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My 390's timing is apparently moving on it's own, I set it at 10 degress, but it won't stay set. When I hit the accelerator hard from a dead stop, I notice a change in the performance, sometimes it will bark a tire, sometimes it will have much less power, or sputter. I pull it back into the drive way, and sure enough, I find that the timing is either lower or higher. When I rev the engine to around 2-3000 rpm and check it, it changes, sometimes lower, sometimes higher.

Timing chain? Or is it something else?
 

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If you haven't changed the chain from a factory one yet, its more then likely the problem. The original gears were plastic coated and the plastic tends to go away causing abnormal timing issue. The chains also stretch over time.
 

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Both of the above posts are good answers!

What about info on your engine? Age, work done to it, stuff like that.

Sometimes it is ABOVE the base timing you set it at and sometimes it's below? Is that fo-rizzle?

I mean if you set it at 10* and mess with the engine driving, do you have to re-set it at 10* again afterward, and if so do you always have to move the distributor the SAME direction to get that thing to set back to 10*?

If you have to adjust it either way to get your 10* back then it's not the roll pin holding the gear on the shaft and it's not the timing chain.

If it simply bounces up and down while holding higher rpm's then it's 95% sure to be your timing chain.

If it is for example at 25* at 1800 rpm one time, then 20 * at 1800 rpm in another time frame, then I say you got sticky advance mechanism.

If you do your testing with the Vacuum advance removed, you will better be able to detect flaws in the system.
 

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If you buy a new Timing set (cam gear, crank gear and chain) and find the new Cam gear has a cast integral spacer SKIP the ford factory spacer! I fell for this, and didnt check end play on cam. Ooops. Same crappy performance.ODd that timing light was all over. some cam followers were riding onthe wrong lobes. Cam was sliding in and out of he block , sending the timing all over.
 

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pull the fuel pump and lookin there if you see missing plastic teeth on the cam gear, better pull the pan and get those pieces out. they will get sucked up and jam the oil pump
 

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LOl. But good answers never go out of style.
 
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