Straight Six Engine Rotor Arms - Dizzy {P}
A member of the Triumph 2000 Register has reported that his car has been suffering intermittent misfiring and multiple condenser failures, in one instance needing RAC recovery.

Replacing the coil, points and king lead failed to cure the problem and it was eventually found that the engine had been fitted with a 4-cyl rotor arm instead of the correct 6-cyl one and this was causing excess arcing which was destroying other ignition components.

The incorrect 4-cyl rotor arm had been bought from Halfords where the parts list is said to wrongly show it as correct for the 6-cyl Triumph. Apparently they no longer sell the correct 6-cyl rotor arm. I expect this problem also applies to any other six cylinder car with Lucas ignition.
Straight Six Engine Rotor Arms - jc
Just cut off the bit that sticks out to the side and it will be suitable for a six.
Straight Six Engine Rotor Arms - Cliff Pope
Just cut off the bit that sticks out to the side
and it will be suitable for a six.


I wouldn't do that - rotor arms are balanced.

Straight Six Engine Rotor Arms - jc
Balanced??? Who are you trying to kid;it's only going about 2000rpm. at a radius of about 1 inch!!
Straight Six Engine Rotor Arms - Cliff Pope
Balanced??? Who are you trying to kid;it's only going about 2000rpm.
at a radius of about 1 inch!!


Just my observation of the dozens of different rotor arms I have seen. Makers originals, and good replacements like Lucus, usually have a balancing lug of plastic opposite the arm. Cheepo Halfords substitutes often don't.

Another factor in this interesting thread might be the correct engagement of the distributor and drive gear. It is easy when re-engaging the gear to get it one tooth wrong, because of the helical twist. The timing can often still be set correctly because most distributors are adjusted by rotating the plate inside, not the whole distributor. So the timing can be correct but the position of the rotor slightly off. This would tend to exaserbate the effect Dizzy is noticing, and would as he explains be more critical with a 6 than a 4.

Incidentally, wouldn't this apply to any 6, not just a straight?
The 2 V-eights I have owned (LandRover and Triumph Stag), had 8-lobed cams and pencil thin rotor arms for this very reason.
Straight Six Engine Rotor Arms - John S
Cliff

Ah, yes, the Stag! I guess yours had the double points system? What fun. I fitted electronic ignition to mine. You are of course right; this is a problem with any 6, or 8 come to that, not just straight sixes.

Regards

John S
Straight Six Engine Rotor Arms - jc
Some Delco-Remy didn't have a balancing lug of plastic!!
Straight Six Engine Rotor Arms - jc
The spark will only occur when the points open-the rotor arm and the points are on the same shaft and will only move with centrifugal advance-vacuum advance will move the whole plate anyway.You can put a distributor in in any position you like because of this and move the leads round to suit.As long as the timing is set correctly the actual position of the distributor body will make no difference.
Straight Six Engine Rotor Arms - John S
Dizzy

Yes, I believe these 4 cylinder rotors are made with a long section at the end to minimise kick back at starting. If it kicks back, it ensures it fires on a cylinder which stops the engine. With the closer spacing of the electrodes in the distributor cap on a six, with a 4-cylinder rotor I can see the spark jumping to two plugs at a time, which won't help the running. Removing the extra section of the metal contact will fix the problem.

Can't see why it damages ignition components, other than perhaps the distrubutor cap, though. Only time I've had a condenser failure recently was when the ignition coil developed an internal fault, which allowed the HV to jump the gap to the LV winding, and so back to the points etc. This did the condenser no good at all! Might be worth a check.

Regards

John S
Straight Six Engine Rotor Arms - Dizzy {P}
Likewise, I can't see why running with the wrong rotor arm damages ignition components - but I'm not so hot on electrical theory. However the owner said he had changed almost everything in the ignition circuit including the coil, and had even bypassed the Lumenition system, but still the condensers kept blowing until he fitted the proper 6-cyl rotor arm.
Straight Six Engine Rotor Arms - Cliff Pope
Sorry JC, I don?t agree. Yes, the positions of the lobes in relation to the rotor are fixed, but the point at which a lobe actually opens the contacts can of course be moved by rotating the mounting plate using the vernier adjuster. The full range of adjustment will alter the timing point over perhaps the whole arc of the rotor tip, which must therefore be long enough to ensure that the rotor is still pointing to a contact when the spark occurs.
What can happen is that a distributor that was correctly set up is removed and replaced for some reason, but is mis-engaged by one tooth. The consequent error in the timing of the spark can sometimes be corrected by turning the vernier adjuster, but that only moves the contact assembly, not the rotor arm. The spark may still occur while the rotor tip is aligned with the cap contact, or it may have ?moved off the scale?.
The effect of the advance/retard mechanisms may mean an engine runs normally and has correct static or idling spark, but misfires at the extreme range of the automatic timing adjuster because the rotor tip is not correctly aligned.
This can be an extremely elusive fault until the original alignment mistake is realised.
Straight Six Engine Rotor Arms - jc
Most distributors have at most 15 deg. of mechanical advance and 10 deg. of vacuum.The mechanical advance is roughly proportional to rpm.The vacuum varies between 0 & 12 ins. Hg.The only time when you will get large quantities of advance(caused by maximum mechanical and vacuum advance) is on the overun at high engine rpm. when the spark is not critical;normal running will be comfortably within the range of the rotor arm.Modern engines with minute combustion chambers do not need large amounts of advance;the flame travel in the chamber is almost instantaneous.
Straight Six Engine Rotor Arms - jc
If you had genuine Lumenition ignition(which I have found excellent)there should be no condenser as such in the circuit;it's all in the black box-do you mean one of the cheap copies?
Straight Six Engine Rotor Arms - Civic8
I think you are right incorrect parts fitted will cause a problem.I am surprised some reply`s say cut bits off.that is often wrong as a coil output will differ from engine to engine and the output from the coil may kill off rotor arm and segments in the distributer cap
 

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