Erratic timing at +3500 rpm

[rdl001]

Although my engine feels like it is running okay through the rev range, I thought I would check the ignition timing.

My TDC pointer is spot on after checking with a depth gauge on cylinder 1. With the vacuum advance removed and a warm engine, the static timing moves between -6 and -8. At 3500 rpm the timing is very erratic between -25 and -35.

Does anyone know if this is normal or do I have an issue with the distributor weights, other ignition components etc or do I just have a 'cheap and nasty' strobe gun?

[phillpot]

rdl001 said:

do I just have a 'cheap and nasty' strobe gun?

That's where my money is

[Sardonicus]

What he said ^ and just for clarity static timing is just that static (engine not running) more applicable to points systems , where as running timing figures are dynamic

[ChimpOnGas]

Have you checked your distributor shaft for wear?

Wear in the main distributor shaft causes oscillation which can be worse at certain RPMs, this alters dwell/timing on a traditional points distributor but will influence timing stability on a hall type electronc distributor too.

Timing chain lash and or distributor/cam gear wear are other elements to check, from crank to rotor arm there's quite a lot of potential wear points that can all add up, this is why distributorless ignition systems give way more accurate timing, the signal is taken from source.... ie the crankshaft.

With a distributor some bounce seen under a timing light is inevitable but if you're seeing excessive jumping about when you use your strobe you should investigate all potential points of wear as these could well be your issue.

[Sardonicus]

A worn electric ign dist (knackered bearings) seldom causes scatter/wandering ign timing but may cause poor starting or misfiring if bad enough

[rdl001]

Thanks for the information ChimpOnGas,
With the rotor arm removed, I was a little surprised that the toothed disc (the bit that makes/breaks the hall/proximity sensor) around the distributor shaft is able to rotate up to +/- 5 degrees. Is that normal?
What is the best way to check the slack on the timing chain?
Regards,

[blitzracing]

The slop you are feeling is the play in the advance and return springs, not wear as such. It wont be an issue as the moment the distributor starts to rotate the weights will move out against the springs and take up any slack. You can certainly check for any side to side play as this would indicate worn bearings. The RPI web site wittered on about lack of distributor lubrication on the preserp engine that can lead to gear wear on higher mileage engines, but I think that was fixed on the serp. Have a read on this whole page:
,
http://www.v8engines.com/engine-4.htm#improved-oil...

If you genuinely have 10' of timing scatter this is huge, something would have to be very worn. Its possible if you have something wrong with the HT system it will cause the strobe to misfire, as the strobe picks up the emf pulse off the HT lead as the plug fires. If you have an open circuit or short circuit this will alter the point the peak HT takes place and could give a mis reading, so it may not be mechanical.

Edited by blitzracing on Wednesday 27th November 21:49

[rdl001]

Hi Blitz,
I understand the 'feeling is the play in the advance and return springs', but I also have movement in the 'trigger rotor with 8 lobes on it (marked with red dots on this image). This piece rotates on around the shaft about +/- 5 degrees.

[blitzracing]

That would make sense, as the lobe alters position in relation to the main shaft as the ignition advances with the movements in the weights. Play in the springs will allow the weights to move a bit as you twist the lobes.

[Penelope Stopit]

rdl001 said:

Hi Blitz,
I understand the 'feeling is the play in the advance and return springs', but I also have movement in the 'trigger rotor with 8 lobes on it (marked with red dots on this image). This piece rotates on around the shaft about +/- 5 degrees.

Nicely done you've found a big problem

The toothed ring shouldn't be moving around the shaft it is fitted to

[Sardonicus]

Penelope Stopit said:

Nicely done you've found a big problem

The toothed ring shouldn't be moving around the shaft it is fitted to

fk didnt read that bit yes that a problem that should be pegged/fixed to that short jack shaft if you have movement then that an issue and needs correcting cant remember how as this is a gutted out unit I did ages ago for an alternative purpose but image may help

Edited by Sardonicus on Thursday 28th November 12:08

[Penelope Stopit]

Sardonicus said:

fk didnt read that bit

Obvious that you hadn't read it, easy peasy to miss innit

[rdl001]

Thank you Penelope and others,
Yes, this is my issue.
As suspected, this lobe rotor should be connected to the shaft. I presume that centrifugal force my throw it in the same direction as the shaft spins, but I think I may just try setting it in the middle of it's movement and gluing in place rather than take it all apart.

[blitzracing]

Had to go and check my own sanity for a moment there- having had another look, you cant see the shaft the lobe rotates against as its below the screw in the central shaft below the rotor- so as you chaps correctly say the rotor lobe should not rotate against the shaft that locks the rotor arm. The lobes are locked in place by a small nylon ring with lugs below the lobes- its the cream bit you can see here:

Edited by blitzracing on Thursday 28th November 16:57

[ChimpOnGas]

rdl001 said:

Thanks for the information ChimpOnGas,
With the rotor arm removed, I was a little surprised that the toothed disc (the bit that makes/breaks the hall/proximity sensor) around the distributor shaft is able to rotate up to +/- 5 degrees. Is that normal?
What is the best way to check the slack on the timing chain?
Regards,

As I said, some bounce is normal but big swings in timing seen under the strobe needs investigating.

Wear can occur in many areas as previously listed and can also be an acumliative thing, but theres more.... advance springs can become detached, become weakened or break as can many other internal components within a distributor.

So the only realy good advice was to investigate, which is exactly what you've done and it does seem this investigation has lead you to the potential issue.

Good luck with it.

Dave.

[Penelope Stopit]

rdl001 said:

Thank you Penelope and others,
Yes, this is my issue.
As suspected, this lobe rotor should be connected to the shaft. I presume that centrifugal force my throw it in the same direction as the shaft spins, but I think I may just try setting it in the middle of it's movement and gluing in place rather than take it all apart.

Is there any way you can set it up in the correct position with silicone sealant or the likes of, once you know it's correct it could be marked before carrying out a more permanent bond

[rdl001]

I set it to the mid-position of it's slack and have bonded it in place with epoxy. On start up, the the timing at base rpm and 3500 rpm is now nice and stable. Seems like my erratic timing is could be fixed Test drive planned for tomorrow.

Note: I have now had to adjust the timing (by rotating the dizzy anti-clockwise) to get back to the -6 degree at base idle which then produces only -20 degrees at 3500 rpm. Should I be advancing a little further?

[ChimpOnGas]

You're looking for a maximum of 18 degrees of mechanical advance.

At 10 degrees at idle this will give 10 + 18 = 28 degrees at 3,250rpm when the distributor is fully advanced.

It sounds like you're only seeing 14 degrees of mechanical advance which will be a performance loser.

[rdl001]

ChimpOnGas said:

You're looking for a maximum of 18 degrees of mechanical advance.

At 10 degrees at idle this will give 10 + 18 = 28 degrees at 3,250rpm when the distributor is fully advanced.

It sounds like you're only seeing 14 degrees of mechanical advance which will be a performance loser.

So how do I get the extra 4 degrees of mechanical advance?

[Belle427]

Give it 14 degrees at idle, I used to run around 18 degrees at idle but did the vacuum advance modification that’s detailed on here somewhere.
The car ran much better with this modification.
Edited to add thread.
https://www.pistonheads.com/gassing/topic.asp?h=0&...

Edited by Belle427 on Wednesday 4th December 05:51