If you have an inductive timing light, try clipping it on each of the six wires to see if each will fire the light. If you have any cylinders that don't light, try swapping wires and spark plugs with ones that do using a process of elimination to determine if you have any bad wires or plugs.

I recently had a unique problem tuning a 235 with brand new AC plugs. I couldn't get my timing light to fire when connected to #1. I figured that my timing light had gone bad. Spent a half hour trying to figure out what had happened to the light before I clipped it to #2 and it discovered that it worked perfect. Turns out that the brand new spark plug on #1 was a dud. Never had that happen before. We take spark plugs for granted except when they get fouled and sometimes forget that they can be defective. I was lucky that the dud was on #1 because a dead plug on another cylinder that wouldn't have had the timing light attached to it might have gone undiscovered for a while.

Matt