1973 400 timing spec
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MSEE
Joined: Apr 2004
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From: Austin, TX
1973 400 timing spec
What is the timing specification for this motor? The engine decal says 12 BTDC, but my Chiltons says 6 BTDC. I was listening for spark knock at 12, but I can't tell--my engine is old and has some light lifter tap anyway, so a lot of the noises I hear at 12 I hear also at 6. How do you distinguish between spark knock and valve tap?
Posting Guru
Joined: Aug 2000
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From: (near) Denver USA
Spark knock is most common when the engine load increases faster than the carburetor can enrich the mixture. Examples are off-idle starting from a dead stop, sudden acceleration (opening the throttle quickly) from steady cruise, and climbing a hill without shifting down. Towing or carrying a heavy load will aggravate these conditions.
If you hear a faint "rattling" sound from the engine, and it correlates directly with one of these conditions (i.e., appears only when you induce the condition, and goes away as soon as the condition is relieved), that is knocking.
Usually, valve train noises correlate only with engine speed, regardless of loading, throttle position, etc.
If you hear a faint "rattling" sound from the engine, and it correlates directly with one of these conditions (i.e., appears only when you induce the condition, and goes away as soon as the condition is relieved), that is knocking.
Usually, valve train noises correlate only with engine speed, regardless of loading, throttle position, etc.
Elder User
Joined: Jun 2003
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From: Houston/Hope BC
I don't know this for sure, but read on some web site history of the 400 it didn't start going into trucks until 75 or 76 or maybe even 77? (can anyone confirm this?) when the 390 got dumped. So I guess yours was transplanted. I would trust the sticker before I trusted chiltons. The 6 degree number is prob for the first 400's with higher compression. If your engine is modified you can throw the book and the sticker away as far as timing is concerned. Keep going up on oil weight to try to quiet the lifters. Add a can of STP to bump it some more if 40 weight won't do it.
Senior User
Joined: Oct 2003
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From: in a house
how many miles on your old 400? did the heavier oil quiet things down any? your valve train componets could need inspecting or replacement. you can do a search for the details and procedures.
I've always determined and compensated for spark knock on a test drive it, bump 2* and test drive again procedure to find that sweet spot. then fine tune from there
I've always determined and compensated for spark knock on a test drive it, bump 2* and test drive again procedure to find that sweet spot. then fine tune from there
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MSEE
Joined: Apr 2004
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From: Austin, TX
It just turned 146,100 miles today. I recently changed the oil to 20W50 with a bottle of STP. The oil light used to come on at idle after trips but it hasn't been on since.
I think now that the noise in question is an exhuast leak rather than valve noise. It's more of a flutter sound. I pop the hood and listen around there and its a pretty silent motor...you can't really hear the noise until you get farther down the car. I had an exhuast backfire last summer that a friend heard 3 miles away (there goes the muffler) and those right manifolds are notorious for cracking.
I started out at 12B as the decal says, and brought it down little by little until it stopped dieseling. It is at about 7B now and there doesn't seem to be any drop in power. It hasn't dieseled yet but I'm keeping an eye on it.
I think now that the noise in question is an exhuast leak rather than valve noise. It's more of a flutter sound. I pop the hood and listen around there and its a pretty silent motor...you can't really hear the noise until you get farther down the car. I had an exhuast backfire last summer that a friend heard 3 miles away (there goes the muffler) and those right manifolds are notorious for cracking.
I started out at 12B as the decal says, and brought it down little by little until it stopped dieseling. It is at about 7B now and there doesn't seem to be any drop in power. It hasn't dieseled yet but I'm keeping an eye on it.
Logistics Pro
Joined: Jun 2004
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From: South Central Utah
1977 was the first year for the 351/400 in a PU. Timing should be at 12B at the specified rpm like the sticker says. The Emissions Label alway takes priority over all other publications unless it is in a factory TSB (technical service buletin).
I would make sure your idle speed and carburetor mixture is set correctly . You may also want to change brands of gasoline to cure the dieseling. Dieseling was never a big problem with 400s. Also make sure the engine is not running too hot.
Generally, you will hear a ping more when the engine is hot. They seldom ping when the engine is cold unless the timing is WAY over advanced.
I would make sure your idle speed and carburetor mixture is set correctly . You may also want to change brands of gasoline to cure the dieseling. Dieseling was never a big problem with 400s. Also make sure the engine is not running too hot.
Generally, you will hear a ping more when the engine is hot. They seldom ping when the engine is cold unless the timing is WAY over advanced.
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