MG Midget Forum

When I'm looking at the Lucas tables for distributor advance for my setup, are the numbers relative to the advance from TDC with the vac plugged in? In other words, if my distributor 41271A with 5-8-3 vac advance is showing 13 deg +/- 1 at 2800 rpm, that would be with the vac plugged into ported relative to 0 (TDC). So, if I set my static at 10 deg per se, I should be seeing 23 degrees on the flywheel at 2800rpm?

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1972 MG Midget MKIII "The VB Weller"
55,400 Miles
Watkins Glen, NY



Edited 4 time(s). Last edit at 2024-04-15 09:20 AM by scotty1998.

Now knowing those tables, in general.

-Tuning will give you iniital advance and often Total Advance which =Initial + Distributor Mechanical Adv at Crank.
It Does NOT include Vacuum Adv.
Total advance is always set with NO Vacuum Adv connected and the vacuum port on Carb/Intake capped.

-Advance Tables for the Distributor usually show the Mechanical Advance of the distributor which is 1/2 the advance at the crank.
So a distributor that has 12 degrees Mechanical advance at the distributor is 24 degrees mechanical at the crank on your timing light. Your Initial Advance will be in addition to that. So if you have 10degrees at Idle+24 degrees Dist Adv at the crank you have Total Timing of 34degrees.

Vacuum advance will be stated separately.
You can check your Vacuum Adv amount by setting your Total Advance without VA and not what you have. Then plug in your VA and increase RPM and your VA will come on.

I see. So, I've been following your advice on other posts regarding timing. I did the John Twist mod on the stop by welding a bead to limit the total advance. I've been capping off the vac advance from the port side with a golf tee and revving the engine to about 3500rpm, then setting total max mechanical advance at 34 degrees.

At idle (around 1,000 rpm), the timing looks to be around 20 - 22 degrees which I think makes sense with what you're saying. So, the table shows mechanical advance of 8 degrees distributor (16 crank) at 1150rpm which might be something like 7.5 at 1,000rpm (15 crank). 15 + my initial static at 10 deg is 25 btdc? However, because I'm 3 degs shy of 25, does that mean my static was actually something like 7 or 8? (The flywheel marks are pretty tough to read).

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1972 MG Midget MKIII "The VB Weller"
55,400 Miles
Watkins Glen, NY



Edited 1 time(s). Last edit at 2024-04-15 09:50 AM by scotty1998.

You're plugging the Vac port on the Carb/Manifold, not the tube going to the Distributor Right?

Many set their Total Timing that way, 34 is fine. With NO Vac Advance,

If your timing at Idle is 20 degrees that means you only have 14 degrees Mechanical advance at the Crank unless your distributor is already advancing at idle, which is not what you want.
You should have NO Mechanical Adv at Idle.
Idle is all Initial timing which does not include the Mechanical/Centrifigal Adv of the Dist.

The Stock timing mark on the 1275 that I have (69) is a small notch on the front pulley with the timing indicators on a sheet metal tab on the bottom side of the Timing Front cover.
What flywheel marks are you referring to?

I really think you need to get with an expert here on what you have going on. I am not clear on it.
Jeff at Advanced Distributors would be the guy to rework your distributor and straighten out what you have going on.
He usually keeps an eye on Distributor Posts and may drop in here.

Thanks for the continued response. Yes, the vac advance is plugged from the carb side. I guess I'm confused on how the mechanical advance works then. If the chart says that the distributor advances 7.5 degrees at 1,000 rpm, then aren't I getting mechanical advance at idle? Maybe the chart means that the vac is engaged? Hmm. Don't know. I have been having carb tuning problems due to a faulty fuel pump where it leans and enrichens causing the idle to swing up and down a bit. That really just shows up as a jumpy timing mark though.

I could put a vac gauge on the carb port and just make sure I'm not getting a signal at idle. If I put my finger over it, there's barely a draw though.

I guess I meant the pulley, not the flywheel. I have marked the notch on it but it's just difficult to see in the crazy small window in front of the car where it could possibly be off a degree or two. Certainly not enough to cause an error like I'm talking about.

