1951 500 Twin and DKX2A Dizzie

[classicrider]

Is anyone familiar with this,I'm repairing and using the shaft of the Triumph 6T which is the same as the Enfields,except for the ignition advance.The triumph one has 12 deg stamped,so 24 total.Anyone know the correct figure on the 500 Twin one?
classicrider

[Tom 60 Chief]

My oldest Lucas book is 1958 to 1962, but I don't think the 500 Twin changed much from 51' to 58', at least in the information you need.  In any case, the 58' Twin, Meteor Minor and the 58' 700cc Super Meteor all used the DKX2A distributor part #40550A (40610) and have an ignition advance listed as
11 1/2 deg - 13 deg.  Perhaps someone with 51' specific information will chime in.  Regards, Tom

[Arizoni]

Just for giggles I did a little button poking.
Using the stroke listed for the 500 twin in Roy Bacon's book "Royal Enfield, The Postwar Models" I find that a 12 1/4 degree advance equates to the piston being .034 from TDC.

Interestingly, the book also shows the ignition timing on a 1949-1958 500 Twin to be "TDC retard" if that makes any sense.
Oh, the points gap is listed at 0.012.

[rotorwrench]

From the information that I have, there were three or four different versions of the P/N 40168 (DKX2A distributor) for the early Enfield 500 Twin with the distributor dyno set up. The 40168A was listed as having 9 to 11 degrees of advance. The other two or three (40168B/D and 40168E) all had 18 to 20 degrees of advance. The early 500 was set at TDC and the later ones had 1/32 inch BTDC when the points open. Most of this info comes from the 1936 to 1958 Lucas Manual and the rest is from Enfield 500 manuals. The 1954 manual listed 1/32 inch advance. A riders manual listed TDC but I don't know what date it was made. I figure it was an early one. Timing adjustments are performed with the retard mechanism held in full retard position. It even suggests making small incremental changes to eliminate pinking and mentions to scibe a mark at the distibutor base at normal position then goes on to mention that 1/32 inch movement off this mark will change the advance by 6 degrees so you can't move it much.

The later 500 Twins with either mags or the 18D2 distributors with Lucas alternator sets were set at 3/8 inch advance. All the Indian Tomahawk information lists this figure but I don't think its accurate for the ones with the distributor dyno electrical sets.

[classicrider]

Thanks for the info,i have a manual which gives 2 different timing figures,TDC and 1/32" after TDC,both retarded.Full advance,i think 5'16  to 3/8" advanced(i'll check).In order to figure that out in degrees i suppose i'd need the conrod length for centers then draw a circle of stroke diameter.
I'd better get this right as 9-11 would only give 18 to 22 which doesn't sound right.That's a big difference between settings compared to the 18-20 of the other 3 which would give 36-40 advance.
Looks like i'll need conrod length first if anyone has that available.
regrads     

[rotorwrench]

The old timers used to tell me that the early 500Twins had design problems that made them run a little on the warm side. The way the oiling system was routed for the top end was likely the cause. Because of this, they probably held back on the timing to keep it from the dreaded ping. You have a lot of leeway to experiment with it after you get it running. Even as funky as the early 500s were, they were still a formidable player in their class in 1951. If ridden sanely, they were still reliable enough to keep most owners happy.

I went back and reread the timing procedure and finally caught the 1/32 ATDC figure (not 1/32 BTDC). It took a while to sink in. That figure could have a tendency to reduce the amount of total advance even further. You can tell by reading the whole procedure that they sort of recommended fine tunning of timing after warm up by the amount of pinking that ws going on under road loads. A person might be better off rigging a degree wheel to work for trial set ups rather than using piston location. The later engines with the 3/8 inch BTDC figure were different bore & stroke than those older machines plus they had higher compression pistons to boot. They needed more advance. Their top end oiling system was all external. The later 500s were adjusted to 1/64" BTDC at full retard with the 18D2 distributor and 3/8" BTDC at full advance for the magnetos. On the auto retard sprocket for the mags, they were held in full advance manually for this adjustment.

[Arizoni]

The length of the connecting rod shouldn't have an effect on the number of degrees the crankshaft must rotate to move the piston a certain value.
The length of the stroke however has a direct relationship because it determines the radius of the crank pin from the center of the crankshaft journals.

In the same book mentioned above by Roy Bacon it shows the stroke of the 500 twin (1949-1958) as 77mm (3.0315 inches).
That would make the radius of rotation 1.5157 inches.

Using the formula: (R-d)/R = Cos A, where R= radius of rotation, d= drop of the piston below TDC and A= angular rotation of crankshaft, for a piston drop of 1/32" BTDC or ATDC I get the following:

(1.5157- .0312)/1.5157 =  1.4845/1.5157 = 0.9794 = Cos A.
Using the Cos^-1 function for 0.9794,   A = 11.6453 degrees.

