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MG TD TF 1500 - Advance Curves vs. 30° @3000

I'm having problems creating the popular 30°advance at 3000 rpm. My TD (Lazarus) sorts a late TD engine with a 40368 distributor. As built, it doesn't reach 30° until 5000 rpm. What makes me think that I would have reasonable advances at every day engine speeds if I set it for 30°@3000? Looks like I would need the mechanism a 40089 YA/early YB. Or, would I need a Jeff Schlemmer special, or one of Len's CFI's? Bud

Bud Krueger

Hi Bud,
The initial slope is built from the first spring.
The second slope is built with the sum of the two springs
And the knee between the 2 slopes is determined with the initial play of the second spring. Actually, this is why I call it the second spring.

And last, the maximum advance (flat) is fixed by the diameter of the holes in the advance plate.It is a stop which prevents the bob weights to go further.

If your advance is 30° only at 5000RPM then you need weaker springs. One or both.

You don't need to change the mechanism. Just select the right springs.


Laurent.
LC Laurent31

Thank you, Laurent. Your description of the mechanics is terrific. It almost seems as if what I need to do is to find a very weak secondary spring to reach the final advance at 3,000 rpm. That would still allow the reasonable advances for normal driving below 35-40 mph.

Thanks again, Bud Krueger
Bud Krueger

The more I think about it, the more important it is for me to have Lazarus being able to easily roll along at a walking parade pace. Rapid acceleration isn't as important as being able to roll along at idle speed in first gear without having to keep goosing the throttle with the clutch in. Lazarus and I do at least 2 parades per year (July 4th and November 11th). Been doing this since 1995. Same clutch disc.
Lazarus will be 72 years next Monday (Oct. 9th). The odometer is at 49991, 32,000 of which I've put on since I put him back on the road in 1995.

What are the important ignition and carburetion parameters for rolling along at a walking pace?

Bud
Bud Krueger

Bud,

Do you have the original 5.125 rear axle? if not then that will help in keeping the road speed down.

John

J Scragg

I've had a set 4.3's for quite a long time, John. With an idle around 700-800 rpm I can roll along with parade walkers with just a wee bit of clutch depressing for most of a parade. Things get a bit ugly when the coolant temperature gets up into the 90's. Have to goose the throttle and clutch too much. My coolant system can handle coolant temperatures up into the oil pressure reading. I recently reinstalled my Arnolt heater for added cooling. Bud



Bud Krueger

Hi bud,
Not so frequent to optimize an XPAG for a walking pace !
I would not bother about the maximum advance and even the slopes in that case.
Static advance is the important parameter. And whatever the ignition curve, I would tune it at idle to get the maximum RPM. I think it would indicate the best yield from the engine and thus the best ignition advance for idle.

Laurent.
LC Laurent31

British Vacuum Unit can set up any curve you need. You may want to play around with optimal timing at idle RPM to find where it can idle all day with the least heat buildup. Then build in the rest of the curve to suit normal driving. No need to go to any fancy electronic stuff unless you want it for other reasons.
Steve Simmons

I need to find a driveable piece of territory where I can emulate a parade for an hour or so. Preferably a place with a few small hills on it. The weather here is low 80's in mid-to-late afternoon. I'd try moving along in low gear at idle with occasional stops. Which raises temperature more, higher advances or lower?
Bud
Bud Krueger

Generally speaking, retarded ignition raises temperature more than overly advanced.
Steve Simmons

OK with Steve. And the reason is clearly explained in the excellent book of Paul Ireland about modern fuel in our Xpag. P55. OR in this article he published in TTT2 :
https://ttypes.org/manchester-xpag-tests-fuel-and-tuning-part-3-cyclic-variability-ignition-timing/

Laurent.

LC Laurent31

That's a great article, Laurent. I've begun searching through my (complete) collection of TTT2's to find the next issue of his writing. Bud
Bud Krueger

The latest changes from Len Fanelli's guidance - Installed a set of NGK 4085 plugs (1/2" reach in my late head??). Prestone Cleaner in the radiator. After a lot of fiddling with timing (now at 5°) I can idle at ~ 500 rpm for well over an hour with coolant staying at 80°C. (A little hesitance when accelerating.) Timing at ~17° at 2500 rpm. Will be draining the cleaner later and replacing it with 25% Prestone. I'll go and hunt up my practice parade ground.
(Must see if there's a 3/4" reach version of the 4085 plugs. Must admit that I do tend to have carbon on the lowest couple plug 3/4" threads.)
Bud
Bud Krueger

Try a bit more advance at idle. Modern fuels burn a lot differently than older formulations. I generally run at least 7* with around 32* at 3500RPM.

I don't feel that the type of plug is likely to make much of a difference if any. But 3/4" will definitely work better. Just grab a set of standard plugs unless your engine has special needs. I've been running the stock type Champion plugs for the past 20 years with no issues.
Steve Simmons

Yikes!! I've been using the wrong reach plugs for almost 30 years. Lazarus' head is a 168422 casting #, a late, 3/4" reach version. BUT!! I had .100" removed from the head to up the compression ratio. The thread on our spark plugs has a pitch of 1.25mm per thread. Two threads=2x1.25mm=2.5mm 2.5mm/2.54cm/inch=0.098". That explains why I've had carbon on the bottom two threads of my 3/4" reach plugs. Len was right after all. Bud
Bud Krueger

Once you cut a bunch off the head, that changes things!
Steve Simmons

Bud, I'm confused. Shaving a tenth of an inch off the head seems a lot but, presumably, that didn't get up to where the end of the plug gets into the combustion chamber. I've attached a pic of my old style (banana holes) head. If memory serves, the plugs enter on the top of the head as shown in the pic so shaving the head should not change the depth of the threaded hole that the plug goes through. Also, and I'm probably wrong on this too, I thought the older head (banana hole) used 3/4" reach plugs and the newer heads used the 1/2" plugs but I could easily have that exactly backwards. Help out a befuddled old guy.

Thanks.

Jud

ps: That head has since been reconditioned and it looks a lot better now than it did then.


J K Chapin

Early heads were 1/2" reach. Later heads 3/4". I've never seen one shaved so far that the plug had to be shortened though. The plug holes are at such an angle that taking 1/8" off the head doesn't really change much at the plug itself.
Steve Simmons

Thanks Steve. I figured I'd get it backwards so in my shop I have a big sign that reminds me to get 1'2" plugs for my old style head.

Jud
J K Chapin

This thread was discussed between 02/10/2023 and 09/10/2023

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