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1972 MG Midget MKIII "The VB Weller"
55,400 Miles
Watkins Glen, NY

No, there should be no mechanical Adv at Idle. (Unless you have a high idle that is higher than where Mech Adv Starts)
Your table says it starts Mech Adv at 1000 rpm? You sure you're not looking at it saying you set Initial Adv at 7.5 degrees at 1000 rpm Idle?
If true as you say and that's advance at the Dist Table that means you have 15 degrees advance at the crank at 1000rpm, which ain't right.

At idle your advance is all Initial advance set by rotating the distributor.
The Mech Adv will start at the specified RPM above Idle.

Do not have your vac connected when setting up timing.

So I'm afraid I'm confused and don't understand what's going on here. Probably my fault.
Don't want to lead you down the wrong path.

Why don't you post a couple picture of the manual so we can see what you're looking at.

You are correct, the mechanical advance does come on at idle (unless you have an idle below 600rpm).

Looking closely at what it says in the Bentley manual, Introduction section, which includes the specs for every year and destination, it gives a setting for static timing advance (no engine rotation) and then it gives a dynamic timing at a certain RPM and at the specified idle RPM the timing advance target matches what the static timing + mech advance at that RPM.

Example on pages 37 & 38:
on page 37, for 25D4 distributor, at 600rpm it ways 0~1deg advance and at 1000rpm it says 10~14deg advance.
Then, on page 38 it restates that in a slightly different way: static ignition timing 7deg BTDC, 13deg at 1000rpm (25D4 distributor).


Note that the timing given in the manual was only optimized for that spec (and with compromises for emissions, etc.). Your particular engine may run best on different timing. In general, your goal is to use as much advance as your engine and fuel will allow without detonation.

This is the sheet I was referring to. I think I just have an misunderstanding on what the thing is telling me. I'm thinking it means that this is the performance with vac advance engaged.

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1972 MG Midget MKIII "The VB Weller"
55,400 Miles
Watkins Glen, NY

There ya go. Thanks, oops Norm



Edited 1 time(s). Last edit at 2024-04-15 11:16 AM by Mtn Sprite.

So, Kerr, based on setting by total mechanical advance of 34 and then checking the timing at 1000 and seeing say, 22 degrees, makes sense then? I do have jumpy idle right now so, it could technically be a bit lower. So, I'm not crazy?

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1972 MG Midget MKIII "The VB Weller"
55,400 Miles
Watkins Glen, NY



Edited 2 time(s). Last edit at 2024-04-15 11:11 AM by scotty1998.

Scott,

It's great to understand how all this works, but you are really overthinking this. Set the distributor where the car runs best (no matter what the numbers are) and forget it.

Rick

Ok!

It's a fine tuning sickness of mine. Mostly I was concerned that the John Twist mod I made on the stop was too much, or too little for the advance to work correctly. I figured I'd use the tables to check and when I was getting some wonky idle numbers I figured I'd ask.

Hopefully will be back on the road this weekend with a new fuel pump and tuned carbs and then I can put it under load.

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1972 MG Midget MKIII "The VB Weller"
55,400 Miles
Watkins Glen, NY

Total timing above 3000 does NOT include vacuum, since that only comes on under no-load conditions and won't induce pinging/detonation.

Your timing at idle will be moving around a bit if you're into the advance curve already.

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jeff@advanceddistributors.com

Thanks Jeff. I'm so close, yet so far with a bad fuel pump. Once I get everything back together and on the road, then we'll see how things go.

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1972 MG Midget MKIII "The VB Weller"
55,400 Miles
Watkins Glen, NY

Also keep in mind, that table is a bunch of info someone copied and typed from old Lucas books that had plenty of typos to start with. There's a ton of mixed up data about rotational direction being wrong, as well as distributor and crank rpm and advance numbers getting transposed. And then people swap parts in their distributor or modify them like I do. Its rare to see one here that hasn't been modified in some way.

What you want is good power at all rpms, stable timing marks at idle, all-in of 34 degrees at 3000-ish. Hopefully 14 or so at idle?

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jeff@advanceddistributors.com