For the 1958-1963 Meteor Minor the book shows the stroke as 64.5mm or 2.5394 inches.

[rotorwrench]

I'm sure that Royal Enfield degreed the crank and measured the distance to give those figures. The rotation off of TDC goes through a good bit of travel before the piston starts to go down and the same at BDC befor it goes back up. 20 to 25 degrees BTDC is a close "full advance" figure for most overhead valve, 4-stroke, air cooled motors depending on compression ratio and what kind of cam is installed.

In mid 1957, they changed the 500 to the larger bore and shorter stroke. These short stroke engines share a lot of components with the 700 twin. It made for better interchangeability and actually more reliability. Those mid 57 & later 500s were hard to kill. The earlier ones are getting hard to find parts for.

[classicrider]

Thanks for the detailed response,so it looks like the first 1/32" ATDC should take up close to 12 deg(11.6453 degrees),to get back to TDC,and the 20-25 should give a total of close to  37deg so the 19 deg stamped on another shaft i have would give 38 deg total advance.So all i have to do is drill the Triumph shaft stamped as 12 deg,out to the same as the 19 deg shaft,for a total of 38 deg between 1/32 ATDC to 3/8" BTDC.Thanks for the assistance.
regards
classicrider.
ps i checked and the manual says advanced timing is 5/16"-3/8'.And yes my engine has the internal oiling to the rockers.

[rotorwrench]

I can see what you are trying to state about the total number of degrees of rotation. When you use the term "advance" you have to be careful not to mix advance with retard. Anything before TDC is advance and anything after TDC is retard but this is really only semantics since it's understood what you mean.

What is evident is that the distributor may be capable of delivering more advance than is necessary for the engine so in order compensate for this, RE set the full retard at some degree level after TDC. I have no idea what 1/32" of piston travel ATDC equals in degrees but it would be best to use a degree wheel on the crankshaft to determine that. Since you are, in escence, setting up a new distributor, I can see your concern. Just keep in mind that the early 500 probably won't need any more than 20 to 25 degrees advance total in order to operate. If it pings a lot under load, you will have to retard the timing a bit to lessen that to a livable degree.

Most of these old Lucas units allowed the points cam to turn a set amount in the little slots that it drops into on the main shaft plate and different units had larger or smaller slots for the cam to rotate in. A person could alter the slots if they had the specs to alter them to.

[classicrider]

Rotorwrench,your last paragraph is exactly what my first post was asking.But instead of slots these dizzies have 2 holes drilled in the advance plates,Lucas simply drilled diferent diameter holes to alter the advance range for any particular motor.And they stamped the deg range on the plate,as above the 6T Triumph was stamped 12 degrees.Not nearly enough for the 500 Twin though.
I've now drilled this out and will test  it tomorrow.But one thing i can tell you now is the motor labours if i set it at 1/32" ATDC at idle.Tomorrow i will test for actual timing with the  cam wedged fully advanced.

[rotorwrench]

I checked to see about the advance springs on the two types of distributor models between the Triumph and the Enfield and they did use the same fly weights but different springs. I hope you still have the springs for the Enfield 40168 distributor since they are different than the ones for the Triumph. On my experiences with automotive and Harley Davidson advance springs, there are different tensions to give either faster rate of change or slower rate. You can get springs in different tensions but I have no experience with the DKX2A distributors so I don't know how they compare to other makes & models of ignition systems or whether something else could be modified to work.

[classicrider]

Thanks rotorwrench,yes it appears the Triumph has 2 small springs of equal size,and the RE used 1 small and 1 larger spring which i have fitted.And i've drilled out the Triumph advance holes to the same dia (.423") as the one which has 19 deg stamped on it.I'm counting on that being the correct RE one,fingers crossed.I'll rebuild that one later on down the track,the shaft is quite a mess.
 

[carlo]

I'm reviving this topic because I'm about to fit a DKX2a distributor to my 1966 Interceptor as a quick and inexpensive way to replace a shorted out magneto.

Based on what I've seen here, this model of distributor has quite a few degrees of advance available, so it's necessary to set the initial timing to after tdc rather than before.
Would it be best to use the figures listed in this thread for the 500 twin?
Incidentally, this distributor came out of my 1958 Tomahawk (which is still too far from completion to be needing it until after I've figured out a more permanent replacement for the dead magneto),  and as far as I know, is the original equipment for this bike.
Also, does anyone have a scanned copy of the Lucas manual which addresses this particular distributor?
I'd like to know the recommended method for disassembling it to check the bearings in it.