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Rob J

How do you properly set timing on a 401?

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Did a full tune up today, but have gotten some conflicting data on setting the timing on a Nailhead 401. I'd just like to clarify the correct procedure.

Is it correct to disconnect the vac advance line to the distributor, plug the line, then check and set timing while engine idles at around 550-600 rpm, or do you disconnect the line, but not plug it. Also, the manual says to set it a 2 1/2, however, I've heard from different people that with todays gas, timing should be set at around 6-8.

One more thing. I still have no power brakes, so I think my booster is bad. When setting the timing, should I disconnect the vac line to the booster and plug it as well, since I believe I have a vac leak through the booster, or just leave it attached to the booster?

I got her running, but something just doesn't seem quite right. She's idles fine, but she stumbles on initial acceleration, and it seems to me that the power is down.

This is the first time I've messed with a Nailhead motor, so I'm learning as I go along.

Thanks in advance.

Edited by Rob J (see edit history)

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Yes, you pull & plug the vacuum advance line to your distributor when setting the timing. I believe I'm running at 2 1/2 degrees & my nailhead runs fine on any gas stronger then 87 octane..

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the timing specs for various vintages of nailheads can be different and depends also on if you have manual or auto trans. use the book specs for your year (2.5 deg. advanced).

Make sure that you have full manifold vacuum on the vacuum hose to the distributor when it is hooked up. Not while setting the timing, but when reconnected.

The vacuum leak at the booster will not affect timing. It might make the idle rough or stall but won't impact timing. Keep the rpm when setting timing low (about 500 rpm).

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Remember every degree you increase the initial timing setpoint beyond 2.5, it is added to the total timing advance. Too much total timing can/will cause detonation. When detonation starts is also dependant on the condition of engine internals and fuel quality so not every situation will be the same.

The easiest way to check total timing is with a dial-back type timing light. First check the total mechanical advance which also allows you to confirm it is working properly. Look at the advance charts in the shop manual.

Once you determine with a vacuum gauge you have acceptable engine vacuum, connect the vacuum back up to advance module and recheck the total advance at around 3900-4000 RPM. It is not uncommon for replacement vacuum modules to add too much advance leaving owners wondering why they can't get rid of pinging without performance suffering from backing off the initial setting.

Opinions vary but 32 degrees total for initial + mechanical, then another 10-12 for vacuum is a good starting point.

Edited by JZRIV (see edit history)

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