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MG Midget and Sprite Technical - Advance curve - definitive answer

Good afternoon folks,
Timing is on the agenda today, as I got a new adjustable timing light for Christmas (sad old sod).

The distributor fitted to my 72 RWA midget is an original Lucas 25D, marked 41270 D 16/72.

The engine is an original 1275cc 12V-586 unit.

Static ignition timing is 7 deg. BTDC
Dyno. timing stated as 16 deg. BTDC @ 1000 rpm, although the car is currently set at 13 deg. BTDC

I am Checking if the old dizzy is advancing OK.

Looking at various sources for advance curve data, different figures appear, some quote:-
20-24 deg. at 3600 rpm, while others state:-
20-24 deg. at 5600 rpm?

Presumably you have to add the static onto the above to get the total advance at the crank? (with the vac. disconnected)

Anyone know the definitive answer so I can proceed?

Regards, Tony

Tony G

Tony. There are a number of methods of listing the distributor advance curve and you have to know which one is being used to make sense of the information.

From the distributor maker (Lucas) the advance figures are given in degrees of distributor advance at various distributor rpms. When installed in the engine, the distributor rotates at one half engine speed and one degree of distributor advance equals two degrees of advance seen at the crankshaft. (e.g. the distributor reads 500 rpm and five degrees of advance on the distributor test machine. When reading the figures with the distributor installed on an engine and measuring at the crankshaft pulley, the readings would be 1,000 engine rpms and ten degrees of mechanical advance.)

Figures given in the factory workshop manual may be either the mechanical advance curve for the basic distributor (as measured at the crankshaft pulley) or it may be the combination of the mechanical and static advance (i.e. what would actually be seen when the engine is started after static timing). Generally, by the late 1960's, all of the workshop manual figures are the static plus mechanical advance and indicate the readings you would expect to get with your timing light measuring at the crankshaft pulley of your engine.

One extensive list of Lucas distributor specifications has been removed from the internet and the page I have book marked is no longer there. The next most complete list, still in existence, does not show a 41270 specification, but does show a 41271 which is listed for the 71 Sprite. That advance curve is:

4 deg at 600 rpm
19 deg at 2,300 to 2,500 rpm
30 deg at 4,300 rpm

Most people seem to feel that the A series engine (high compression) runs best with about 32 degrees of mechanical advance at about 3,500 to 4,000 engine rpms. Why not set your engine up at 32 deg at 4,000 rpms, then read what your actual advance is at 3K, 2K and 1K rpms? That should give you an idea of what is happening and, after a test drive, you can modify the advance if necessary.

Yes, at the crankshaft pulley, you are reading the static advance and the distributor's mechanical advance when the engine is running.

Yes, always test/set the distributor with the vacuum advance line removed from the dizzy end and plugged.

None of this is absolutely exact and a little testing with your new timing light will give you a better idea of what is happening and how well your distributor is working. Old springs lose their tension and allow the mechanical advance to come on too early. Worn parts can operate in an erratic manner and not be as consistent as when they were new. Parts that need to be lubricated operate differently when they have not been lubricated in several years.

Let us know what you find and we can supply more (and better ideas). Hard to be definitive when you do not know the history of the distributor and how well it has been maintained over the years.

Les
Les Bengtson

If it is a 25D distributor the 13 deg stroboscopic would appear to agree with the 67 owners manual.

The 20-24 deg at 5,600 rpm again agrees with a workshop manual I have here for a 25D distributor.

The 41270 D 16/72 would tie back to documentation to the springs spec and total mechanical advance of the distributor.

Again all measured with the vac. hose off and yes the mechanical advance is additive to the strobo figure.

Due to modern fuels etc these figures are a guide and I tend to give the distributor a tweek to get it running at the highest speed.

Then check the engine for pinking on a run.

The two differing length springs and the distributor bearing will also be a factor in the equation due to age and wear.

eddie
Eddie Cairns

This list has details of the 41270 distributor but the author advises caution as he says the data is from various sources.

http://www.teglerizer.com/mgstuff/advance_curves.htm

It suggests max advance at 3,600 rpm which is not in agreement with the 1972 British Leyland workshop manual!!!!!!!!!!!

It is possible one or both are in error.

Enjoy your Midget.

eddie
Eddie Cairns

Hi, thanks for your input, I will try the max of 32deg.
at 4k revs and see how it runs and what the 1k readings are.
The distributor is original and 60k miles, with very little play.

