XJ40S wrote on 07.04.2012 at 20:50:05:


Well, let's keep it simple. Just ask that source WHY he thinks that cutting off the top of a distributor cap on an AJ6 engine helps in any way in determining the correct alignment of the rotor to a plug lead pick-up.

What does he hope to see which he cannot see with the cap off and the rotor arm aligned with a scribed mark on the distributor body in line with the plug lead pick-up.

I think he is pulling your leg! 

XJ40S wrote on 08.04.2012 at 19:39:01:


As well as  'selling' you a totally irrelevant and unnecessary procedure for setting the distributor rotor arm alignment on an AJ6 engine, I bet he gave you a cracking deal on the Forth Bridge or the Eiffel tower!   

Leo_Denmark wrote on 09.04.2012 at 13:17:10:


Leo,

What you describe is common practice if your motor struggles to rev above 2 or 3 thousand RPM or is otherwise running a little lumpy. It indicates that the rotor arm was not correctly aligned in the first place and judicious adjustment of the distributor body within the range permissible by the slot in that fixing hole is a good way of determining if the rotor alignment is at fault.

On my car, I can turn that distributor body through the full arc of that slot and the engine will still run OK but at one extreme it starts to sound sluggish at higher RPM so returning it to its central position in the slot and it runs and revs perfectly because that is how I set up the rotor arm in relation to TDC #1 cylinder when I re-fitted the distributor - a simple scribe mark on the body to indicate the position of the plug lead pick-up for #1 cylinder.

As you say, no need for cut-off distributor caps or any other nonsense. 

Roly, now we are getting slightly OT, but what you mean is something like this ?

Ho Ho Ho - I won't be the first person who has been mocked for suggesting something others can't grasp the significance or usefullness of, for example, the uproar when someone suggested the Earth was round when most thought it flat . Or the air ministry & RAF when they rejected Frank's jet engine .
I am not going to go into all the details because I don't want to write a massive post.
The nominal ID of the HT pick ups in a distributor cap is 76mm. In practice it tends to be larger as the pick ups erode and effectively larger still due to the insulating deposit.
The nominal swept Dia of the rotor arm is 75mm (37.5 Rad) with a cord length of 16mm but the are Lucas arms on the market that have a Rad of only 36.2 and a cord length of 15mm. That means there is a gap between rotor arm and pick up of at least 1.3mm 9much greater than plug gap) and that is before wear and eccentricity is taken into account. Looking at the rotor arm compared to the notch in the distributor to check distributor position can result in parallax error because the are a long way apart (a cut away cap avoids this). Now suppose an engine has an intermittent misfire or backfire on one cylinder (part of 40 minute discussion referred to in previous post), this wears and can distort the distributor drive and change the relative position of pick up and rotor for that cylinder only. This would not be detected by the scribing a line at No 1 cylinder method but would be seen using a cut away cap and checking each cylinder.
An engine may continue to run and apparently quite well with an excessive rotor to pick up gap but a Jag ignition system can produce over 100kV (if rotor gap excesive) and in time this will damage the ignition amplifier and ECU.
Putting it simply, cutting the top of and old (or new at only about £10) and checking your distributor alignment, rotor arm gap etc may well save you a lot of time and money long term and save fuel in between.
Cheers