Timing light doesn't work on #8 wire

[ghiggi]

76 Scout, 345, 727
pertronix ignitor and flamethrower coil
new heavy duty wires and new plugs

I was trying to time my engine (using the power timing guide) but I was still having some problems getting the engine to smooth out, and I was curious to see where the timing was. I connected the timing light to the #8 wire and it wouldn't work.

At first I thought it was broken, but then it flashed once. As I watched it, it would flash irratically every 10-30 seconds. I switched over to the #6 wire and it worked fine. I checked all of the other wires and it wouldn't work on #8 and two of the odd # wires.

I had to run an errand and didn't get back to the truck for about an hour. When I started it again and checked, the light was working on all of the wires.

So, the question is, what would cause the timing light to stop working on only a few wires and only when the engine was hot?

Thanks,
glenn

 

[Chris Pucci]

I had the same problem over the last week. Man it drove me crazy!!!

Check your distributor cap. Mine was in bad shape. With a new cap it seems to have solved the problem.

 

[Michael Mayben]

Guess this means ya got the tranny back in the sii glenn?? Hope it worked out ok, if not...ya need to call me so we can handle that!

Chris' analysis is one thing to try! A distributor cap can have a crack in the phenolic material that cannot be seen with yore eyeball and will really make a motor act stoopid!

Also, the inductive pickups on some timing lights are very "touchy" about exactly where they are placed as they can pick up rfi from an adjacent location which will either false trigger or attenuate the actual firing pulse through the cable. And the pickups are polarity sensitive which means if they are not installed the correct direction they will either not pick up the signal, or will false/erratic trigger. Snap the clamp several times and see if the erratic operations fixes itself.

If ya think the rig will make it up here ok, come on up after work some day and we'll put it on the scope and take a look at the ignition system! That's the ez way to diagnosis this stuff! No charge for that of course since we're neighbors!

I just picked up another toy...a handheld, portable digital scope or "micro-scope", a tool that was never actually used by the shop that previously acquired it from snap-on. And it does have an inductive pickup probe with it that has never been un-wrapped. So we can put that on your rig also, along with my oldskool analog scope and take a looksee at the ignition system.

And bring your timing light so we can make a comparison with mine also.

 

[ghiggi]

I checked the cap and rotor and found some carbon build up and scortch marks, so I cleaned those up and started up the truck. It was running fine for a minute then it just sputtered and died. It turns out, I'm out of gas. I'll try again when I get back from the gas station.

Mike, the transmission is running great, even better than after the rebuild from two years ago. It shifts smoothly and has great pick up.

I May have a day at the beginning of next week available to bring the truck over. I just have to figure out what is causing it to run fine when it's cold but start acting up when it is at running temp.

I'll let you know.

Thanks