Unleaded - Ignition Timing

[PeterPortugal]

Gents,

There are loads of posts about using unleaded fuel and it's impact, or not, on valves.

What about ignition timing? I set my car up as per the BBB but suspect that it is knocking / pinking.

Does anybody have any specific values to advance the timing by?

Regards

Peter

[Benz Dr.]

I try to obtain around 38 degrees at about 3,000 RPM with all vacuum lines connected and working properly. The three main types of ignition systems all function differently on our cars but still should produce the same full advance numbers. 

[Cees Klumper]

The thing I worry about just a bit, from time to time on older cars like ours, is whether the timing marks we use on the harmonic balancer/crankshaft pulley is actually in the 'spot' it is supposed to be (0 at TDC compression stroke cylinder 1, normally I think), or whether due to some mistake, timing chain stretch or what have you, it is off. Because if that is the case, naturally we are not setting the timing correct.
So, experts like Dan - would you advise us to always double check the timing marks by manually setting top dead center (spark plug out, feeler inside cylinder no. 1 with a dial indicator to tell us exactly whether the 0 mark is in the right spot)? So far, I have always relied on the timing marks being spot on, but also note that, when I finally replaced my timing chain, it did affect the running characteristics, as it moved all the components (camshaft, valves, pistons, ignition timing) closer to spec.

[Benz Dr.]

Replacing a timing chain will only affect those things that are run by the chain - pistons would not be one of them. The crank assembly should never really change orientation but chain wear will affect ignition, cam, and IP timing to a certain degree. If you hear pinging under load then you should retard your ignition timing 2 or 3 degrees or find higher octane fuel.

[PeterPortugal]

Gents,

thanks as always for the advice. I subsequently got round to following the description in the technical manual to set the idle fuel/air ratio.

I leaned out the pump by about 6 or 7 clicks overall, adjusting the air at every iteration. It brought my warm idle down from about 1000 to 750 rpm and the car sounded and drove a lot better.

I will be honest and say I am not sure it was pinking now, I think it was struggling due to being laid up plus it needed a bit of a tune up.

Is 6 or 7 clicks a lot?

Regards

Peter

[PeterPortugal]

What I haven't done is tried to start the car cold yet. That will happen tomorrow.

Maybe it was set up rich to disguise a cold starting issue?

[Cees Klumper]

You only affected the idle mixture setting with that adjustment. It should not really affect normal driving, I think. Also, I don't expect it will have much of an impact on the cold starting and warm-up characteristics (but I could be wrong!).

@Dan, I meant, that a stretched timing chain causes timing of the cam, ignition and pistons to be out of sync, but I understand that the vibration dampner is directly bolted onto the crankshaft that in turn drives the pistons.

[stickandrudderman]

The timing marks on the damper are perfectly adequate for most but if you want absolute accuracy then you need to mark points a known distance either side of TDC.
The simplest way is to insert a threaded rod of the same thread pitch as the plug thread into No 1 plug hole to a sufficient length such that it will foul the piston.
Turn the engine over with a socket  until No 1 piston hits the rod and then mark the damper against the pointer. Now turn the engine backwards until the piston hits the rod again and mark the damper again. Measure the distance between the two marks and divide by two to get TDC.
A more absolute way is to use a dial guage in the plug hole and measure (eg) 2mm BTDC and ATDC, marking the damper as before and deducing TDC.
This is a process normally reserved for race engines or engines that don't have timing marks.
If you get pinking when the timing is correct you may be suffering with lowered octane due to oil contamination of the fuel mixture, commonly from failed valve seals.

[PeterPortugal]

Just following on from this post where I suggested I substantially leaned out my mixture....I have just used a basic Gunson gas tester at home but got a reading of 2.5% CO at idle (fully warmed up).

Is it too low? maybe I did go to far with my 6 or 7 clicks.

[stickandrudderman]

Yes it is too low.it should be around 4%.

[PeterPortugal]

Hi Colin,

Thanks for confirming this. I am going to take it for MOT and if they confirm my Gunson reading is accurate on their professional emissions tester I will adjust the fuel pump richer by a few clicks.

Setting the pump as it is was the only way I could drop the revs at idle to 750 / 800 rpm. Maybe it wants to idle a bit faster than this.

Cheers

Peter

[Pawel66]

Faced similar issue. I ended up suppressing air to bring the revs down, getting to CO at idle at 5 or slightly above, which, I think, is fine. Actually, for automatic it is more important, I think, what is CO reading when in gear.