The following info. is what I found:-

1275cc 12CE/CD/V Midget MkIII 41270(HC)
(25D4)
0-1 @ 600
2-6 @ 800
4-8 @ 1000
10-14 @ 2000
20-24 @ 3600
Static 7 BTDC
Dyno. 13 BTDC @ 1200

The above taken from:- http://www.teglerizer.com/mgstuff/advance_curves.htm

and:-

LUCAS 25D4 41270A
2800 rpm 11 deg.
1150 rpm 7.5 deg.
400 rpm 3 deg.
<1971 MG 1275cc 12CE/CD/V Midget MkIII (HC)

The above taken from :-
http://www.mgaroadster.co.uk/Technical_Information/lucas.pdf

My distributor curve fits very well with the first list and not the higher end of the second.

Regards Tony
Tony G

NO...use only dynamic timing stats, NOT static timing

static timing is only used to start a new rebuilt engine for the 1st time...once its started, the engine is retimed to.dynamic stats

prop
Prop and the Blackhole Midget

So there are
Lies,
Damn Lies,
Statistics
and Lucas Distributor advance listings!

eddie
Eddie Cairns

Hi, started with a check of the existing advance curve for the 41270D Lucas 25D (vac plugged):-

1000 rpm - 13 deg. BTDC
1500 rpm - 14 deg. "
2000 rpm - 17 deg. "
2500 rpm - 21 deg. "
3000 rpm - 26 deg. "
3500 rpm - 31 deg. "
4000 rpm - 36 deg. "

Bit surprised it advanced up to 36 degrees.
The distributor is stamped 11 deg. inside. I thought the total advance should have been 2x11 deg. plus 7 static, i.e. 29 degrees total. Maybe its knackered?

I don't know where the quoted 16 deg. at 1000 fits in?

What do chaps you think?

Tony
Tony G

Tony. Any distributor can be rebuilt and, after more than 40 years, many have. A well maintained distributor will, in my experience, require rebuild by the 60K miles point. One that is not lubricated on an annual basis will require rebuilding before that point. So, some 40 years after your car was built it is hard to determine, except by actual test, what your distributor may be, regardless of the specification number on the side.

The specification you list, in your previous post, for the Lucas 41270 distributor is what you would see on a distributor test machine. Figures, both rpms and degrees of advance, should be doubled to convert to engine rpms and degrees at the crankshaft.

The 11 degrees stamped on the plate does indicate a total mechanical advance 11 degrees distributor or 22 degrees at the crankshaft. However, these were mass produced items, along with all of the other distributor parts, and an error of one or two degrees (two to four at the crankshaft) is not unknown. And, if the distributor has been rebuilt, you do not know if the stamping on the plate is correct--some internet articles write of filing the plate's stop arm to increase the overall mechanical advance.

I can see doing one of two things:

First, set the overall mechanical advance to 32 degrees at 4,000 rpm and see how the engine performs.

Second, replace the distributor with a rebuilt one which has been tested to give the original factory advance curve.

Of the two, I would suggest plan one would be the best first step. It is commonly understood that modern fuels, particularly blended fuels, burn differently than the straight petroleum products available forty years ago. Many have found that the best engine performance was obtained by having a distributor rebuilt to a custom curve as determined by a rolling road session. There are, today, more modern distributors which may be set up with any of several different curves (choose the one that works best) or are completely programable and can be set up to duplicate, almost exactly, what was found to be the ideal curve on the rolling road.

"I don't know where the quoted 16 deg. at 1000 fits in?" It fits in with the old days when a dial back timing light was not available and the engines and distributors were relatively new. Just as static timing fits in with the even more remote days when ownership of a stroboscopic timing light was something that only the more advanced professional mechanics were willing to pay for. Times have changed.

Les
Les Bengtson

I remember reading that in the 60s the dissy could be well out with just a couple of years use

and most owners wont have bothered to lubricate at all let alone each year/12k-miles as the good book tells us

I'm totally non-technical so don't know but wear on other components and changes in parts and components mean take figures with a degree of latitude and as a starting point only, when it sounds like it's running well then it probably is

after that its a rolling road session where you can get enough figures and stats to keep a maths enthusiast happy
Nigel Atkins

Hi Tony

Don't assume your timing light/TDC mark etc are spot on. You have a guide. You also need accurate rpm readings. Plugging the vac is rough and ready as the base plate is free to wobble without a little vac. remember you are dealing with shades of grey.

Peter
Peter Burgess Tuning

Tony

In the first of the two spec lists you give above I think there is a typo in the last line. The BMC workshop manual has identical figure for the first 4 lines, but the last line is 20-24 deg at 5600 RPM (not 3600 RPM).
This is exactly the same as the second list's 11 deg at 2800 RPM as the figuresin that list are in distributor degrees & RPM, not crankshaft like the first list.
(The second list is identical to the spec given in my very handy late '70's Lucas book listing the spec of every distributor for the previous ~25 years)

It is instructive to plot distributor advance figures on a graph as it gives a much better feel for what's happening than just trying to read numbers. I've done that for the workshop manual figures and the Lucas book figures and (unusually) they do show a small variation - but only at very low speeds (1000 RPM crank and below). In an operational sense this isn't significant other than if setting timing at idle with a light.

