Timing problem on a 400
Senior User
Joined: Nov 2007
Posts: 340
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From: Near Madison, WI
Vacuum advance is only there to increase fuel economy under part throttle cruising. Unhook it, plug the vacuum line from the carb and drive it. If that doesn't fix the problem, we'll know it's not the vacuum advance.
-Jared
-Jared
Laughing Gas
Joined: Nov 2003
Posts: 898
Likes: 1
From: southeren Oh
Is the engine and ignition system in a stock configureation? Use the stock timing specs if so.
If not I'd time by ear myself.
Backfiring out the exh I'd think the timing is way to retarded if it was the ignition.
If not I'd time by ear myself.
Backfiring out the exh I'd think the timing is way to retarded if it was the ignition.
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Tuned
Joined: Sep 2006
Posts: 250
Likes: 5
The truck has stock ignition, as far as the motor- I have no way of knowing- looks and sounds like a mild cam- towing I expect. I have been messing with the timing all day- I was always taught advance it until it pings under load and back it up a little. It seems to run better advanced past the the stock 12 BTDC. I was also taught throw the book out once you put the an aftermarket cam it- is this correct- should I be timing by ear? Should I push advance it as far as I can? Also can I disconnect the vacuum advance and plug it? If that is so why do they still have advance on performance motors- I have a 351c in a 72 Mach1 that has mechanical advance? I alway thought it was there to keep the timing right as the RPMs increased?
Senior User
Joined: Nov 2007
Posts: 340
Likes: 4
From: Near Madison, WI
Mechanical advance and vacuum advance are two different animals. The quick version looks like this. Initial timing or base timing is what you set with a timing light. From there, mechanical advance adds timing advance as rpm rises, at least until the system of weights and springs inside the distributor reaches its full advance. All of that is adjustable depending on your combo. Vacuum advance adds still more advance under part throttle cruising in an effort to increase mileage. Many high performance dizzy's do not have vacuum advance, but they still need mechanical advance to run optimally, unless they are electronic or are locked out, but that is something else entirely. Unhook the vacuum advance, you still have mechanical advance, go drive it and let's eliminate one thing at a time.
-Jared
-Jared
Laughing Gas
Joined: Nov 2003
Posts: 898
Likes: 1
From: southeren Oh
.....I was always taught advance it until it pings under load and back it up a little. It seems to run better advanced past the the stock 12 BTDC. I was also taught throw the book out once you put the an aftermarket cam it- is this correct- should I be timing by ear? Should I push advance it as far as I can? Also can I disconnect the vacuum advance and plug it? If that is so why do they still have advance on performance motors- I have a 351c in a 72 Mach1 that has mechanical advance? I alway thought it was there to keep the timing right as the RPMs increased?
(What would be ideal would be to have a Timing adjustment on the dash like the MSD makes).
I have a 77 F150 with a 351m or a 400 (not sure which) that is not stock and I use it as a work truck I left the vac advance hook to the ported vac so when the load is light I get plenty of timing and as much MPG as I can get. But at near WOT and lower vacume and RPM (high load) it will not ping.
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Tuned
Joined: Sep 2006
Posts: 250
Likes: 5
OK- disconnected and plugged the vac advance- and drove the truck for 20 miles- back roads and highway. It did backfire for a few seconds at 60 mph on the highway, but after that I could not get it to miss again. No miss or backfire in third as before and seems to run all around better- So does that tell me that I have a problem with the actual vac advance on the distributor?
Laughing Gas
Joined: Nov 2003
Posts: 898
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From: southeren Oh
Look at the wires that go from the mag pick up to the module. Between the mag pick up and where the wire are held down just before they go though the dits body. some times the wires can break with out the insulation looking bad at all. Friend of mine had the same problem years ago on a 302. Every time the mag pickup moves because the vac advance pulls it the wire bend. The wires can only bend so many times. If you had a tach on it I bet when it would be missing and backfireing the tach will jump all over the place.
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Senior User
Joined: Jul 2008
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Sounds like all great advice..the vacuum advance should be plug in on the front of your carb.. Not the tree behind the carb...but nobody talking about fuel....How much fuel pressure are you getting....what size carb?
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