Timing Marks Jumping around
This might be a silly question, but in timing my 66 L79 the timing marks appear to jump around a little... every second or so. I assume that this indicates that the ignition system is misfiring. The former owner installed a Pertronix module in the distributor. I have installed new plugs and coil so far. The skipping occurs on both #1 and #6 plug wires so I assume it is not plug wires. Cap and rotor look new. And I am experiencing a strong vibration that starts at 4000 rpm that is engine related not drivetrain related. My next step is to look at the distributor . Any ideas?
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Re: Timing Marks Jumping around
Some symptoms of a jumpy timing mark can be worn/loose timing chain, or a distributor that is not properly shimmed.Dave, 1969 427, 1957
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Re: Timing Marks Jumping around
David and William-----
I'd go with the worn timing set diagnosis. This is, by far, the most common cause of the "jumping around" timing marks. I don't know if your engine has ever been rebuilt. However, 1966 was the first year for the use of nylon toothed cam sprockets in small blocks. If there's one of these still installed in your engine, I can almost guarantee it will be missing teeth.
In any event, even if the engine has been rebuilt at some point, if it's got enough miles on it since, there's a very good chance the timing chain will be "stretched" or the sprockets worn. A nylon-toothed cam sprocket might even have been installed as a replacement.In Appreciation of John HinckleyComment
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Re: Timing Marks Jumping around
Joe, the engine was supposedly rebuilt 5 years and 2500 miles ago, but I have no idea what was done to it,or if it was done correctly.When the timing marks jump I can see the idle falter very slightly. I want to get a look at the distributor next, but if you say that the problem can be caused by the timing chain system, I will believe you. Can that cause the engine vibration at higher rpm only.Comment
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Re: Timing Marks Jumping around
William-----
It's very had to imagine that you'd have a timing set problem with an engine with 2,500 miles on it, assuming that they replaced the timing set at rebuild. Even if they had used a new nylon cam sprocket, I wouldn't expect any problem, at all, at 2,500 miles and no matter how many years since rebuild. So, I'd say that the problem must lie elsewhere.
Under any circumstances, I would not expect a timing set problem to cause engine vibration at higher RPM. More likely, that's due to slight engine imbalance. I wonder if it was competently balanced at rebuild or if that step was left out? Usually, a "standard rebuild" will not necessarily include balancing.In Appreciation of John HinckleyComment
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