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Distributor advance curves

As we all know it’s the combination of a petrol / air mixture with a spark that produces the power in an engine. If power isn’t made in the combustion chamber it isn’t made anywhere else and you can only save it through friction reduction etc elsewhere in the engine. So it’s important to get that spark at just the right time to get the most power from that expensive petrol mixture as well as minimising consumption and pollution. With the side valve engine the location of the spark in the combustion chamber - the spark plug position—also has a bearing, see another page relating to cylinder heads and chamber shapes.

 

The spark has to be just at the right time and that does vary with engine speed and to some extent the load or throttle position. Although the speed of the engine suggests that combustion is instantaneous, it isn’t and the spark timing can be optimised to give the flame time to fully combust across the chamber and exert the maximum combustion pressure onto the piston at the right time. Combustion flame speed in the chamber doesn’t vary much with engine speed so if the engines going twice as fast the flame has to be ignited earlier with ignition advance—the spark is earlier, hence the advance mechanism in distributors and the advance lever on earlier cars.

 

Early A7’s had a magneto with a manual advance and retard control. Later cars had a Lucas DS4 or DJ4 distributor with manual advance and retard control and then an auto advance distributor the Lucas DK4A. The advance curve controlled by the centrifugal weights and springs were designed to deal with the characteristics of the standard engine. Tuning the engine changes that. Bigger valves and ports with free flow manifolds and carburettors allow more air / petrol mixture into the combustion chamber. Higher compression and different chamber shapes change the way the combustion flame front goes across the chamber. All these affect the optimum timing, some aspects require less advance, some more, often in different places on the engine speed. Another factor is that your standard distributor is liable to be at least 50 years old and could be worn, it may not be delivering the standard advance curve anyway.

Making sure your DK is OK

Most distributors are now 50 plus years old. Remarkably there are still some good ones about. Things I look for starts with a spin on the gear, it should be smooth with a nice clicking sound from the cam and points. Next check the wear on the gear teeth, a shiny set of teeth without undue change of shape along their length from wear or corrosion is good. Check the up and down movement of the gear, there should be just a little. Any more with a visible gap will give unpredictable timing when the gear meshing moves up and down but this can be adjusted out. Lift the cap and twist the rotor arm with the gear firmly held, it should move and then smoothly spring back proving the advance and retard. With the rotor arm off try lifting the cam spindle up and down, a little is OK but some move quite a way, again this can be adjusted out. Wiggle the shaft from side to side, again a little is OK but the less the better. Look for broken bits on the bakelite baseplate, the lip round the edge and the bedded in threaded parts are particularly vulnerable. The baseplate can be replaced but that’s more expense………. Make sure nothing is missing, the little top hats and washers around the points spring may have been lost over the years. The subtle check is on the rotor plate and weights. The advance of the distributor is set by the movement of the weights pegs inside the two drilled holes. These tend to wear little dints in the edge of the hole—giving more advance, this knowledge can be useful to us, read on below. The standard A7 distributors have 3 or 8 degrees advance AT THE DISTRIBUTOR, that equates to 6 and 16 degrees at the engine. 3 or 8 should be stamped on the rotor plate. If there’s something else this may not be an A7 distributor.

Notice the wear in the rotor plate hole, the distributor advance degrees should be stamped on the plate as well though not all are.

 

This is the way an A7 rotor weight hangs with the pivot at the left hand end under the spring anchor. This one is quite grotty but notice how the spring is stretched with the coils parted. The shape of the spring ends shouldn’t be distorted either or the advance will probably come in too early.

These types of distributor were widely fitted to British cars of the thirties and forties. There were 4 and 6 cylinder versions as well as clockwise and anti clock rotation. You can make a none A7 distributor work for you. The key thing is that the rotation is right. Look at the picture above, the weights need to fit in the same way as in the picture above. There should be a thin brass washer between the weight and spring quadrant on the top.

 

Stripping down is fairly straight forward. If the gear looks OK match mark it with a centre pop on the gear and spindle so it goes back on the same way round. File off the protruding part of the retaining pin and knock it through. Dress the through hole edges so that pulling the spindle out of the distributor body doesn’t damage the bearing bushes. After removing the baseplate pull out the spindle. Remove the screw under the rotor arm and lift off the cam spindle. Lift off the weights and spring assemblies. Give everything a good clean up.

 

The rotor plate is crimped on the spindle. I have seen some that are loose and wobbling about. You may be able to recrimp them with a well aimed hammer and small chisel but before you do that degrease thoroughly with petrol or thinners and apply a little Loctite. Obviously make sure its square. If the holes are worn as in the picture above they could be welded up and reset as described below.

 

Reassemble the cam spindle onto the main spindle and check the up and down float. You can reduce it by lightly dressing the top of the main spindle where the screw goes in.

 

Side to side float of the main spindle in the body can be rectified with new reamed bushes as long as the spindle isn’t worn which most of them aren’t.

 

The key parts are the two springs. Taking them off the pegs on the weights invariably stretches the eye of the spring. Previous owners may have done this by inserting a screwdriver between the coils which stretches the spring, see the picture above. If the springs and weights look OK, clean them up and leave them alone, you only need to remove them if there’s something wrong.

 

One part to watch out for is the cam spindle. For some reason the plate with the two weight pegs is often slightly bent in my experience. It may seem silly but make sure the cam only has four lobes! There are a few variations on the four lobed cams, despite what was written in the fifties on them I haven’t found any problems with any shapes.

