New cap and rotor ... tuning??

[ATG]

Advice for a numpty please ...

Background: car failed to start ... eventually I noticed arcing down the side of the distributor cap. Thorough cleaning plus dose of WD40 got it running ... but the contacts in the distributor cap and the tip of the rotor arm are all pretty warn so I thought I'd replace them.

So, the question. If I fit a new rotor arm and distributor cap, I assume I'm going to be advancing the timing a bit, but I have no idea if it is worth worrying about? Might the tuning need a bit of adjustment too?

[number 7]

Well the rotor arm comes off vertically, and is only held on by pressure from a piece of spring steel - you'll see it on the new one - so there's no movement of the distributor shaft to change the timing. The cap can only fit properly one way because of a peg in its edge whcih matches the distributor, and the whole thing is secured by the spring clips which have locating points on the cap. As long as you mark the plug numbers on the old cap before swapping them over, and refit the leads in the same order, the timing can't change.

7.

[ATG]

Yeah, but no, but yeah ... yes, everything will line up etc, but the rotor arm I'm replacing is quite worn. The new one will be wider at the tip and surely will therefore make fractionally earlier contact with the contacts?

It might well be that it's such a small difference that it doesn't matter.

>> Edited by ATG on Friday 20th January 16:20

[Tyre Smoke]

ATG said:

Yeah, but no, but yeah ... yes, everything will line up etc, but the rotor arm I'm replacing is quite worn. The new one will be wider at the tip and surely will therefore make fractionally earlier contact with the contacts? It might well be that it's such a small difference that it doesn't matter. >> Edited by ATG on Friday 20th January 16:20

It will make feck all difference - except the contact will be stronger between unworn parts.

Take one lead at a time off the old and position on the new dizzy cap in the same position.

[ATG]

Oh ... hang on ... I reckon I've been talking complete bollox. It's just struck me that the whole coil firing thing probably happens after contact has already been made in the distributor cap, so the timing won't be effected at all.

[Cider Andy]

There isn't actually any contact made. The rotor arm passes near the electrode in the cap and the spark flies across the gap, on its way to the spark plug. There should be no reason to adjust the ignition timing after replacing the rotor arm and cap.

[greenv8s]

The timing is defined by the position of the upper dizzy shaft (which is moved around by the mechanical advance) relative to the pickup (which is moved around by the vac advance). The rotor arm tip has to be wide enough to ensure that it lines up with the terminal insude the cap when the spark occurs, but this isn't what triggers the spark. So just replace the rotor arm and cap, ensure the carbon brush hasn't falled out of the middle of the cap and the lug on the cap is aligned with the notch on the dizzy, and transfer the HT leads across one by one making sure they go to the correct terminal on the new cap.

[ATG]

Makes good sense. Thanks for clearing that up.