Insulation of the contacts of the distributor cap burned

[Bertson]

Dear All,

I have some problems with the isolation of the contacts of my distributor cap.

The insulation of the contacts in the distributor cap burns off after about 2000 km. After about 3000 km misfires start. It seems as if the spark hits the insulation before reaching the contact and slowly destroys the insulation there, i.e. the spark ignites in the direction of rotation. The pictures show it most clearly.




It is an etype S1, 2+2, year 1966, 4,2l.
Due to the Weber carburetors, which do not produce vacuum, the previous owner has installed a distributor from an XJ after Bj. 1974. This was equipped with a Pertronix Igniter 1, instead of the breaker contact.

Spark plugs, ignition coil Lucas Sport, ignition cable, plug, rotor, distributor cap all new from SNG-Barrat. (Of course matching the XJ distributor).
Ignition with ignition gun checked everything O.K.

So far I have worn four distributor caps, but despite troubleshooting found nothing.

Maybe someone here in the forum has already seen this phenomenon and has a tip....

So slowly I run out of ideas and patience and I tend to install a 123 distributor, but actually runs the Pertronix super.

Thanks in advance ...

Robert

[bitsobrits]

Seems to me your ignition timing must be off to cause that type of erosion.

Have you checked your timing? If so, how much initial advance are you running?

[christopher storey]

I cannot resize the images so I cannot actually see any damage, but from the description it sounds to me either as though

1.your distributor is slightly mistimed so that the spark is occurring when the rotor is between 2 terminals rather than pointing towards 1 terminal ( on the face of it unlikely although your strange mixture of XJ distributor, pertronix etc might account for this ) or

2.You have excessive High Tension voltage and/or current . A visit to a garage with an engine analyser should be able to tell you - what is needed is perhaps 6KV to 12KV - some modern electronic modules give much more than this, in the 20KV upwards to 40KV range .

3.An analyser would also tell you whether for any reason your spark duration is excessive, which might also account for the spark from rotor to terminal being "dragged round" in the direction of travel of the rotor

[MSM]

Christopher has suggested that the timing may be out such that the spark is occuring when the rotor is between 2 terminals but surely if that were the case the car would run very badly?

I do not believe this problem is anything to do with timing, that is simply the position of the distributor when the points open.

I think this might well be a mechanical assembly issue, at the moment when the points open, the rotor arm is not directly opposite the connection inside the distributor cap and the spark jumping the gap is causing the damage.

You say - the previous owner has installed a distributor from an XJ after Bj. 1974. I don't understand what Bj means.

Is it possible that the distributor cap and/or rotor arm do not match the distributor body? Has the distributor shaft been dismantled and incorrectly reassembled?

[christopher storey]

Mike makes good points which can be looked at in addition to mine. Might I also suggest that you observe the engine running in conditions of total darkness - it is amazing how this can reveal tracking along surface irregularities in the distributor cap, which might reveal exactly what is going on

[bitsobrits]

I think this might well be a mechanical assembly issue, at the moment when the points open, the rotor arm is not directly opposite the connection inside the distributor cap and the spark jumping the gap is causing the damage.

Would that not show up as incorrect timing? The flash from the strobe would occur before the correct timing mark.

The original poster may also want to check to see if the distributor advance mechanism is stuck in the advanced position.

[Kember17]

I think BJ (at least in this context!) is a contraction for the German compound noun for Build Year.

P

[Durango2k]

Yes, BJ means Baujahr which means year of built.

I thought maybe the dissi drive is 1 tooth of inside the engine ?

Carsten

P.S - my car is also a 1966 / March S1 2+2. I am sanding the bonnet for painting this summer, and I’m still waiting for the windscreen „to be made in England“….for about 7 months now.

[MSM]

Yes, BJ means Baujahr which means year of built.

I thought maybe the dissi drive is 1 tooth of inside the engine ?

Carsten

If you mean maybe the distributor drive gear has been inserted one tooth out, that maybe a possibility.

Are the engine timing marks correct? Does indicated TDC happen with pistons 1 and 6 at the top of their travel?

Are you still using the Pertronix ignition system? is the plate inside the distributor installed correctly?

[abowie]

I think this might well be a mechanical assembly issue

Short answer:

remove Pertronics; run with points for a while and see what happens.

consider buying a quality rotor and cap from the Distributor Doctor, not SNGB etc

Long answer:

The distributor shaft top section is a solid unit, and the six sided cam section can't be moved relative to the notch at the top that locates the rotor arm. The top section positively locates on the lower shaft and is held with a screw. It does rotate with centrifugal advance but this doesn't affect the relationship between cam and rotor arm. So under normal circumstances these cannot become mistimed.

It is possible for the distributor drive shaft to be incorrectly timed relative to the crankshaft. We see this pretty commonly on cars we work on.

This either occurs because allowance isn't made for the rotation caused by pushing the drive gear home on the crank, or simply not aligning the shaft properly, up to and including getting it 180 degrees out. However none of this will affect timing per se because you can compensate for it by altering lead position and rotating the distributor.

Furthermore as above the relative positions of the rotor arm and cap are fixed as both are located by notches on the shaft and distributor body respectively. With points fitted I don't think it's actually possible for the spark to be mistimed mechanically.

However the OP has a Pertronics unit fitted. If I understand them correctly, they work by the Hall effect. The trigger is a wheel which is pressed down over the cam section with a magnet located to trigger the unit. Now if this wheel is mispositioned, or the unit itself is in the wrong place on the baseplate then you can well imagine that the spark might be mistimed between the rotor and the cap. If the Pertronics units provide a stronger spark as well (not sure about that) this would potentially worsen any problem.

