timing mark jumps around
Hotshot
Joined: Feb 2004
Posts: 11,697
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From: Marengo, Iowa
If that isn't it, the timing chain has slop in it. You can check that by taking the distributor cap off, and move the crank backwards until the rotor moves, this will show you just how much slop you have. As the chain gets more time on it, it stretches, giving the slop, which will affect timing, it bounces around trying to stay tight. There was no automatic tensioner in these motors yet.
Hotshot
Joined: Feb 2004
Posts: 11,697
Likes: 3
From: Marengo, Iowa
Oops, you're right, if it is a 300 (4.9) If it has the factory nylon gear, the teeth could be breaking up, and need to be replaced. I have seen some engines with the nylon gear take off teeth to the point the cam was no longer turning.
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Post Fiend
Joined: Mar 2002
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From: Bonita Springs FL
The steel gears are much better than the nylon ones for this reason. They're noisier but worth it.
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Brett Foote
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From: Sand Lake, MI
You didn't say how much it varied each way, 1, 2 degrees? 5,10 more? Knowing may help narrow it down.
Yea it could be the cam shaft gear is worn, bring #1 piston up on the compression stroke several times and mark the exact point the rotor points on the rim of the distributor. If you find it points to a sightly different place each time, I'd suspect the timing gear/s.
Could also be caused by a worn out distributor, If its bushings are bad it can spin off center constantly changing its path as it rotates and varying the timing.
Would not be as evident when turning the motor over by hand but would play with the timing when its running. Pull it out and see how much play it has in the bushings.
A bad/worn cam shaft thrust plate may cause it also. Allowing the cam shaft to drift forward and aft slightly as it spins. Unlikely but a thought.
At the mileage you state it has on it I would suspect the distributor bushings and or drive gear are worn out if it is the original distributor to the truck.
Yea it could be the cam shaft gear is worn, bring #1 piston up on the compression stroke several times and mark the exact point the rotor points on the rim of the distributor. If you find it points to a sightly different place each time, I'd suspect the timing gear/s.
Could also be caused by a worn out distributor, If its bushings are bad it can spin off center constantly changing its path as it rotates and varying the timing.
Would not be as evident when turning the motor over by hand but would play with the timing when its running. Pull it out and see how much play it has in the bushings.
A bad/worn cam shaft thrust plate may cause it also. Allowing the cam shaft to drift forward and aft slightly as it spins. Unlikely but a thought.
At the mileage you state it has on it I would suspect the distributor bushings and or drive gear are worn out if it is the original distributor to the truck.
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wook540
1987 - 1996 F150 & Larger F-Series Trucks
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Oct 5, 2011 12:37 PM