 

Other bits and pieces are fairly obvious, look for wear or bent / broken bits.

The upshot is that the distributor may not be delivering the best spark timing for your engine. Those with manual advance and retard can fiddle around with the lever and perhaps the good news is that the A7 engine is very tolerant, it will still go as long as the distributor is in reasonable working condition. You just may not be getting that last bit of ‘sparkle’ out of it.

By this point on a page I’ve usually gone through the standard references on the subject. The fact that I’ve got this far suggests that there aren’t any! OK, info on the distributor and the static timing plus all the usual ignition fault finding information is covered in many places but I haven’t really found anything that describes resetting this vintage of distributor. One firm do offer a breakerless conversion for it and Distributor Doctor does offer overhaul and resetting. Up to date information has to be absorbed and applied to the DK if you want to reset the curve. The ones I’ve found useful are Tuning Lucas Ignition systems, another of those useful little Speedsport books from the seventies. Also Tuning BL’s A Series engine by David Vizard (Vizard the Wizard) does go well into this for the Mini etc sixties and seventies Lucas distributors. These can be extrapolated across to the DK.

 

The one key reference worth reviewing if you’re wanting the most from your A7 ignition is in the now dormant Motoring Pages website. I’ve unashamiably ‘borrowed’ the information from there.

Resetting the DK4

Whilst springs do set the advance curve I haven’t experimented with them. You may be lucky and find some old/new stock part number 404461 or 416042/S but I’ve always tried starting with a set that look OK visibly from an existing distributor. What can be adjusted is the maximum advance that the distributor will go to. Remember the photo above with the little worn dint where the weight bumps up against the edge of the hole? By adjusting how big that dint is sets the total advance from the distributor.

With the bottom of the spindle firmly but considerately clamped in a vice and the advance mechanism built up; set up a geometry protractor with some sticky tape on top of the rotor arm. Set up an indicator finger from a piece of welding wire or similar pointing to ‘zero’ on the protractor. By carefully twisting the cam the advance is pulled out against the weights and the total advance can be read when the end stop is hit. Standard is 3 or 8 degrees, the information from Motoring Pages suggest 10 to 11 degrees for a tuned engine with the extra advance in the higher revs.

The advance can be increased by filing out the two holes, be patient and retry the advance on the protractor, you don’t want to overdo it! Do each weight separately, they assemble individually then they’ll both end up the same such that both weights bottom at the same time. I also err on the light side, the stops will bed in slightly when working so I stop about a degree short of what I’m really after.

I have toyed with the idea of setting up the distributor with a variable speed drive—a drill or sewing machine (!), and with a coil and Timing lamp getting the original curve set by the springs. From there it should be possible by tweaking the springs or trying alternatives to come up with setting a more ideal advance curve. Taking this a step further with the engine on a dynamometer the maximum power can be set at each part of the revolution range using a fixed distributor recording how much advance is needed at each speed. A DK4 can then be built to match.

Rebuilding

Rebuilding the DK4 is a straight forward reverse of taking it to bits. Things to set are the lift in the main spindle by fitting a thin washer behind the gear and the lift in the cam spindle by gently filing the top of the main spindle. Take care, both should have a little clearance and with the cam spindle make sure the weights aren’t nipped. The square spring quadrants on top of the weights should have a thin brass washer underneath them and also make sure the spring is in the right hole in it. See the photograph. If you’re fitting the original gear make sure it goes on the right way round to line up the holes. Either a spring roll Bissel or Mills pin is a good way of retaining them firmly together or use a soft steel pin well pened into place at both ends. In both cases drop of Loctite avoids any unplanned movement. Loose gears seem common. If you’re fitting a new gear make sure the end clearance is small and redrill across the gear and spindle with a size that you’ve got a good pin for. It might seem a bit basic but trimmed down round nails make good soft pins

Alternatives

Those that I know driving cars with magnetos either have no problems and run for ever or alternatively have no end of trouble and despite whoever the reputable rebuild man was they just can’t get any mag to work reliably for any length or time. There is an alternative. Willie McKenzie makes a mag to distributor conversion to fit in place of the mag. A Bosch distributor is fitted into a housing similar to the mag body with a Simms coupling to marry to the engine. A little tweak on the wiring and who will notice apart from you not breaking down every trip?

On side entry distributor caps, there is one available for the DS4 and DK4, it was fitted as standard to Morris 8 Series E I believe. There’s still a few new ones about at autojumbles, the part number is 409635 or 400316. These are ideal for a special with a low bonnet line. Watch out when fitting the cables, they have to be impaled onto spikes. Not only will you not have enough fingers to keep them all in line when you’re fitting them together but if you try to pull the lid down on all five at the same time the Bakelite will invariably crack. It takes longer but fit and squash them into place one at a time.

Bosch distributors can also be fitted in direct to replace a DS4 or DK4. They’re a Bosch 009 for a VW or some such. The body and spindle diameters are right but the body has to be machined back to give the right position for the drive gear. Again Willie McKenzie does them as does John Barlow. The red distributor cap does give them away though a black one is also available as is one with side entry cables for low bonneted specials. Although our experience has been very good in terms of performance, we’ve had a problem with the rotor arm. There’s a resistor for suppression or some such in the brass conductor on top of the rotor arm. We burnt one out and had the unfortunate experience of realising the spare (good—at least we had one!) was slightly different (Arrrgh!). It seems that it doesn’t like high voltage coils so watch out and make sure the spare you carry is a direct fit.