Strange knocking from vacuum advance...

[Pat# 25912]

[Timothy B.]

Re: Strange knocking from vacuum advance...

Pat,

Did you apply vacuum on the distributor machine and check how much advance the can is providing. Most of these replacement cans come without a rubber stop bushing and the ones I have worked on needed that bushing to be near spec. Where is the initial timing set and what is total idle timing?

If the spec for B26 is 8 distributor degrees than it is providing 16 crankshaft degrees and without a stop bushing and with high initial timing the engine may have to high total idle timing. I guess a quick way to check would be to retard the distributor after it knocks and see if the noise stops or verify thye advance on your distributor machine.

[Pat #25912]

[Wayne W.]

Re: Strange knocking from vacuum advance...

I have experience a knock when the vacuum is wavering very close to the cut in point. The advance mechanism was vibrating.

[Pat #25912]

Re: Strange knocking from vacuum advance...

Wayne, that is what I suspected as well, but I can't make it knock while on the machine.

Using the "full vacuum advance at no less than 2" less than typical idle vacuum" rule (the B26 spec is 11-13" and my idle vacuum is 14-15"), maybe I am too close and the thing flutters or vibrates.

I will try the used 446 15 can (different make, similar specs) tonight.

Thanks. Pat

[Duke W.]

Re: Strange knocking from vacuum advance...

You can check for VAC flutter by checking total idle timing with the VAC connected. The timing should be steady. Also check manifold vacuum for steadiness.

Try activating the VAC with a vacuum pump. Does it hold vacuum? Does the problem go away?

Duke

[Pat #25912]

[Duke W.]

Re: Strange knocking from vacuum advance...

18 (crankshaft) is a little too much. If you conclude that the 12" VAC is not aggressive enough, I'd just go back to the 8" B28. As long as there is no transient detonation as you add throttle, it should be okay.

Try pinching the signal line to nearly closed while the engine is idling. This will provide a damping effect and steady the signal to the VAC. Typically 8-cylinder engines give a fairly steady signal, depending on how the vacuum port is plumbed in the carburetor, but this could be an issue and you should test for it.

Duke

[Pat #25912]

Re: Strange knocking from vacuum advance...

The knocking was indeed the vac can vibrating.

Following Duke's suggestions (thank you Duke)...

At about 900 rpm with the VAC (B26 VC1765) connected, the timming is not steady. It fluctuates slowly by a couple degrees on the tab. The vacuum is not steady either. It fluctuates rapidly between 14 and 15".

As soon as the vacuum drops below 14" the knocking starts. Or, drop the rpms below 900 and the knocking starts.

Then, I pinched the signal line a bit with a spring clamp. It's a miracle!
Duke's a genius, I was thinking. Knocking stopped, vac signal was steady.
I lowered the idle to 800 (below 750 the VAC would rattle) and the vacuum signal was a steady 13".

Next I removed the clamp and inserted a short length of smaller diameter hose inside the vacuum signal line for a restriction. So far so good, still no rattle. Disconnected my guages and went for a road test.

Not so good. The car jerks under light accelleration. I think the advance is comming in and out. With the vacuum now only pulling a steady 13" at 800 rpm, maybe the B26 isn't the best choice?

Tonight I will try the B28 again and leave the restriction in the vacuum line to steady the signal.

Thanks for all the help, I am learning a lot. Pat

[Wayne W.]

Re: Strange knocking from vacuum advance...

See if you can find a VC 778 for an early Pontiac. It has pretty good specs.