Volvo Repair: 760 miss firing, vacuum leaks, ok check

Question
I have a 1986 760 turbo, it had developed a bad miss along with spark knocking . Checked the plugs, wires, dis. cap all seems to be fine.It seems to be firing on all cylinders.  Any sugguestions. Thanks

Answer
   Pull  all spark plugs to compare all readings, one more or less wet is culprit, look in that area.
The cap and rotor needs to be removed and checked to make sure no carbon tracking or cracks. If these and wires are ok, check for vacuum leaks by spraying some carb cleaner around the hoses and intake manifold area. Next listen to make sure all injectors are firing but listening for click at injector. Do this by using stethoscope or screwdriver placed at injector and ear at handle end of screwdriver to listen for clicking.