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200 6cyl '68 - when at speed 65-70 mph with light gas pedal the engine will begin to tap or knock. if you shower down past 70 the knock goes away as long as you continue to accel or if you back down below 65 it disappears. head has been redone too many times to think about. no spacers or multiple head gaskets used...can anyone think of what the knock is ? I'm planning on replacing the head with one that hasn't been shaved, but obviously would love to know if this is going to solve the problem. will also replace the oil pump when I tear it down (could the knock be oil starvation at speed ?). don't really want to get into rebuilting the bottom if I don't have to.
 

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The plugs should give you some idea if the knock is ignition related. It could also be in the big end or the little end of a rod.

Before you replace the head, learn how to read your spark plugs, or find someone who can. It would be no fun to replace the head only to find the problem is in the bottom end. You might also try running a better fuel, or a fuel additive to see if that helps the knock.

I did a very quick search on the web for "reading spark plugs" and got a few matches. For example www.nightrider.com/biketech/spkplghnbook.htm#We
 

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I had a problem like that and it turned out to be the vacuum advance. Yours could be that the compression ratio was raised and it can't handle the decrease in volume. Try better gas. Check your timing and back it down some to see if that changes it. And check your plugs, it could be running lean. It could be a combination of some of these things or just one.

If you have a vacuum advance, pull the hose and plug the hose then take it out for a drive and see what happens. If that doesn't work try backing off the timing 3 or so degrees and drive it again. If no change try 3 more degrees, and still no change, then pull a plug or two and tell us what they look like.
 
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