<VV> Curious about Scope patterns

[Doug]

I am curious.  What exactly is so good about a scope?  What kind of patterns 
are you looking for?  It is not that I doubt it - it is that I never really 
knew the value.

Thanks,

Doug
>
> I used to see weird problems all the time, a good oscilloscope would show
> the problem quickly. Lacking that and going by your description, and I
> assume you are using points, you should check the following. The fact that
> it takes time or heat buildup and was significantly cured by a high output
> coil is the clue. I had in years past seen  lower quality point sets
> actually have the point arm start to bind on it's pivot post at high speed
> or heat condition. Sometimes a tiny drop of oil on the pivot cured the
> problem and confirmed it. If the points are binding you could watch the
> dwell period/cam angle start to drop on a meter as the points start to 
> stick
> open longer. Do not confuse this with the normal dwell change as the point
> plate advances/retards with distributor vacuum changes. The point plate 
> does
> not pivot around the shaft but from it's own offset spring loaded pin 
> which
> causes point gap to change as it pivots and is designed to provide a dwell
> change. As the points stick open the dwell lowers (small number = large
> point gap) indicated by less point closed time the coil has less time to
> saturate and the spark weakens. Your high output coil could be correcting
> the problem by concealing it with higher output.
> I had seen similar problems on early electronic ignition systems, a
> defective ignition control module that would break down under 10 minutes 
> of
> high speed only (such as turnpike driving). I had also seen high output
> coils that overheated when used with electronic ignition modules which 
> they
> were not designed for, they would leak out their insulating oil when
> overheated and short out internally, initially only under high load or 
> heat.
> High output coils also have a higher current draw which fried some
> electronic modules in time. This is why your see epoxy coils now in modern
> electronic ignitions. Performance problems that initiated tune-ups were
> usually point problems 9 out of 10 times. There are other things that can
> cause problems such as deteriorated plugs or plug wires. Excessive rotor 
> to
> cap gap, occasionally seen in mismatched cheap brands. Bad distributor
> ground or high points circuit resistance. But those problems are usually
> constant by their nature, not intermittent. Actually improper heat range
> plugs can cause a similar problem too. Years ago Champion consolidated 
> heat
> ranges and inventory. They came out with copper core plugs that they 
> claimed
> would cover a wider heat range. In reality the consolidated numbers were
> usually a higher heat range and were fine around town on short trips. On 
> the
> road they overheated and damaged the resistor core causing a crack in the
> internal carbon resistor. This would start to arc and burn out, 
> effectively
> increasing the plug gap and causing hesitation, stalling and lack of power
> on a otherwise normal appearing "new" plug. Only a oscilloscope would show
> the ruined plugs problem that could occur after one turnpike trip. In 
> other
> words, cross reference charts lie.
> It is difficult to accurately diagnose your problem here as visual
> inspection is critical. I could give you things to look for in scope
> patterns if you have one available.
> Good luck
> Chet Motyka