Adding Tony's as-measured figures to this graph reveals two things:

1. There is an unusual flat spot between 1000 and 1500 RPM. This suggests that there may be no advance happening until nearly 1500 RPM, though there could be other explanations. It would be interesting to investigate this further. For example, a reading at idle (700-ish RPM) and 1200 RPM and check the static setting. if they are all round the same figure it would indicate no advance early, alternatively somemething else might show up.

2. While the remainder of the curve tracks the spec ones well, it does so only up to the point where the second spring should take up. After that it just keep advancing at the same rate as the initial part of the curve. This suggests that the second spring has worn and is not taking up until much later.

Combining these two with Tony's observation that there is another 7 deg of advance from somewhere, my expectation is that the extra is coming from an increased static setting - see (1) above. This is exactly what would happen if the spark is set at idle with a strobe. (I'm not saying doing so is a bad thing, just that with a worn dstributor however you set the spark it will be right at that point and vary from spec everywhere else.)

There is another source of additional advance: Backlash between the distributor drive gear and the one on the cam. For some reason, backlash here always results in addtional advance at higher engine speeds. It is very common in B series where the cam gear has a lot of load from driving the oil pump as well and so wears much faster. The presence of this can be detected from plotting a graph, which shows a vertical step at one stage (usually between 2000 and 3000 RPM) and at the same engine speed the ingnition timing will fluctuate wildly. I've seen +/- 15 deg on B series at 2500 RPM. Many distributors have been overhauled because of this but to no effect - a replacement cam is the only cure. Fortunately it is very rare in A series.

Solutions:

For Tony's situation, I wouldn't get too hung up about the low speed variation. For the continued advance at higher speeds, it is usually necessary only to tighten the hook of the secondary spring to make up for wear. In this distributor the second spring should be taking up after the distributor cam has moved about 60% of its travel towards the stop. A fiddly job (I grip the base of the hook - where it joins the main coil of the spring - in the corner of a vice, then tap the hook end-on with a small hammer)which has pleasing results.

Taking this approach will sort 90% of the variation and not cost a thing.
Paul Walbran

We must have gone to the same school of keep em running Paul!

Peter
Peter Burgess Tuning

Thanks for all your input, greatly appreciated.

Took the distributor off yesterday to inspect.
Found the inside very grubby and gungy, the two weights were not moving very freely.
The two springs appeared in good condition and tight on their posts.

Before adjusting for any wear, I measured their free length and the minimum distance between the posts as follows:-

Light spring (primary) 17mm long, 15.5mm between posts, so under slight tension when in position.

Heavy spring (secondary) 17.33mm long, 16.96mm between posts, a good fit with no free play.

Although the plate is stamped 11 deg. I measured the total advance from rest to the stop at 14 deg.(see pic)

Maybe this would explain the total advance of 35 deg. i.e. 2x14 plus static of 7 = 35 deg.?

Apart from a good clean and light lubricate, replaced the dizzy and measured advance curve again, quite a difference:-

Before RPM After
800 11
13 1000 13
14 1500 16
17 2000 20
21 2500 24
26 3000 27
31 3500 32
36 4000 35

Should I shorten the secondary spring as Paul suggests and/or seek out a replacement advance plate to give the correct max. figure? or any other ideas?

Regards Tony

PS never thought a new timing light for Christmas would get me learning so much about my distributor!






Tony G

I'm not into maths or engineering but I do enjoy driving my Midget, I got a fully electronic 123 dissy, what a difference, much better

I later had a Peter Burgess rolling road tune, even better still

the only thing I have to measure and gauge now is refilling the full tank
Nigel Atkins

Tony

What are you asking here.

It could be is this advance curve at the optimium for this specific engine that is working with modern fuel.

The answer is you will know only if you put the car on a rolling road to check it and adjust the distributor every 500 rpm or so to see if the power goes up or down when you advance and retard the timing by rotating the distributor.

When you get the answer you will find if it really makes much of a difference to performance.

Then you either put on a 123 distributor that can take those exact settings or you send your existing distributor off to the "distributor doctor" to have the curve set up on his testing rig and have the advance springs changed or tweeked to get as near as possible to the desired curve if the rolling road run shows that this would make an improvement.

So to date you have got a lot of data that only says that the distributor is working but maybe not to the optimium setting for your specific engine.

