MG-Cars.info

Welcome to our Site for MG, Triumph and Austin-Healey Car Information.

Parts

MG parts spares and accessories are available for MG T Series (TA, MG TB, MG TC, MG TD, MG TF), Magnette, MGA, Twin cam, MGB, MGBGT, MGC, MGC GT, MG Midget, Sprite and other MG models from British car spares company LBCarCo.

MG MGB Technical - Modified distributor

I am looking for some advice on 'tuning' a Lucas 45D distributor for use in a modified engine.

I am reconditioning a standard disy, and have a number of 25Ds for parts with various bob weights and springs.

Not using vacuum advance.

I am sure Willy (Revitt) will have some suggestions??

Colin
Colin Parkinson

Colin. Until Willy gets around to answering, the following might help.

First, you need some idea of the modified advance curve you believe you need. This would be laid out similar to the advance curve specifications listed in the factory workshop manual.

Second, you need some for of testing machine so that you can operate the distributor at various engine speeds and use a timing light to determine what actual advance you have. (Remember, total advance is static advance plus the mechanical advance of the distributor.) For a one-off, use your engine as your test machine, assuming you have a dial back timing light and a good quality tachometer. Some tachs in the cars are off by several hundred rpm, not what you want when setting up a one off.

Third, the total mechanical advance is controlled internally within the distributor by the mechanical advance cams and plates. These can be interchanged among the same model distributors.

Fourth, the timing of the mechanical advance is controlled strictly by the two springs which work against the outward movement of the weights. Some distributors have two springs of the same weight, others have two springs of different weights. You will have to find what weight of springs, and what combination of springs, work best for you.

In general, you need to make sure the bronze bushing in the distributor body is new or as new to prevent distributor shaft wobble. You need to determine that the distributor shaft is perfectly straight or it will cause the timing to shift from one cylinder to another. (I have seen shafts with sufficient bend in them that the points, on one cylinder, barely opened and on another cylinder did not open at all.)

All of the other parts should be in excellent condition, particularly the pivot post holes in the weights and the pivot posts the weights fit onto. If they are not in perfect condition it will introduce slop when working. The holes in the weights can be bushed and redrilled, but this would require some careful machine work, preferably involving a lathe and either a milling machine or good quality drill press.

Years ago, Paul Hunt, Bob Muenchausen, and I did a lot of work and experimentation with the Lucas 25D4 distributor. Paul made up his own distributor test machine. Bob bought and refurbished a Sun distributor test machine that I later bought from him. But, we were all a lot younger back then.

Les
Les Bengtson

Even once you have determined what 'curve' you need it will be a long job with a lot of messing around without a distributor test machine. Or just send it away saying what curve you want e.g. Distributor Doctor.

You imply you will be using 25D parts in a 45D, there may well be some incompatibility between 25D parts and 45D parts, particularly the weights as the centrifugal mechanism is quite different between the two.

If you can run to it a programable distributor would be far easier, allowing to to make changes 'on the road' without taking it out and opening it up all the time.
paulh4

Thanks Paul, no cannot run any sort of electronic disy, never mind programmable! Take your point about 25D, will have another look.

And Les, thanks for all that, the engine is a new build, not running yet. I am just trying to get something to get it started. Looking for about 34 - 35 degrees total. No vac.

Colin
Colin Parkinson

This may help with the total. There is a stop-plate on the upper section of the spindle and the gap between that and the stop peg determines the max - the bigger the gap at rest the greater the max will be. In theory you can work out what that should be to give 35 degrees using the distance from the stop peg to the centre of the shaft plotted on a protractor - but I've not done it! Remember distributor degrees are half crankshaft degrees, and it's usually the latter used in timing 4-stroke engines.

What rpm that figure is reached is controlled by the strength of the 'heavy' spring. That is usually loose on its pins at rest, so the strength of the 'light' spring controls how rapidly the curve rises initially, then when the the slack in the heavy spring is taken up the curve flattens out and the rate of increase reduces. So it's more of two straight lines at an angle than a true curve.


paulh4

Hi Colin
Bit of a can of worms to be getting into from half a world away but, some info
Is this a road car or a race car --
Engine size-
What compression ratio-
Camshaft specs-
What fuel will it be running on
Cheers
willy

William Revit

Hi Willy, 1820cc, 10.75 cr, 285cam, 95 octane.

This is the MGA rally engine with the Pauter rods!

Colin
Colin Parkinson

Don't forget 95 octane is E10. These days the higher octanes at E5 are recommended for cars of our era, as well as giving better performance by allowing more advance without pinking.
paulh4

Sorry Willy / Paul petrol would be super unleaded 97 /98 octane.

