Timing
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Senior User
Joined: Jan 2004
Posts: 166
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From: Masontown, WV
Timing
I decided to check the timing on my engine today and found something strange. For starters, the vacuum advance housing is turned all the way over touching the gooseneck. When I checked the timing it was at the last mark from the btc mark and when I moved the distributor vacuum advance away from (counter clockwise) the gooseneck the timing moved further past the marks. Then I hooked up the vacuum line and gased it to see what happened and the timing moved the wrong way. ??? Am I doing something wrong? The truck runs ok.
77 F150 4x4 351m or 400 auto
77 F150 4x4 351m or 400 auto
Posting Legend
Joined: Sep 2001
Posts: 30,255
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Scroob, the vacuum advance mechanism always advances the timing. It does not matter if it is hooked to the full manifold vac or venturi vacuum as far as the mechanism is concerned. The vacuum mechanism is meant to be connected to the venturi vacuum port on the carb during normal operation, the "part time" vacuum port, in order to provide the proper ignition timing. If the system is set up properly there is a switch that switches the vac source to the distributor vac adv mechanism depending on engine temp.
Another possibility here is that the timing chain is worn and has slipped a tooth. There are many posts here on how to test for timing chain failure.
Another possibility here is that the timing chain is worn and has slipped a tooth. There are many posts here on how to test for timing chain failure.
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Senior User
Joined: Jan 2004
Posts: 166
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From: Masontown, WV
Well, heres what I found.
I do'nt know what marks I was looking at but I have found the correct ones. I had to use a small disk sander on the balancer to find the marks. The timing was at about 0 deg. as near as I can tell. The marks are very rusted so I cant really tell. I checked for play in the timing chain and found about 2 deg. This seemed ok to me so I found tdc and then marked it on the distrbutor. Then I pulled the distrbutor and rotated it from the gooseneck ccw to the next tooth. Made my own tdc mark and set the timing to around 10 deg. WOW does the truck run better.
Thanks again for your help!
Now I have another question. Like I said the timing marks are prety much gone but I was able to mark them with an awl. Are they measured off by twos or threes or what? I was going by twos.
I do'nt know what marks I was looking at but I have found the correct ones. I had to use a small disk sander on the balancer to find the marks. The timing was at about 0 deg. as near as I can tell. The marks are very rusted so I cant really tell. I checked for play in the timing chain and found about 2 deg. This seemed ok to me so I found tdc and then marked it on the distrbutor. Then I pulled the distrbutor and rotated it from the gooseneck ccw to the next tooth. Made my own tdc mark and set the timing to around 10 deg. WOW does the truck run better.
Thanks again for your help!
Now I have another question. Like I said the timing marks are prety much gone but I was able to mark them with an awl. Are they measured off by twos or threes or what? I was going by twos.
New User
Joined: Oct 2003
Posts: 20
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From: long island
I've been trying to get my timing right also. the marks are in 2 degree increments. I have a 77 f250, 400 4x4. The dist. was frozen with the timing at about 24 btdc. I freed it up and went to 12 btdc, not perfect , but better. Gonna try 10 btdc tomorrow.
good luck
good luck
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Elder User
Joined: Feb 2002
Posts: 987
Likes: 2
From: East Bend, Carolina, U.S
A vacuum advance canister "dashpot" can in one situation cause retarded timing.
The original post stated that the timing went the wrong way with the vacuum line hooked up, so this is indicating the vacuum had this effect on timing. If manifold vacuum is applied to a vacuum advance it will initially advance the timing during idle. Then as the engine rpms increase with the openning of the throttle plates the manifold vacuum begins to dissappear. This will cause the vacuum advance to release the timing it had been holding at idle and the timing will begin to retard at this point. If the centrifugal advance system is working it may overcome this a bit, if it isn't then the timing will only be retarded if the hose is attached to the manifold vacuum.
The original post stated that the timing went the wrong way with the vacuum line hooked up, so this is indicating the vacuum had this effect on timing. If manifold vacuum is applied to a vacuum advance it will initially advance the timing during idle. Then as the engine rpms increase with the openning of the throttle plates the manifold vacuum begins to dissappear. This will cause the vacuum advance to release the timing it had been holding at idle and the timing will begin to retard at this point. If the centrifugal advance system is working it may overcome this a bit, if it isn't then the timing will only be retarded if the hose is attached to the manifold vacuum.
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