Cost of doing this work and the rolling road say 200 to 500 depending on what you do.
Eddie Cairns

I've only got the standard 123 dissy

for (probably a lot) less than 500 you could get a (Peter Burgess) 123 programmable dissy and have it rolling road tuned by Peter Burgess

perhaps it's more expensive in Scotland
Nigel Atkins


if I can make one suggestion...

123 TUNE from peter burguss...NOT ebay

this is a great dissy and is completely tune able with a laptop computer, and can even do multiple advance curves at the press of the button

the bad news and I hope this has been remeded....there customer sevice sucks...you are on your own

BUT...if you purchase 123 TUNE thur peter burguss and NOT thur ebay... you have an ace up your sleve, as peter has the long standing reputation of doing and offer any.help thats needed...you wont get that from the 123 factory

sometimes more expensive is the best economical value of the day

just saying

prop
Prop and the Blackhole Midget

Nigel please do not misquote me on the prices I mention. I said between 200 and 500 and will explain as you obviously did not take in what I said.

Say take the car to Peter Burgess and apart from a few rolling road runs he would probably spend some time on carb needles and say the odd spark plug or lead to get the car running properly.

Assume there is no advantage to be gained from modding the distributor I cannot see the price being below about 150 especially if he has to replace the odd needle or plug lead.

I guess a rebuild of the distributor if needed by the Distributor Doctor would be circa 100.

A 123 Tune is in the region of 350 with VAT and the USB lead from Peter Burgess

So we have Derbyshire prices of between 150 just a rolling road and tweek or up to 500 if we add on a 123tune. The preset 123 distributor is 298.

I think my estimate is good and I have not suggested this should be done just what would have to be done to make sure that the engine is setup in the best way possible.

Eddie Cairns

Hi Eddie,
I didn't mean to misquote you and I didn't, I'll be honest and say I had no idea that the Tune was that expensive, is that list price?

if the car isn't too far off standard then stick with the 300 123

I've had 4 RR set ups on my present Midget alone and I do know what can be involved especially for those who don't keep on top of their servicing and maintenance - I usually say that the (whole) car should be fully serviced before a RR set up is practical (see my previous post if you disbelieve me)

I've been to an A-series specialist tuner where your 150 could easily be correct

I've also been to Peter Burgess, twice

I've quoted on this BBS more than once (and elsewhere) that the A-series specialist and Peter Burgess spent an equal amount of time doing almost exactly the same work - the difference was Peter Burgess done an excellent job and charged considerably less (and I mean considerably)

Prop,
sorry I didn't make it clear, yes buy the 123 tune from Peter Burgess and have him set it up on his rolling road
Nigel Atkins

yepp the 123 TUNE...IS the most advanced.and.newest.in the 123 family of dissys

it really is a great.piece of work, id think the time savings alone in.set up is worth the price of admission...its all computerised with a laptop...no actual hands on adjustment

that to me is just. so cool

prop
Prop and the Blackhole Midget

Tony
Mybe you Christmas present may not end up so cheap after all! 123 is good and worth it, but if you don't want to go there at theis stage (or ever) you can inexpensively get the distributor performing much closer to spec than it is now by working on the springs.

Whether tightening the secondary spring will work depends on the point at which it takes up. As I noted above, when I plotted what yours is doing it appears to be following the original spec primary line well on past the point where (originally) the second spring would take up. Before acting on this assumption it is first necessary to check this is actually the case by advancing the cam realative th the spindle until the second spring just takes up and measuring (or estimating) how much of the cam's total advance has been used at that stage. If it appears it is taking up later than it should. then tighten the hook.

However, yours is more complicated in that there is additional advance to waht there should be (probably someone ground the cam's stop away in the past). In this case teh effect of the extra 3 degrees is that you'll need the 2nd spring taking up after only half the total advance, not 60% if things were as original.

If you set it to this, then that extra advance will not start to occur until high engine speed - remember that spec has max advance at 5600 RPM, so above that, at the rate of an extra degree per 500 RPM. At max revs on a standard engine there would be only a couple fo degrees extra advance, which is much better than your current situation. (The full 7 degrees extra wouldn't be reached until 9000 RPM so becomes rather academic.)

On the other hand, if you find that the second spring is already taking up by 50% total advance, then there is an issue with the spring not being spec and tightening the hook won't help. In my experience the springs don't lose their rate, so if this is the case then perhaps it was changed when the stop was fiddled, suggesting that in an earlier life the engine may have been tickled a bit.
Paul Walbran

This thread was discussed between 09/01/2013 and 12/01/2013

MG Midget and Sprite Technical index

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