Re: IML: Unleaded gas effects

["Brent Covey" <brentcovey@xxxxxxxxxxx>]

I think maybe its time for some reflection-

If some valves are receding and others arent, whats different about the
situation? They all ran the same RPM on the same fuel at pretty much the
same temperature, so what is different enough that some failed and others
didnt?

The weakness of original carb jetting and the common need to slightly retard
ignition settings from the original spec to prevent ping both increase
exhaust valve temperatures enough to impact longevity, and might bear a
little extra attention- the standard rule of thumb is re-jet any 'smog'
carburetor about 3-5 thou richer than 'stock' spec. on the primary circuit
to restore the original thermal energy of the non oxygenated, high Btu fuels
that were availible prior to the late 1970's. Current fuels are quite a bit
weaker than 1960's blends many places, as much as ~10% or so. That you're
getting 17 mpg at 75+ mph suggests its running pretty darn lean indeed. That
will make a blowtorch out of any valve face/seat flaws in a hurry.

The fact that valve work was performed 16,000 miles prior is a prime
suspect- the removal of seat material may have allowed some pitting in the
seats to surface which will adversely effect some valves and not others.
Depending on the workmanship and techniques used, the seats might have been
poorly matched to the valve faces or may have been ground out of round from
indexing the seat cutter to a slightly worn valve guide- theres a ton of
variables when you service heads that are not much of a factor in factory
assembled heads. Sometimes the valves dont have enough margin to cover the
seat face adequately to protect it from the blast of combustion. Even weak
springs might cause pounding damage from valve lofting at speeds not much
over 3000 rpm if the engine is operated for extended periods at highish
temperatures, or perhaps the valves are not rotating adequately from the
weak springs . A used engine with even modest oil consuption (say a quart in
1200 miles or so) blows enough oil thru the exhaust valve to provide many
times the seat protection any amount of lead ever could have provided.

I would not implicate the unleaded fuels in the seat damage. There is likely
a combination of causes for the damage where it has occurred, and they
probably can be corrected pretty easily. Hardened inserts are a pretty good
solution however there are other risks related to them, such as cracking the
heads or hitting the water jackets by using too deep a seat, or having them
drop out of the head entirely from inadequate tension. Charging up the
mountain one side getting everything hot and coasting down the other with
closed throttle cools the seat faster than the head and can occasionally let
an insert drop. After recession has already occurred you're kinda stuck with
inserts if you plan to retain the heads of course.

I certainly do not mean to criticise your workmanship but the head service
likely is where the problem originates and the operation of the car at
elevated speeds and temperatures just aggravated a flawed situation. I
belive your original point of view regarding the lack of neccessity for
leaded fuels is correct and feel you'll likely discover other reasons for
the seat recession if you inspect the heads and engine calibration much more
closely. I dont think leaded gas would have saved the day for you, Dick-

Very sorry to hear theres been troubles whatever the cause,
Brent Covey
Vancouver BC


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