MGB & GT Forum

I am doing a minor cleanup and inspection of two distributors: one 40897 B 265, and one 40897 D 1265. The only obvious difference is a lubrication hole for the main shaft of the later "D" distributor.

After reinstalling the weights, cam and auto-advance springs, I saw that both springs on the "D" are always under tension, even when static. The secondary (heavier) spring on the earlier "B" is slack until the cam is turned about a third way to the auto-advance stop-peg. The primary (lighter) spring of the "B" is always under tension.

As I understand, having both springs always under tension will give a straight-line profile for the auto-advance. On the other hand, having the secondary spring come into play at higher revs will yield a bend in the profile. What this all means is way beyond my knowledge.

I don't know anything about the later "D" distributor except that hidden in the thick layer of grease was a small washer that clipped the plates as they spun. I'm fairly certain that the earlier "B" distributor is original apart from condenser, contact points and rotor arm. Both distributors seem in reasonable condition.

I see that Moss sell a set of five different auto-advance springs for this distributor.

I'd much appreciate any advice or further knowledge about this subject as I really have no idea what I'm doing.

With luck, Jeff Schlemmer will chime in, and all your questions will be answered.

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Chris Howells

1968 MGB Purchased already dis-assembled but which is largely back together so I'm a lot less ignorant.

Chris, This topic does need some expertise, and I might bump it along for a while to see if I can get a response.

I suppose my query is really about distributor settings and modern fuels. The earlier 40897B distributor seems to be close to the original settings of the nineteen-sixties and it hasn't seen the light of day since before the introduction of unleaded fuel. I don't know about the later 40897D which seems to be incorrectly set, but may have been adapted for modern fuel. That's part of what I would like to know. And what's the story with the Moss set of five different springs? Trial-and-error to see which combination of springs work best?

I once had a friend (long deceased now) juggling springs in his car's distributor. After many days he finally settled on a set he modified himself. Since he never used a dyno it was always touch and feel. MOSS basically advises the same, but in careful wording advises starting with the strongest ones (lowest advance curves) first. So if you've got time to spare go ahead. The risk being you end up with a pinking engine by too high advance without knowing it, so have a dependable strobelight ready....

Juggling sounds more like an art than a science. With so many variables, a little bit of art is probably what's required.

Comparing the two sets of springs from the almost identical 25D distributors, it appears that both spring sets are original (Lucas part number 54415428) - same gauge and same number of turns. However, the heavier secondary spring from the distributor with unknown history appears to have been modified to shorten its reach. See the attached photo, second from left. The hooks have been bent further down and the ends of the hooks nipped off. Makes me wonder if this is not an unusual tweak.

I fully agree and that's why I never folowed my friends example. Now with programmable electronic units a click with the mouse defines one of the rpm/advance points and the more expensive units permit many more adjustment variations than any 2 spring mechanical system can cover. And dwell variation over the rpm range as a bonus.
Still I wonder whether such electronic refinements are better choices for modern engine management systems, but a strange part inside our older engine designs with their inherent wider mechanical tolerances

Don

I expect that if the MGB was not such a forgiving creature, there'd be far fewer around.

Posting here because I may have found a more thorough answer to my question while reading through some informative links from tvrgeek (Scott S) on another thread.

The following article "Distributor advance rate adjustments: compensating for high mileage wear" appears to have been written before the new sets of auto-advance springs were available, and it discusses shortening the existing heavier spring to compensate, not for fuel differences, but for wear in the centrifugal advance mechanism, the wear being mostly found in the hook of the spring or the spring's supporting pins. In my case, the previous owner of the distributer may have over-compensated or swapped from another dizzie, leaving the heavier spring under tension even when idle. See:

http://www.mgparts.co.nz/advice/technical-notes/distributor-advance-rate-adjustments-compensating-for-high-mileage-wear/

(BTW: just to give perspective of the spring sizes, tthe matchstick in the photo posted earlier is 50mm, not standard.)

All 25D distributors had the same bob weights so the different advance characteristics required for different models like high or low comp, carburetors, emissions and transmission was accomplished by various springs and the length of the advance stop. The ones with the slack secondary spring usually brought in the advance earlier. Denis

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68 B roadster, Daffodil yellow, supercharger, Burgess SC head, SC cam, Mikuni HSR 48 carburetor and engine built for supercharging.

73 BGT V8 conversion starting with a bare shell. Built the engine "3.9L Rover" early in 2016 with high comp pistons and a few other nice bits, plus a T5 ford trans. Started on the body late 2016 and complete late 2017, Did all the work myself, mechanical, body. paint etc.
Finished and going well, great to drive and quick. Now has a nice 3.23 LSD.

