Timing "knock" solved

[NAM VET]

I am posting this, not because I I can pass on knowledge, (there are others here who are very capable of that), but rather my experience with a perplexing problem which other may someday meet. I found no information on several searches for my problem.

I was tuning a rich idle mixture, when I noted my truck's engine had a light "knocking" sound when I would rev it via the carb linkage. The tapping noise as not present at idle, or with slow increase in RPM, but only when I "blipped" or zinged the RPM. It was not a deep down heavy rod knock, having had that on my British sports cars long ago, and it was not the light tappet sound or clicking. Which was reassuring, as I was dreading having to set my clearances, having done that in the past. I used a stethoscope, but could not really locate the sound's location, but it did seem to be from the upper part of the engine.

So I looked up the possible causes of such a sound. Not rod knock, not heavy like that plus my oil pressure is unchanged. Not piston slap, as my engine has new pistons 1500 miles ago, not broken rings, as my compression is 120+ and even on all cylinders. Not the tappets, they barely clicked at all. Not an exhaust leak. Not valve slop, as Charles Talbert put in new valves and guides not long ago.

But a cause could be a timing too far advanced. So I checked that, and sure enough, my timing was about ten degrees before TDC. But then the spec of 4 degrees TDC is at a very low idle RPM, and my engine just doesn't like to idle at 550 or so, it is much happier and smoother at about 700 or so. Meaning there is some centrifugal advance already possible. So I set my idle as low as possible, about 600 RPM, and set my advance at 4 degrees.

Not an easy thing for a sole person to do. It would be a lot simpler to be able to turn the distributor with one hand and loosen and tighten the lock on my stud and be done with it. So it was trial and error from below, made more difficult with the coolant filter and lines Charles T installed when he more correctly re-built my motor. To get enough "swing" on the distributor's adjustment, had to make a minor adjustment on the "coarse" adjustment on the underside of the distributor. But eventually got it done, and now my motor does not have the "knock" or tapping with blipping my engine.

My theory is that when the RPM is more slowly raised, the advance wt's are accurate, but when zinged, they fling themselves out to too much advance, hence the pre-igniton and knock. All I know is that there is no knock now with a correct timing.

All the best, and be safe. Hal Copple, UpState SC

[RMS]

how much slop is in your dizzy weights ? if you grab your rotor and turn it back and forth how much free play exists ? ive seen loose rotors and dead springs cause such symptoms. with a timing light how much does the mark wonder at a given rpm?

[NAM VET]

probably not much "play", as Charles T rebuilt it, with a Pertronix. I think reving the engine from Idle prematurely advances the springs, hence the upper engine tapping. But then, all I know is that it is gone with resetting my timing. My engine has a new timing chain, and new cam and crank gears too. It would be interesting if there was a scale of the spec timing advance at assorted RPM, up to when it was "all in", as the expression used to be. I remember long ago when gas stations had a big Sun device for checking and setting the advance, or "curving" distributors. Now days, with the ECM control of a car's ignition, it is fixed for life, unless one has an aftermarket Tune on their car. I just found on Ebay a used MAC tool 7/16 distributer wrench and ordered it. May make it easier in the future. No one now makes a 7/16 such wrench. Obsolete size, I guess.

Just putting out there what make my engine not rap anymore. I doubt that unless one blips the linkage by hand over the motor, it wouldn't be audible in the cab, especially with the the much slower increase in RPM when out driving. I hadn't heard it. Of course, I have quite a hearing loss too. Army thing.

Be safe, HC

[NAM VET]

Oh, to add, there was NO free play in my distributor with my manipulation, and NO wonder under my timing light. HC