system to dual exhaust.
A ruptured vacuum advance diaphragm also will contribute to
premature valve burning, especially if the vehicle is subjected to hauling
heavy loads, such as towing a trailer. This is due to the retarded spark
where the burning mixture is going out the exhaust, rather than
propelling the vehicle. Fuel mileage will be reduced if the spark advance
systems are not working properly.
Above, the final reading shows this cylinder is “healthy.” Right,
this reading shows this cylinder is “sick.”The state of California mandated exhaust emission controls on new
cars beginning with 1966 models. These controls focused on reducing
unburned hydrocarbons and carbon monoxide output. A few years later,
it was learned that these early modifications greatly increased oxides of
nitrogen (NOx) output, and California began a program to retrofit pre-
1970 cars and light-duty trucks with NOx devices. All of these devices
involved some form of disconnecting the distributor vacuum advance.
Most of these retrofit devices brought about an increase in engine
operating temperatures, and many of the poorly engineered or cheap
devices did not have temperature-override provisions. Those devices
without this provision could cause an engine to suffer from prematurely
burned exhaust valves.
Valve clearances should be periodically checked and adjusted. Valves
that are set too tight will prematurely burn. When I worked in the tune-
up field, I would purposely set valves a tad on the loose side —
especially exhaust valves. This resulted in slightly noisier valves, but