Most of the advance curves show a bend or "knee" shape implying that the secondary (stronger) spring is slack up to the point of the bend whereupon the rate of advance slows (the curve becomes flatter) because the stronger spring comes into play at those higher RPMs. A secondary spring that is always under tension, even when idle, seems to make the need for a primary (weaker) spring redundant except that the rate of advance is always restrained by the combined force of the two springs.

The appended document can also be of help for those who want to try compensating for wear or modify a distributor behaviour. BTW the springs shown in the earlier posts
seem to be of two equal sets, to be expected for 45D versions with equal springs. MOSS E supplies 5 different spring types, without any indication but "start trying the strongest ones first"

Still, without any testrig available I would hesitate in entering such a process. If I wouldn't have lost my original 25D I may have sent to Jeff instead and put a 25D Accuspark in afterwards.

Now I make do with those aftermarket ones which give me the best driving experience. 123/CSI are quite expensive units and I am not a fan for such technologies in an oldfashioned car
It is just for those who do not mind going full electronic in an oldtimer that my remark on freedom of tuning advance curves to your liking was directed......

What I notice from some advertisements for the 123 is the suggestion that a major step towards advanced motormanagement is made with these units. This is not true, one just gets some control on centrifugal advance curves, none on vac advance but use it or buy one without and this dwell issue. Motormanagementsystems in modern cars measure actual engine temperature, take into account time, driving situation and also steer fuel injection and ignition systems in conjunction. Their programming is much more extensive and the lambda sensor for checking the effectiveness of the injection and catalysator function can also be used to help sensing pinking danger on the spot and correct it. All not provided for by just exchanging the dizzy technology by electronics

PS I have dowloaded an another, rather thick document on ignition settings in relation to modern systems. It is no help with the B, but makes one understanding the issues better

Don

Thanks for the article. Im a little uncomfortable with the conclusion, "What is clear is that ignition advance curves are simple to change and assuming the correct advance curve is known, it is easily obtained." This assumes a ready supply of advance springs with various known ratings and lengths.

The photo of the advance springs that I posted shows the sets of springs from two 25D 40897 distributors. The two secondary springs are on the left and the two primary springs are on the right. I was trying to provide a comparison between an untouched secondary spring on the far left and a modified secondary spring, second from left.

I'm happy to start with the article linked earlier "Distributor advance rate adjustments: compensating for high mileage wear", which gives as a guide for resetting the secondary spring, "the cam should have moved about 1/3 of the way to its stop when the heavy spring takes up."

I repeat my check with MOSS Europe listing TT1903 "Advance spring set Lucas distributors" not visible in the overview, but when ordered on part number they are there in limited stock.
When available they are with "5 specially selected distributor advance springs" and the text ends slightly different from my earlier quote: "We suggest you start with the heaviest two springs"
The MOSS wording suggests equal springs to be used in a 45 D configuration, not the unequal spring setup so customary with the 25D. You have to find your own way...

All the best with you efforts, it may take some time you will surely find a good setting in the end

PS In this Lucastuning doc it is assumed you can not only calculate the spring tensions if knowing the spring steel properties but measure them too. The diagrams are not the difficult part IMHO, but how to reach any given diagram with the springs available to you and test it in a testrig (assuming measurement when in the engine is too cumbersome, especially at high revs)

Are the springs between the 25 and 45 the same basic length? In other words, would the 25 spring kit be possible experiments in a 45?

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Cogito ergo sum periculoso

Don your information on the 123tune is not correct as there is full vacuum advance plotting together with many other useful and extremely easy to use features. Great for tuning modified engines, A novice can simply copy a known good graph or someone with a bit of experience can plot their own and all within minutes. Even a feature that gives a acceleration time readout in one gear , sort of a private dyno if you can find a quiet road. A great easy to use unit but I would agree not necessary on a stock MG Denis

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68 B roadster, Daffodil yellow, supercharger, Burgess SC head, SC cam, Mikuni HSR 48 carburetor and engine built for supercharging.

73 BGT V8 conversion starting with a bare shell. Built the engine "3.9L Rover" early in 2016 with high comp pistons and a few other nice bits, plus a T5 ford trans. Started on the body late 2016 and complete late 2017, Did all the work myself, mechanical, body. paint etc.
Finished and going well, great to drive and quick. Now has a nice 3.23 LSD.

3X for the 25D Accuspark distributor. Makes a big difference compared with my the old wiggling Lucas one. Fine running at all revs and steady idle.