Distributor rotor tang pattern

[Mike E.]

I pulled this rotor out of a 71 big block that wants to cut out above 2800 rpm. I have never noticed a pattern like this on a rotor before--maybe I have just missed it. Do the small dots (8 in the circle and 1 in the center) indicate anything to those of you who are for more experienced with engine diagnosis than I am? (Other than it is an 8 cylinder). Car has original coil, replacement TI amp, new GM TI pickup coil, new GM braided plug wires, new AC plugs. Ready to replace rotor with extended tip one. Any help much appreciated.
distributor rotor pattern (1).jpgdistributor rotor pattern (2).jpg

[Donald O.]

Re: Distributor rotor tang pattern

What does the distributor cap look like on the inside for the center post?

[Dick W.]

Re: Distributor rotor tang pattern

I think your tang might be a wee bit short from the looks of the pattern. Go to your local NAPA and get a ECH RR1670 rotor and I believe it will solve at least that problem

[Larry M.]

Re: Distributor rotor tang pattern

Never liked the later replacement gray or white colored rotors. Buy a NOS Delco-Remy or an extended tip rotor made in the USA. Then give it another try.

Agree to check cap center electrode also for issues. Who made the cap??

Larry

[Mike E.]

Re: Distributor rotor tang pattern

What does the distributor cap look like on the inside for the center post?

Looks just fine--like it should.

[Mike E.]

Re: Distributor rotor tang pattern

As I indicated, I'm installing an extended tip one. I'm asking about what is the read on this one besides cheap and poor quality.

[Owen L.]

Re: Distributor rotor tang pattern

As I indicated, I'm installing an extended tip one. I'm asking about what is the read on this one besides cheap and poor quality.

I don't recall ever seeing a pattern on a rotor -- just a dot where the coil's center tower is. It kinda looks like the rotor is walking around in a regular pattern. When the cap is screwed on, does it seem to wobble or is it pretty stationary with respect to side:side movement? Does the fit for distributor in the intake feel secure side:side without the clamp? Perhaps pull the distributor and check the drive gear as well as the shaft's play in the bearings.

[Joe L.]

Re: Distributor rotor tang pattern

As I indicated, I'm installing an extended tip one. I'm asking about what is the read on this one besides cheap and poor quality.

Mike------


I don't know what could have caused this pattern on the contact. One might opine that it was caused by some "arcing" between the rotor contact and the carbon contact in the distributor cap. However, I don't see how that could create the pattern seen since the contact point should be the same throughout the rotation of the rotor.

This does not appear to me to be a Delco rotor. I have never seen a Delco rotor of this light gray material and I've not seen a Delco "light colored" rotor which used a rivet to retain the brass contact. You could check for the possibility it is a Delco piece, though. Usually, these will have the Delco logo on the underside. No logo=not Delco.

[Ray K.]

Re: Distributor rotor tang pattern

Does not look like Delco rotor to me. I would try to locate a Delco black rotor if possible. Also, as pointed out, make sure that the distributor cap is also a Delco Remy unit.

Ray

[Jim L.]

Re: Distributor rotor tang pattern

Mike------


One might opine that it was caused by some "arcing" between the rotor contact and the carbon contact in the distributor cap. However, I don't see how that could create the pattern seen since the contact point should be the same throughout the rotation of the rotor..

I think you've explained it, Joe.

If the axis of rotation of the rotor were to be aligned with the tip on the inside of the cap, there would, indeed be a single point of contact. I believe that rotor axis is NOT aligned with the contact tip causing the rotor movement to look more cam-like than rotor-like. So each time a plug fires, the tip inside the cap is making contact at a different point on that tang, producing the 8 little arc dots.

That's my theory and I'm stickin' to it.

[Gary B.]

Re: Distributor rotor tang pattern

One dot for each spark plug wire location on the cap. When the rotor turns and catches the terminal in the cap, other's cross-fire to the center.
Replace the cap. If you have an extended tip rotor, use that also.
Gary B

[Joe L.]

Re: Distributor rotor tang pattern

I think you've explained it, Joe.

If the axis of rotation of the rotor were to be aligned with the tip on the inside of the cap, there would, indeed be a single point of contact. I believe that rotor axis is NOT aligned with the contact tip causing the rotor movement to look more cam-like than rotor-like. So each time a plug fires, the tip inside the cap is making contact at a different point on that tang, producing the 8 little arc dots.

That's my theory and I'm stickin' to it.

Jim------


That was my original thought, too. However, I then figured if the rotor was turning with this much eccentric, the engine would be running horribly.

[Jim L.]

Re: Distributor rotor tang pattern

Jim------


That was my original thought, too. However, I then figured if the rotor was turning with this much eccentric, the engine would be running horribly.

Weeeellllll, I'd say that cutting out above 2800 RPM qualifies as "horrible".

[Joe L.]

Re: Distributor rotor tang pattern

Weeeellllll, I'd say that cutting out above 2800 RPM qualifies as "horrible".

Jim-----


I would think that it would be far worse than even that.

[Dan D.]

Re: Distributor rotor tang pattern

Looks to me like the contact piece is placed too far in on the rotor. This is putting the axis of rotation off center, as previously said, and causing an excessive gap at the plug wire terminal. I would say the rivet hole in the contact is not in the correct location, putting the contact too far in and increasing the spark gap to the plug terminals. This will cause a misfire.

-Dan-