fire the coils. The reason the voltage isn't sufficient is because the alternator can't keep up with the demand. The combination of low output, small gauge wiring and a joke of a fusebox all conspire to drain a charged battery pretty quickly if the bike is ridden at lower speeds and in traffic. The solution is a higher output alternator and ideally a heavier gauge wiring harness to the battery. The popular solution is an ST2 alternator rotor or an aftermarket unit with stronger magnets and if possible the ST2 stator along with a better rectifier/regulator. A search on this site will yield more info on the solution of this problem.