[johnetype]

I think this might well be a mechanical assembly issue

However the OP has a Pertronics unit fitted. If I understand them correctly, they work by the Hall effect. The trigger is a wheel which is pressed down over the cam section with a magnet located to trigger the unit. Now if this wheel is mispositioned, or the unit itself is in the wrong place on the baseplate then you can well imagine that the spark might be mistimed between the rotor and the cap. If the Pertronics units provide a stronger spark as well (not sure about that) this would potentially worsen any problem.

Misplacement of the trigger wheel on the shaft seems the most likely cause and to check I suggest you do a static type of timing check where you rotate the engine by hand with the ignition on to determine the firing point (use a neon indicator in the lead from the coil or monitor the current flowing into the coil). At the point the distributor fires, stop rotating the engine and remove the cap and see where the rotor arm lines up to relative to the contacts in the distributor cap and you'll likely find your problem.

Reverting to points doesn't really help as you'll be sidestepping your problem and the weaker spark is less likely to cause any contact burn.

[abowie]

Reverting to points doesn't really help as you'll be sidestepping your problem and the weaker spark is less likely to cause any contact burn.

I disagree. It removes the whole Pertronics unit which is the most likely cause of the problem. You can always refit it later and see what happens.

[cactusman]

It seems to me as though the spark is being initiated at the correct time (or the engine would run like a tractor) but somehow the rotor arm is not correctly lined up with the contact in the cap.....long shot but is the rotor arm correct? An arm that physically fitted but was intended for a distributor that revolved the opposite way may cause the above as most arms have a trailing edge....which would be a leading edge if it was the wrong arm.....just a thought!

[Bertson]

Seems to me your ignition timing must be off to cause that type of erosion.

Have you checked your timing? If so, how much initial advance are you running?

No timing is o.K. in idle and with higher revs, checked with ignition pistol.
The problem only occurs after approx. 3000km...
Best Regards
Robert

[Bertson]

I cannot resize the images so I cannot actually see any damage, but from the description it sounds to me either as though

1.your distributor is slightly mistimed so that the spark is occurring when the rotor is between 2 terminals rather than pointing towards 1 terminal ( on the face of it unlikely although your strange mixture of XJ distributor, pertronix etc might account for this ) or

2.You have excessive High Tension voltage and/or current . A visit to a garage with an engine analyser should be able to tell you - what is needed is perhaps 6KV to 12KV - some modern electronic modules give much more than this, in the 20KV upwards to 40KV range .

3.An analyser would also tell you whether for any reason your spark duration is excessive, which might also account for the spark from rotor to terminal being "dragged round" in the direction of travel of the rotor

Hello Christopher,

if my distributor is misstimed, I must have everytime problems with bad ignition, I have checked ignition timimg with an ignition pistol and additional the problem only occurs after approx. 3000km...before the car runs "well".
I will biuld back to my previous ignition coil and check again, could be that the Lucas sports coil has too much voltage, thanks for this tip...
I think the Pertronix has a problem.
Best Regards
Robert

[Bertson]

Christopher has suggested that the timing may be out such that the spark is occuring when the rotor is between 2 terminals but surely if that were the case the car would run very badly?

I do not believe this problem is anything to do with timing, that is simply the position of the distributor when the points open.

I think this might well be a mechanical assembly issue, at the moment when the points open, the rotor arm is not directly opposite the connection inside the distributor cap and the spark jumping the gap is causing the damage.

You say - the previous owner has installed a distributor from an XJ after Bj. 1974. I don't understand what Bj means.

Is it possible that the distributor cap and/or rotor arm do not match the distributor body? Has the distributor shaft been dismantled and incorrectly reassembled?

Hello MSM,

I no longer have a breaker contact, I have a Pertronix (Ignitor 1) that works with a Hall sensor.
Sorry, Bj is german and means year of manufacture...
I have just drilled a big hole in an old cap and the position was O.K.,
The problem occurs after 3000km, before the car runs "well", distributor was not disambled, but I will check the centrifugal weights.
Best regards
Robert

[Bertson]

Mike makes good points which can be looked at in addition to mine. Might I also suggest that you observe the engine running in conditions of total darkness - it is amazing how this can reveal tracking along surface irregularities in the distributor cap, which might reveal exactly what is going on

Hello Christhoper,

yes, but then the last five caps must have problems...but I will check, thanks for the tip...

Best Regards
Robert

[Bertson]

I think this might well be a mechanical assembly issue, at the moment when the points open, the rotor arm is not directly opposite the connection inside the distributor cap and the spark jumping the gap is causing the damage.

Would that not show up as incorrect timing? The flash from the strobe would occur before the correct timing mark.

The original poster may also want to check to see if the distributor advance mechanism is stuck in the advanced position.

Hello MSM,

no, advance mechanism is not stucked in advanced position, timimg is correct, checked with strobe pistol, 10° in idle and in high revs up to 26°, did not rev to the limit...
But will check the function of the centrifugal weights, thanks for the tip...

Best Regards,

Robert

[Bertson]

I think BJ (at least in this context!) is a contraction for the German compound noun for Build Year.

P

Hello Kember,

Bj was my mistake, is german and means year of manufacture or in german, Baujahr => Bj

Best Regards,

Robert

[Bertson]

Yes, BJ means Baujahr which means year of built.

I thought maybe the dissi drive is 1 tooth of inside the engine ?

Carsten

P.S - my car is also a 1966 / March S1 2+2. I am sanding the bonnet for painting this summer, and I’m still waiting for the windscreen „to be made in England“….for about 7 months now.

Hello Carsten,

nice another 2+2 driver, my etype is born in may 1966..with some special modifications, but all depending to the year of manufacturing.
If the distributor is twisted by one tooth, I would have ignition problems all the time, but I only have misfires after about 300 km and my strobe pistol will show me a wrong timing...thank you for the tip I will check everything...


Best regards,

Robert