I'm trying to work this out. As far as I knew, timing is set by the points or hall sensor making and breaking - thus static ignition timing can be done with the distributor cap removed.  It can be set precisely with a bulb on the low tension side.

 

So does rotor arm positioning on the shaft affects the timing or not?

 

I have had some cheap rotor arms developing play on the shaft (can be turned a bit either way) and the performance seems worse. But there is no test as far as I know to check that the arm is positioned within 1 or 2 degrees of correct, yet this is set for the low tension side.

 

I'm sure it's an easy answer but I can't find it! Cheers. 

Incorrect positioning of the rotor shouldn't directly affect the ignition timing - but you may possibly get a weaker spark as the gap increases from the rotor to the required spark lead contact.

Ideally, the points should open while the rotor arm is closest to the external plug contact, but there will a fair degree of tolerance because rotor contacts are normally curved.  

Long time since I played with a distributor. But turning the distributor body (hence adjusting the rotor position) is how the dynamic timing is set once you have it started.

 

Strobe light on the crankshaft timing marks for the right setting.

 

 

 

 

6 hours ago, Tailhappy said:

I'm trying to work this out. As far as I knew, timing is set by the points or hall sensor making and breaking - thus static ignition timing can be done with the distributor cap removed.  It can be set precisely with a bulb on the low tension side.

 

So does rotor arm positioning on the shaft affects the timing or not?

 

I have had some cheap rotor arms developing play on the shaft (can be turned a bit either way) and the performance seems worse. But there is no test as far as I know to check that the arm is positioned within 1 or 2 degrees of correct, yet this is set for the low tension side.

 

I'm sure it's an easy answer but I can't find it! Cheers. 

Reread CB points and dwell angle until you actually understand it!

16 hours ago, Paws4Thot said:

Reread CB points and dwell angle until you actually understand it!

I have successfully set timing and points for a fair few years now but this question was about faulty rotor arms with rotary play on the shaft. My question was not answered on the Wikipedia link (despite reading twice) so I have ordered a Gunsons timing light and will test myself using the failing rotor arm and see what affect it has on timing.

22 hours ago, Warrior193 said:

Incorrect positioning of the rotor shouldn't directly affect the ignition timing - but you may possibly get a weaker spark as the gap increases from the rotor to the required spark lead contact.

Ideally, the points should open while the rotor arm is closest to the external plug contact, but there will a fair degree of tolerance because rotor contacts are normally curved.  

 

This is exactly my thinking - the tip of the rotor arm is several times wider than the contact in the distributor cap so it should not directly impact timing seeing that the spark is jumping anyway. But I have ordered a timing light and will see if the failing arm has any impact on timing - I will report back here my findings.

It will not affect timing for the reasons you have stated.

 

As vehicles have not had distributors for 30 + years now be very wary of any replacement rotor arms, distributor caps, coils and condensors, the latter were of questionable quality even back in the day.

 

Fit manufacturers original second hand parts if you can find them, it doesn't take Einstein to manufacture a pattern part rotor arm but many are completely non functional.

However, if there is any play in the distributor shaft itself, this could have a significant effect on dwell angle and timing.

Also movement/stickiness in the vacuum advance mechanism and baseplate.

In the 'good ole days' of distributors,  points and rotor arms mechanical wear made timing a little difficult to be precise. I would set the timing with a strobe light and then use a vacuum gauge, which usually gave a slightly different reading and I would then set the timing to the reading on the vacuum gauge.

I have been timing engines with strobe lights and by ear for 50 years and never heard of doing it with a vacuum gauge.

 

OK I admit the last 25 of those 50 years there has been no timing to do but I still have an ear for it.

 

I am assuming that you swing the timing until you get the maximum vacuum reading? - That would correspond with the maximum idle revs before the engine hunts from over-advance, I do that by ear but critically you must then swing the timing back to the point where the revs just start to fall again and lock the distributor at that point, if you set it to maximum vacuum it will be over-advanced which can be catastrophic for the last generation of Kettering ignition vehicles with their lean burn mixtures.

 

Setting the timing by ear on an early 6 volt VW air cooled engine results in far better performance and economy but the engine will not start the solution is to wire in a latching relay to the ignition circuit with a push button latching circuit, you crank the engine which will turn vigorously without the ignition working against it and shout "CONTACT!" out the window when you press the button, preferably wearing an Erwin flying jacket and Biggles scarf 😀

43 minutes ago, J.R. said:

I have been timing engines with strobe lights and by ear for 50 years and never heard of doing it with a vacuum gauge.

 

OK I admit the last 25 of those 50 years there has been no timing to do but I still have an ear for it.

 

I am assuming that you swing the timing until you get the maximum vacuum reading? - That would correspond with the maximum idle revs before the engine hunts from over-advance, I do that by ear but critically you must then swing the timing back to the point where the revs just start to fall again and lock the distributor at that point, if you set it to maximum vacuum it will be over-advanced which can be catastrophic for the last generation of Kettering ignition vehicles with their lean burn mixtures.

 

Setting the timing by ear on an early 6 volt VW air cooled engine results in far better performance and economy but the engine will not start the solution is to wire in a latching relay to the ignition circuit with a push button latching circuit, you crank the engine which will turn vigorously without the ignition working against it and shout "CONTACT!" out the window when you press the button, preferably wearing an Erwin flying jacket and Biggles scarf 😀

This is a typical vacuum gauge.

 

Edited 5 January, 20251 yr by moley

Vacuum gauges are very very useful but Ignition timing is the last thing I would use one for, I'm very very suspicious of that marking, any vacuum leak or valve burning will start with a slight drop in idle vacuum getting progressively worse, it's crazy for them to state the area to the left of the green zone is exclusively indicative of retarded ignition. That goes back to the 60's and 70's when all sorts of claims were made about and plastered over things to increase their perceived value.

 

I would advise the OP to set the timing using a strobe and the manufacturers timing marks, if they want to experiment outside of that setting then only do so if they have a good experienced ear above all be able to recognise pre-ignition and detonation, 40-60 mph timed runs in top gear will reveal the optimum setting which will be a range and I'll bet the manufacturers timing setting falls bang in the middle.

 

Thankfully its not something that many of us have had to contend with for decades.

 

Finally dwell angle on a Kettering ignition system is critical and far more precise than the points gap, a dwell meter is definitely worth investing in, a vacuum guage equally but not for setting ignition timing in my view/experience