C3 Distributor noise

[Andrew P.]

1971, 350 4spd 45,000 miles

Hello,

I've searched previous posts about distributor noise but have come up with no real solution.

Distributor is making a loud knocking noise, can feel it knock with hand on top of cap.

Read that it may be VAC flutter, but noise does not go away with increased idle or applying vacuum to canister.

Any help would be greatly appreciated.

Andrew P
48361

[Patrick H.]

Re: C3 Distributor noise

Andrew,

Silly question... have you removed the cap and looked inside? Did you have someone turn over the motor while the cap is off so you could watch it?

If so, and nothing obvious was flying around, I'd say it's time to pull it out and rebuild it.

Patrick

[Kenneth T.]

Re: C3 Distributor noise

1971, 350 4spd 45,000 miles

Hello,

I've searched previous posts about distributor noise but have come up with no real solution.

Distributor is making a loud knocking noise, can feel it knock with hand on top of cap.

Read that it may be VAC flutter, but noise does not go away with increased idle or applying vacuum to canister.

Any help would be greatly appreciated.

Andrew P
48361

Andrew,

Check the tach drive. You can disconnet it and then listen.

[Andrew P.]

Re: C3 Distributor noise

Thanks for the replies!!

Yes, I have removed and inspected the internals and had someone turn engine over, nothing out of the ordinary.

I have removed the tach drive aswell and no change.

[Stuart F.]

Re: C3 Distributor noise

Andrew;

Does the engine otherwise run normal, i.e. seem to be in time, accelerate normally, Etc.?

The way you describe it makes it sound like you are hearing something more metallic, i.e. like distributor shaft, drive gear or even camshaft related. That's why I ask if it is running normal.

Stu Fox

[Andrew P.]

Re: C3 Distributor noise

Yes, the engine is running fine. Idle is stable, car accelerates fine, no driveability issues. Timing is stable as well.

Thanks,
Andrew

[Kenneth T.]

Re: C3 Distributor noise

Yes, the engine is running fine. Idle is stable, car accelerates fine, no driveability issues. Timing is stable as well.

Thanks,
Andrew

Is the oil pressure stable and at the correct pressure for your car?
The distributer does also drive the pump, which I am sure you know, just
thinking out loud.

Ken

[Jack H.]

Re: C3 Distributor noise

Here's a 'stab'... These cars originally were equipped with an RFI shield inside the distributor cap covering the points/condensor. The shield bolted to the distributor's breaker plate with two screws. It's possible the shield components were removed/discarded and one of the hold down screws has 'migrated' either above or below the breaker plate...

[Paul D.]

Re: C3 Distributor noise

To completely rule out the tach drive, after taking the cable loose go ahead and pull out the cross shaft. I would think you would notice erratic tach operation if the cross shaft was worn enough to cause a noise though.

[Donald L.]

Re: C3 Distributor noise

I have a distributor whose rotors tang is a tad longer than the emission style rotor.The is a slight interference with the cap contacts and the rotor tang. Turns out the condenser hold down screw is contacting ever so slightly the cap . Solution was to hog out a small area of the cap to clear the screw head or use a rotor with a shorter tang.

[David D.]

Re: C3 Distributor noise

Here's a shot in the dark...
Have you checked to make sure that the 'stop bushing' is still in place and that your Centrifugal Advance Weights aren't simply swinging out and clipping the distributor cap?

1) I would open up distributor and check to see if they move freely and stop where they are supposed to.
2) Put an elastic band around them to see if noise goes away.
or
3) While cap and rotor are off, just turn engine over and physically watch to see what happens to them. (See if they swing out to far)

Reference attachment:
Good Luck,
David

[Andrew P.]

Re: C3 Distributor noise

WOW, thanks for all the advise!!!

Oil pressure bang on
Tach operation good
RFI shield in place and secure
I replaced original Cap and Rotor 1000 miles ago. Swapped originals back in just to be sure... still same noise.
removed cent weights and springs and started engine....same noise

I hooked up VAC to manifold vacuum last night and made no difference.
I hooked up my vacuum pump and pulled 20Hg heard a LOUD KNOCK, noise is now gone.

So I'm guessing its the VAC???.... Does anyone know what this exactly means, Diaphragm? plunger?......

Thanks once again for all the replies,

Andrew P
(48361)

[Stuart F.]

Re: C3 Distributor noise

Andrew;

If I understand right, you have narrowed it down to the VAC. I have found a number of old and new ones that make noise, hang up and/or leak. Best bet is to pitch it and replace with one that works correctly. It is a good idea to bench test a replacement before you put it in too. If you have a Mityvac, test to see if it "begins" to move and is "all in" at the specified values of hg. Also see if it holds the vacuum for a length of time (make sure your connection to it is good). Check the alignment of the stem to make sure it isn't dragging on the guide slot and engages the hole in the plate straight w/o being cocked off on an angle, and then moves w/o binding. You also want the stem to pull in straight w/o the diaphram popping or the stem drawing off to the side.

Stu Fox

[Duke W.]

Re: C3 Distributor noise

Test the VAC with a vacuum pump and gage to see if it meets spec, and have the cap off so you can see breaker plate rotation. The breaker plate should rotate smoothly between the vacuum start and end points. There could be something wrong with the VAC or the breaker plate could be binding/hitting something as it moves through its angular travel.

Many VACs are still original, and it's amazing how many lasted this long.

Duke

[Andrew P.]

Re: C3 Distributor noise

Great suggestions!!

I did check earlier to make sure the breaker plate was moving smoothly and it looked ok. The VAC is original to the car, even has a grease marking on it. Where could I get a replacement VAC? (# on can 437)

The noise is gone but after I took it out for a short road test, I now notice the Tach is not smooth anymore, it bounces very rapidly when it sets at a certain RPM. (looks like it has the shakes) I did buzz it up to 4500rpm when shifting. Do you believe I damaged the cross gear?

I can't win for losing.

Andrew P