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Author Topic: Ignition Timing - Late contactless Marelli distributor  (Read 251 times)
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Modano
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« on: October 28, 2021, 10:04:17 PM »

Hi
I'd like to check the ignition timing, in order to rule out ignition in my Carb "debug".
I have a Marelli SM 806 AX (1300 - C3 engine) so no breaker points, but a hall effect. It has inertial advance (up to 18°) + vacuum advance (up to 12°) on top of the "static" 10° BTDC advance.
On these engines, there are 2 marks on the pulley (+ one "production" , somewhat hidden), and nothing on the yellow carter. There is a fixed metal "arrow" ("2" in the below procedure)
The 2 marks are TDC ("Repère PMH" in the below procedure) and 28° ("1" in the below procedure)
I bought a timing light and like your advice, as I never did that before.

The Lancia procedure mentions a simple : At 3200 RPM, check that the advance is 28°, with vacuum off (BTW my vacuum capsule is dead).

Therefore, if I understand correctly, I have to do a white mark on the 28° pulley mark, and on the "arrow", and then, at 3200 RPM, the strobe should make them coincide, right ?
If it does, nice, nothing else to do (except the vacuum capsule, which graph shows a depression in mmHg, which I guess would require a tool to detect this.)
If it doesn't, I guess two options :
- Static timing (10°) is wrong (Solved by rotating the distributor)
- Inertial advance is slow/bad (18° more at 3200 rpm) (solved by greasing/changing the springs, the "flyweights")

Am I right here ?
another question is, with that kind of markings, how can I check the 10° static timing ? With a protactor I check 10° right of the TDC, do a mark , and check at 800 RPM ?




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« Last Edit: October 28, 2021, 10:06:10 PM by Modano » Logged
Modano
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« Reply #1 on: October 29, 2021, 12:04:48 AM »

Something like that ?


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peteracs
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« Reply #2 on: October 29, 2021, 08:16:52 AM »

Hi

There should be markings on your crank pulley for TDC and on the cam cover for 5 and 10 degrees, so using a strobe you can set it up on idle. I guess you would have to mark it yourself for 28 degrees. I would however check that the pulley mark advances as you raise the revs to give you a good idea if the distributor is advancing ok as a first test.

Peter
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squiglyzigly
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« Reply #3 on: October 29, 2021, 09:02:29 AM »

If your Vacuum capsule is dead you must block to air pipe from the carburettor until you repair the capsule.
Otherwise it will be sucking air into the carburettor and causing lean running/misfires etc.

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rossocorsa
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« Reply #4 on: October 29, 2021, 09:30:05 AM »

Hi

There should be markings on your crank pulley for TDC and on the cam cover for 5 and 10 degrees, so using a strobe you can set it up on idle. I guess you would have to mark it yourself for 28 degrees. I would however check that the pulley mark advances as you raise the revs to give you a good idea if the distributor is advancing ok as a first test.

Peter
No markings on the cam covers on later cars, electronic ignition should be set at the higher rpm using the pointer and  markers on the pulley provided.

Regarding static timing there are markings on the distributor inner plastic cover that help but I've forgotten how you do this off the top of my head, so will try to look back on my notes if I have time
« Last Edit: October 29, 2021, 11:17:52 AM by rossocorsa » Logged
Modano
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« Reply #5 on: October 29, 2021, 03:42:19 PM »

Thanks all,

Hi

There should be markings on your crank pulley for TDC and on the cam cover for 5 and 10 degrees, so using a strobe you can set it up on idle. I guess you would have to mark it yourself for 28 degrees. I would however check that the pulley mark advances as you raise the revs to give you a good idea if the distributor is advancing ok as a first test.

Peter
No markings on the cam covers on later cars, electronic ignition should be set at the higher rpm using the pointer and  markers on the pulley provided.

Regarding static timing there are markings on the distributor inner plastic cover that help but I've forgotten how you do this off the top of my head, so will try to look back on my notes if I have time

Thanks, so, indeed a good idea to arbitrarily measure that timing is advancing with the RPM increasing.

Regarding the static timing, does adding a mark past 10° on the crankshaft pulley make sense ?

There was a mark with white corrector at roughly 10° past TDC, so between the TDC mark and the 28° mark, so I think it makes sense to remark at 10° and try at idle.
The timing charts are clear : No advance from the inertial or the vacuum correction systems is performed at 800-1000 RPM with low depression, so it should be near 10° at idle.
Am I wrong here ?


Quote
If your Vacuum capsule is dead you must block to air pipe from the carburettor until you repair the capsule.
Otherwise it will be sucking air into the carburettor and causing lean running/misfires etc.
That is very interesting and makes perfect sense. I'll block the carb pipe. One less factor.


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squiglyzigly
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« Reply #6 on: October 30, 2021, 08:21:45 AM »

Yes you are correct. Setting 10 degrees at idle will be good.
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Modano
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« Reply #7 on: November 04, 2021, 11:47:27 PM »

Ok, so, some news.
I bought an used Timing Gun (Sears), very nice Cheesy

I just tested it and I saw that at idle, the builtin timing arrow was in front of one of the manually white painted marks on the crankshaft pulley.
But here's the twist.... The guy who painted these marks was wrong. He counted 10 degrees counterclockwise from the 28 degrees mark (on late Betas), and not clockwise from the TDC mark.

Now I perfectly remember him saying, 10 years ago : "oh the painted timing marks are wrong, I put new ones". Indeed I can barely see an older mark, which is 10 degrees "before" the TDC, and would be the good one.

So instead of having a static 10 degrees, I'm currently with 18° degrees advance at idle, and with full "inertial" advance, I might go to 36 degrees instead of 28 degrees.
Not sure how the impact is measurable but i's good to put it back at least for the static advance.

Will take a picture.
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Modano
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« Reply #8 on: November 13, 2021, 02:51:35 PM »

Hi
So I made a session today (first time) for ignition timing.
Tests done without vacuum advance, no air box, and hot engine.
Currently , it seems impossible to achieve 10 deg static AND 28 deg at 3200.
What I can do is
- less than 10 degrees at idle (3-6 degrees) and 28 at 3200
- ~ 10 degrees at idle , and more than 28 (around 34-36)

Also , when slightly adjusting the distributor , the idle speed may increase (like when choke is on)
EDIT : that sounds totally normal as I guess I suppose to adjust the carb once the ignition is timed properly and therefore lower the idle speed.

Sometimes, after a quick rev, the engine “takes time” to go back to idle (lazy ignition weights ?)

I have no experience but is that common that it’s not easy to achieve factory specs on an old engine ?
What would be better to do ? Preferring static at idle or 3200 advance ?
Is it dangerous to have low advance at idle, or no advance at all ?

« Last Edit: November 14, 2021, 12:40:07 PM by Modano » Logged
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