John:
First and foremost, make sure your car has all the proper
tune-up stuff:
1. Timing properly set (10 BTDC with vacuum disconnected).
2. Idle mixture properly set - plugs should be a grey/tan color
on the tips.
3. Idle speed set to about 850-900 RPM with vacuum retard
connected.
I would not expect a carb heat shield to have much effect on a
run-on problem. The biggest factors are idle mixture and idle speed for
the usual suspects. Some other stuff that can get you in trouble:
A. Wrong plugs. Champion N12YC for the later cars, N9YC for the
earlier cars. If you are running a hotter plug to avoid fouling, the
tips can run hot enough to cause run-on, so make sure you have the right
plugs.
B. Higher than stock compression. Many cars have modified heads
to get more power and fuel economy - but it increases the risk of run on
- Higher grade fuel can help, but is no guarantee. If you haven't tried
it, use a tank of premium and see if you do any better.
C. Wrong Thermostat. Make sure you have a 180F thermostat in
there. If you have a hotter thermostat, you are more susceptible so
replace your 'stat.
D. EGR valve missing or disabled. The carbs are calibrated to
run slightly lean at cruise for fuel economy and emissions reasons. If
the EGR is disabled, the mixture leans out even more, causing the plug
temperature to go up. On later cars you can adjust the mixture, but this
will reduce fuel economy and requires some fussing as the "lift the
slide test" will no longer give you the correct mixture.
Assuming all of this checks out, and you still have the
occasional case of the "death rattles" read on. For me, it invariably
happened when there was a cute young lady nearby that I was trying to
impress. Nothing takes the wind out of your sales like a loud rattle
followed by a farting sound as you car dies while a young lady points
and laughs. <sigh> The ultimate solution is the factory run-on valve. It
works well, but is a bit involved and VERY easy to miswire. The young
women still point and laugh, however (mumble).
End part 1
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