John---You gave a lot of good information regarding your symptoms. The
one about getting better mileage when the timing is set at 12 BTDC, but
idles better when set at 32 BTDC and returns poorer mileage is
troublesome. Do Frank's suggestion on checking the EGR valve, as it can
certainly affect idle if stuck in the open position, but a real pain to
block this off.
You didn't state when this started, and it would be helpful to know when
this all came about. Did you replace the points with Pertronix when
'normal' points replacement didn't solve the idle/stumble? How about the
advance control springs? Either of these two can cause a low rpm
problem.
What I am going to suggest for your mechanic to check could only
happen during a rebuild, not during the course of normal wear. See if #6
valves are "on the rock" (an English expression that took me years to
figure out) when the timing pulley pointer is exactly on "0" with #1
firing.
We all like feedback on juicy topics such as yours!
Dick T.
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