CP
Colin Parkinson

Or 99 octane or zero ethanol 99 (E5 labelled).

Or if you're in some sort of sport some cheat fuel on the sly, nod's a good as a wink to a blind horse. 😉
Nigel Atkins

You may not be able to run a programmable distributor in competition, but using one to determine a suitable advance curve is probably a good approach. A dyno session has got to be better - and quicker - than what would be politely called "successive approximation", or unkindly "trial and error". Unless you already have a good idea what curve you need you could spend a lot of time swapping springs and weights etc.

As Paul says, once you have a curve then somebody like Martin the Distributor Doctor will likely have a range of suitable springs etc , as well as the data to quickly set up a mechanical distributor.

Jim
J N Gibson

Colin
Nice to see them rods getting a start
Forgot to ask, iron head or alloy-
William Revit

Iron head Willy
Colin Parkinson

Colin, if it is any help here is what we ended up with recently after RR session using a 123 programmable on a similar spec engine (slightly higher CR, a bit longer duration cam - tend to cancel each other out as far as advance requirements go) runningvon 98 octane, so may be a useful starting point for RR:
1000, 15
2000, 19
4000, 26.5
5000, 30 (maximum)
Paul Walbran

Colin
I was just about to fill you up with useless info but i see Paul has posted some specs from a roller session---rollers don't lie so i'd be following his info-
You mentioned 34-35 deg earlier, that's way too much, it'll cost you horsepower and the combustion chamber will run hot and end up in headgasket failure eventually, -32 is a known dead maximum and as Paul has proved on the rollers 30 was the best for his session--I do recall Peter B saying he was running back at 30 and below now on some engines
Just a few points----There is no way that I'd go against what Paul has proven on his rollers but I'll stick my neck out and tell you what I was going to anyway just to give you another base to work from-
If you're going to run without a vac. advance and with your cam which if it's a modern design, I'd imagine has the inlet opening around 35deg BTDC, you need as much initial advance as you can get.
These engines kick back on the starter with any more than 18deg so I'd aim for 17-18 initial and that needs to be solid at that till 1100/1200 rpm so that you have a reliable base for idle setting that'll be exactly the same every time-
You'll need to pull your dist apart and weld up the end of the advance stop so that you get 14deg maximum crankshaft advance in the distributor (7 distributor degrees) You can check that with a protractor, just assemble the weights and shafts together without springs and rock it back and forward and measure --seven degrees---and that'll give you 14 engine degrees-
Then refit the original primary spring but discard the secondary slotted spring and replace it with the lightest primary spring you can find--resulting in two primary springs, the original and the new light one--both fitted with a little tension, not loose like the old secondary--This combination should give a good solid unwavering return tension to get stable idle and then restrict advance from moving till after 1000 rpm
Ideally you're looking for 17-18 deg base from 0 to over 1000 rpm then advance should commence between 1100-1200rpm then advance smoothly to 32 deg full advance at between 3200-3500 rpm, preferably around 3300/32 deg total and stay solid at that for the rest of the rev range
I know this is a bit more aggressive curve than Paul has proven but because you have no vac advance it won't hurt to have a bit more in it in that 3000-4000 for cruising and shouldn't be too much to rob power in that rev range---at worst case and it's too much and pings a little round that 3000 mark on sudden full throttle then the timing can be pulled back a couple of degrees to say 16 idle and 30 flat out bringing it closer to Paul's numbers
Another issue is that with the timing up at idle compared to standard the engine 'can' run on a bit when switching the ignition off, so that's where it's important to have a stable/reliable idle and as slow as possible with a healthy idle mixture of 12.5:1 (5.3% CO)and the same on full throttle or even a bit heavier right up in the rev range on full hook, as much as 12:1(6.6CO) under full load is still ok and certainly shouldn't get back anywhere near 13:1 on full hook if you want it to last
Hope this helps---but yeah, a roller session once you've got it run in is the best

William Revit

Yes, fixed advance below 1000 is the one. The 123 makes that easy to achieve but as Willy has outlined can be done with the original distributor.

Apart from cr and inlet closing point, other things that affect timing are the amount of cylinder filling and chamber shape & swirl, so there is always the potential advance for variation between apparently similar engines, which make rr essential. The engine I quoted above needed less advance than some, others have been closer to Willys figures, and the max advance has been as low as 28 and never higher than 32 on 98 pump fuel.

Paul Walbran

Thanks everyone, particularly Paul and Willy.

All good stuff guys!

But I have located a modified unit at the right price which will do as a starter for ten!

Colin
Colin Parkinson

This thread was discussed between 23/01/2022 and 01/02/2022

MG MGB Technical index

This thread is from the archives. Join the live MG MGB Technical BBS now