Recomendations for distributor rotor arm

[djm16]

This is really for my P3, but the rotor arm off the P4 90 (25D6 distributor works in it).

I have three rotor arms, only one of them allows the P3 to run without missing. It is a genuine looking (old) lucas 418726 (off the P4).

I also have a newish looking red rotor stamped: arrow, 6, DD. The P3 misfires continuously with this one. Ditto with a second original looking Lucas 418726.

I suspect the red one came from Distributor Doctor, which is a little disappointing as I had been led to believe that these were reliable.

So any recomendations for source, make etc for a replacement?

[enigmas]

Hi David, I've made/fabricated/altered a few rotor buttons when fitting non standard electronic ignition modules/senders to various cars and motorbikes.

When assessing a non standard or modified rotor button for use with a distributor, check to see that when the distributor is fully advanced (centrifugal), that the rotor pointer is not in close proximity to the next distributor cap terminal in order of firing. Similarly ensure that there is no overlap with the terminal behind the one that is firing. Modify the end of the rotor pointer to suit the situation.

Fabricate a cardboard template that replicates the distributor cap terminals, then gauge the relative position of the rotor button through the range of distributor advance. Modify the rotor button terminal (brass/metal or whatever your using to convey the voltage to the terminals) to the desired shape, so that it functions within the parameters of distributor advance without "jumping" electrically to an incorrect terminal.

Hope this assists.

[boz]

That's very interesting information Vince, I never thought about rotor arms having that effect on the firing before, especially when an engine will run with extreme movement either way when advancing or retarding the dizzy, on my particular engine I can turn the dizzy about 3/4inch either way, and the engine will still run.
Your explanation could mean that a spark could be jumping to the next terminal, I will be checking my cap and rotor next time I'm in the garage, Boz.

[Phil Nottingham]

Never heard about that - I suppose a quick check can be made by comparing the rota-arm profiles with the known good one?

If the rotaarm has tracked at any time through damp a carbon trail can be left on it which shorts the spark - just cleaning can sort this

[djm16]

The red rotor arm is made from a bright red plastic, and there is no sign of cracking or carbon tracking, nevertheless the engine misfires with this one.

The old lucas old is Bakelite (I presume) and is brown / black. Ditto, no sign of carbon tracks or cracks.

Interestingly, the red rotor electrode has a different profile to the Lucas. Whereas the Lucas electrode maintains a constant gap to the plug lead electrode, the red rotor arm has a progressively widening gap.

Could it be the distributor cap instead? Before you ask, I tried replacing it with no change in the misfire rate.

[percy]

The only comment I would make is to always buy NOS Lucas unless there is no alternative. 

The original composition of the body material was arrived at after much research to ensure that there was the best combination of strength and resistance to unwanted conductivity.  

Apparently, there is a load of rubbish out there causing no end of mysterious electrical issues.

[theroveringmember]

I think a major part of the rubbish is in new Lucas boxes. The company now producing them has nothing to do with the old Lucas company & simply uses the old style box.
Search online for The Distributor Doctor or find new old stock items..

[enigmas]

Are you really sure that it is the rotor button?

Have you checked each of the spark plugs for resistance or open circuit issues. They all should have similar resistances. Similarly the high tension leads per foot of length. Are they wire core, carbon core, spiral core or other?

Have you tried substituting the coil and or the distributor cap? Similarly the condensor if points fired?

Does the distributor cap have a viable/functioning carbon rod and spring at the internal centre coil terminal?

As one cylinder is oiling, is it carbon fouling and causing tracking issues with the plug on that cylinder?

Have you substituted clean spark plugs? Blackened, carboned fouled plugs misfire. If you have a small sandblasting cabinet a quick abrasive clean will fix that issue if the plug is viable.

Has something other possibly shifted to throw the timing out...whether valve timing or ignition timing?

[djm16]

Hi Vince,
Yes to all of the above. I was very reluctant to blame the rotor arm.

This is the P3 and it runs perfectly with the arm borrowed from the P4 (no cylinders are oiling). In addition, the weak or absent spark with the two bad rotor arms could easily be demonstrated with a inline spark tester.

All the leads are copper and show continuity.
Regards
David

[enigmas]

Well then that's a good outcome David. 👍
I'd be interested to see some pix of the rotors, top and underneath/internal where the drive lug is molded within the non conductive body of the button. There must be a logical explanation for the differing performances of each rotor button.

[djm16]

Believe me, there is nothing to see, even with a magnifier. At some point (when everything else is fixed) I may do that test where you hold the coil HT lead over the exposed rotor and see if you can get it to arc to the rotor.

Meanwhile I have just ordered two NOS Lucas rotors. They are the type with a rivet holding the conductor in place. I seemed to remember some years ago the rivet being blamed for misfires, which is why I did not go for them initially.

[Phil Nottingham]

The Lucas versions should have a rivet - a cheap copy some years ago failed mainly because the plastic conducted as well!