> If I
> put in a new set of spark plugs, my 74 TR6 will run perfectly for about
> 500 or so miles. After that it starts missing at low RPM when under
> load. The problem will get progressively worse until I replace the
> plugs. When I remove them, they look clean. The insulator is basically
> white with the slightest dusting of dark combustion material near the
> tip and the electrodes look to be in perfect condition with a slight
> whiteish dusting on them. They are gapped correctly. All of the plugs
> look pretty much the same. I am using non-resistor NGK this time. The
> car has a crane XR700 and putting points back in made no difference. It
> has a new coil but that made no difference. The wires are pretty new and
> came from Moss so I assume they are correct. I can say that this problem
> is quite repeatable. I am guessing that the spark is not happening in
> the correct place on the plug?
>
> I don't get it, without visible wear, what could be going wrong?
One possible explanation is glazing ... basically a conductive coating forms
on the ceramic surface, that conducts enough to bleed off the spark voltage
before it gets high enough to jump the gap. The usual cause is a plug that
is running too hot, so I'd start by trying one range colder plugs.
Randall
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