Advance Mechanism Help Please

[Ian Williams]

On some of the DK series of distributor One of the springs is intentionally loose and only governs advance above a certain RPM..... two stage if you like. There are some very good Lucas instructional booklets written for their trainees which explain all of this very well and are worth searching out and reading. They were available on line a few years ago but I don't know if they still are.

[Chris KC]

There are some very good Lucas instructional booklets written for their trainees which explain all of this very well and are worth searching out and reading. They were available on line a few years ago but I don't know if they still are.

One of the two I have is attached in the thread above Ian - the other was too big a file, but can be found in instalments here: https://mgaguru.com/mgtech/books/lucas400e.htm

Possibly there are others I'm not aware of.

[John P]

Just had a more detailed look and searched the internet for some info.

The distributor is marked DK4A but has a model number BN24 which looks like it could refer to an Austin 8 so possibly the distributor was changed in the past and the replacement was from a more modern Austin 8.

Not sure if, apart from the different toggles, this makes any difference.   Ideally I'd like to have found some sort of comparison between the various springs but no doubt that info no longer exists.

John.

[Chris KC]

BN24 is as good as it gets (n.b. there is more than one variant of BN24...)
Anti-clockwise rotation, 7-8.5 degrees of advance.
The "correct" advance spring set is 405396/S but as I recall the "Distributor Doctor" supplies a substitute 405508.

p.s. I just checked and yes, my DK4A with DHYH4A internals is on Mrs. Lopsided. Please consider that likewise, your unit may say BN24 on the outside, but there's no telling where the internal parts came from. If you suspect they are from a later unit you might want to check the lift & shape of the cam against a reference part, as this may affect how you should set the points. One clue is that you may find the advance angle stamped on the action plate. FWIW my hybrid unit (with home-made springs) has done 1500 miles since fitting and is running nicely - which suggests that being somewhere close is at least better than sheer neglect.

[John P]

Well. I'm mystified!

I checked over all the various pivot points in the advance mechanism and although they are a little worn nothing seems seized.

I put it all back together and set the engine up using a feeler gauge so that the points just started to open at TDC and the engine started and ran quite smoothly at tick over.   On the road it seemed to run OK until I came to even a slight (by Devon standards) incline when I had to change down.   Once the revs were up again it was OK but definitely didn't really have much pull at low revs.

I then tried using some timing marks on the camshaft pully and a rather old timing light which works OK but is a bit of a hassle to use as it needs a 12 volt supply to function.

The timing wasn't exactly on TDC but not that far adrift but when I increase the revs there doesn't seem to be any significant increase in the advance.   To be honest it's not very easy to see what's happening and the timing does seem to be varying slightly even at a constant running speed presumably due to the wear in the distributor.

I'm sure the advance mechanism isn't seized so either the DKY style cam and toggles are the issue or the springs are too strong, neither of which really seems to be that obvious.

I'm beginning to think I'm just going to have to put up with it until I can afford either a better DK4A unit or even a new electronic one.

John.

[John Cornforth]

Hi John

To answer some of your questions I have dug out my notes from the recent rebuild. I measure the toggle holes as 1/16 inch diameter and their centres are spaced 3/16 inch apart. Most of the ones I have seen have a slight countersink, to stop wear on the spring ends presumably.

My two springs are identical and consist of 9.75 turns of 0.037 inch diameter wire. The length from the extremes of the ends is 0.807 inch, the length between the two inner faces of the end hooks is 0.733 inch. The latter stretches by 30 thou when installed and under initial slight tension, i.e. the installed length is 0.763 inches. Your photo is a bit fuzzy but the springs look to be 7 turns or thereabouts, so they may be from another model and be too strong.

I know you have taken it apart, but when reassembled can you easily prise the two weights apart and see the cam advancing ? The cam should be free to rotate on the spindle, there is a shoulder to stop the central screw from tightening it all up. Also, the weights should have spacer washers underneath so that they can pivot easily.

Distributor Doctor always emphasises that exact spring length and tension are critical, and difficult if not impossible to get right without actually having the unit on the bench.

The rim of the pulley on the camshaft is 4.25 inches diameter, and represents 720 engine degrees. If the distributor advances by its spec of 16 engine degrees that's a movement in the timing mark shown up by the strobe of only 0.3 inches.

[John P]

John,

Thanks for all the info.

The springs in the distributor have approximately 7.5 turns and are 0.820" overall.   The diameter of the spring is 0.185 and the wire diameter 0.038" so actually fairly similar to yours apart from the smaller number of turns which would presumably increase the spring rate.

Whilst the distributor was open I tried prizing the weights apart and the cam definitely moves OK but trying to open the weights by rotating the cam is extremely difficult.

I now have to admit to a schoolboy error in that I hadn't thought about the camshaft running at 1/2 engine speed meaning my timing marks were just under 0.6" apart so no wonder I didn't get the full advance I was expecting!

Anyway, I have now played around with the static setting and the car is definitely running better although still not perfectly.

John.