Good Times? Setting the timing on a rebuild can be a hassle

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Hack_Mech_Timing_Lead
Courtesy Pantera International

Don Graves writes:

I have a 1995 Lincoln Town Car with the 4.6-liter V-8. The engine stalls two or three times each morning when it is cold. This has been going on for some time. When the car starts and keeps running, it runs very well. Any suggestions?

Given that the symptoms manifest only during warmup, I’d suspect that the Idle Air Control (IAC) valve is sticking. The IAC is a silver cylinder a few inches long, located at the very top of the engine. You can try tapping it with a screwdriver to see if it unsticks. It’s also an easy part to remove, and a quick test is to gently shake it to see if it rattles. If not, it’s probably stuck. You can try cleaning it with carburetor cleaner or simply replace it, as it’s not a terribly expensive part.

Gil Mares writes:

I own a DeTomaso Pantera with the 351 Cleveland. The spec for initial timing varies from 6 degrees to 16 degrees, depending on compression and heads. There isn’t much stock on these engines after a fresh rebuild, so how do you determine where the initial timing should be set and where the vacuum advance should start?

The engine should be timed so it never knocks. Unless you’re having the distributor rebuilt and recurved, the static timing, centrifugal advance, and vacuum advance aren’t independently adjustable; when you rotate the distributor, you’re changing the sum total of all of them.

Centrifugal advance is a function of engine rpm, but vacuum advance is an inverse function of load (throttle opening) and how the vacuum line is connected. On nearly all cars prior to the incorporation of emission controls, the distributor’s vacuum advance diaphragm was connected to a manifold vacuum at the base of the carb, providing about 10 degrees of extra advance in the lean-running, throttle-closed configurations of idle and highway cruising and dropping to zero at wide-open throttle. When emission controls ratcheted up in the 1970s, many manufacturers changed to “ported vacuum” (using a port on the carburetor above the throttle plate) in an attempt to increase exhaust gas temperature to help burn off hydrocarbons. This essentially defeated the vacuum advance at idle. I believe this is the reason the Pantera has two stating timing specs–the 6-degree spec is for the manifold vacuum configuration, and the 16-degree spec is for the ported vacuum configuration.

Whether your Pantera is set up with manifold or ported vacuum is one of those “decades later many changes” issues. The trend generally is to convert early 1970s-era cars back to pre-emissions manifold vacuum, but some Pantera owners say that ported vacuum works well on a 351C with iron heads and flat-top pistons.

Because of the changes from long-ago stock, as well as the unavailability of leaded gas to act as an anti-knock agent, on any vintage car with a mechanical advance distributor, I recommend: starting with the factory settings; putting an advance timing light on the engine; verifying the distributor is actually advancing with increasing rpm (sticky pivot points can often cause a no-advance situation); disconnecting the vacuum advance; setting the total advance to 32-36 degrees; reconnecting the vacuum advance; then driving the car under a variety of load conditions. If it knocks, back the timing off. If it doesn’t, try advancing it slightly. Then check it with the timing light and see what the total advance is to have that as a reference.

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Rob Siegel’s new book, The Best of the Hack MechanicTM: 35 years of hacks, kluges, and assorted automotive mayhem, is available on Amazon. His other seven books are available here, or you can order personally-inscribed copies through his website, www.robsiegel.com.

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