Engine is still at the machine shop and I’m making my way through various engine-related items. This week’s project was the distributor. I took it apart and thoroughly cleaned it. Only problem was I didn’t pay close attention to the centrifugal weights that provide the mechanical advance. In my disti (DMBZ6A from a '62 Mk2 3.8 liter) the weights can be set to 13 degrees or 25 degrees of mechanical advance…at least that’s what I’m assuming based on the 13 and 25 stamped into the weights. It appears that how the weights are assembled determines how much advance they’ll provide. So, since I didn’t note whether they were set for 13 or 25 degrees when I took it apart can anyone shed some light on how these should be set? The disti has a vacuum advance as well.
Hi Tom, if you don’t get a definitive response from someone knowing more about these dizzies than I do, I could open mine up (it’s not on the car) and send you a picture of weight orientation. Mine is also a 1962 DMBZ6A ex my E type 3.8. Let me know. It also has a high and low value weight. They are set up to give you a “knee point” in the distributor advance curve - ultimately summing to 38 degrees - IIRC. Paul
Personally, I haven’t come across a distributor where the weights were different and I’m struggling to think why you’d want to have alternative curves unless one setting is for use with vacuum advance and one without (that’s just a wild guess). The mechanical advance curve is set by a combination of the weights, the springs and shaft/action plate. The springs are normally different. Usually you can see that one is of a lighter weight and therefore tension than the other. The maximum mechanical advance curve is determined by a physical stop so that when the limit is reached you can’t stretch the springs any further. This is often set somewhere in the region of 2000 - 3500 revs.
There are many different part numbers for DMBZ6A distributors. The common Jaguar ones all use the same weights (41033/S) but have different spring sets according to application. If you tell me the part number of your distributor I should be able to tell you what it would have had when new.
Trouble is, in the 60s & 70s every town in the UK had a Lucas agent and we used to experiment a lot with different weight/spring combinations so after 55 years your distributor may not have the original parts in it.
Most of the early XK engine DMBZ6A distributors have a maximum mechanical advance of 18 degrees. It depends a bit on engine size and compression ratio.
Also, there are several different vacuum units.
As I mentioned, I might be able to help a bit more if you tell the part number and the application (XK150 or 340 or what?).
Even if you use the right weights, the springs will be out of spec after all these years. Send the whole thing to advanced distributors in Minnesota. He only works on Lucas distributors and does a real nice job.
Paul/Eric/John – thanks for the responses. To Eric’s question the disti P/N is 40665B and has a date code of 3/61. I have no reason to believe it is anything other than stock…but who knows as it’s a 55 year old car and I’ve only had it a few years.
I’ve added a picture below to show what I’m talking about. The weights do not change between 13 degrees and 25…just their orientation. As you can see in the picture there’s a stop pin that limits the advance. In the 25 degree setting the profile of where the corresponding stop is on the piece that rotates allows for more travel/advance than the 13 degree setting. Looks simple enough. Yes, the springs are different and, again, have no reason to believe it is not stock…but don’t know for sure.
I’ve seen postings about people running upwards of 40-50 degrees total advance (with today’s modern gas). I’m assuming with the mechanical advance set at 13 this will not get close to this (static + mechanical + vacuum)…maybe this gets to around 30. Does that mean I should be using the 25 degree setting for mechanical?
40665A/B was fitted to Mk IX 9:1 CR and 3.8 Mk II also 9:1 CR. It’s quoted as having a maximum mechanical advance of 11 - 13 deg.
The spec I have gives the following data: no advance upto 225 revs, 3.5 deg at 550 revs, 9 deg at 1100 revs and 12 deg at 2000 revs. The ECM curve is no. 700 if you can ever find a source of these curves - I never have.
The correct weights for the distributor are P/N 410033/S and spring set is 54411290. If you are in the UK you can almost certainly get these from Distributor Doctor.
The vacuum unit is 421189. I don’t have the three numbers for this but if you have an original they should be stamped on it. The first number is the inches of Hg to begin the advance, the second number the inches of Hg for max advance and the third is the max advance. (I think but I’m not certain that this figure is in distributor degrees so double it for crankshaft degrees if it is). So, you can work out the total advance available by adding the mechanical to the vacuum.
Once you get the engine back and running you can check all this if you have a decent strobe.
I hope that this is a least a little bit helpful. The 13 marking on the cam plate makes sense but I don’t get what the 25 means. It’s tempting to think it might be total including the vacuum but I don’t know.
While I haven’t found any definitive documentation on this I believe if I reorient the weights so the 25 will hit the stop pin this would provide 25 degrees of mechanical advance. Since I have it apart again I’ll recheck, but when I was trying to figure this out I’m pretty certain the 25 setting allowed for more rotation (hence advance) than the 13 setting.
Yeah, ultimately when I get the engine back I’ll just have to keep note of this in the event I need more advance.
Its a rare engine that requires 25 deg adv. at the distributor (as its half speed). It would equate to 50 deg at the crank, plus whatever initial advance you set it at.
I think you could blow up your engine pretty quick with that.
13 degrees at the distributor is 26 at the crank with 10 degrees static (initial) would total 36 degrees “all in” at the crank - a more reasonable number.
Be careful until you figure that out.
Dave
Thanks, Dave. That’s helpful. I think I have it set correctly at 13 and will leave it there and see how it runs. Also a good reminder about distributor degrees versus crank degrees.
Vacuum advance is marked 5-12-8 with P/N 54412033. I believe this equates to 8 distributor degrees of advance.
So if this is the case crank degrees would be 10 static + 26 mechanical [2x13] + 16 vacuum [2x8] which is a theoretical total advance of 52 degrees.
Came across this post when trying to rebuild a friends Lucas distributor which he had completely dismantled to clean without making notes (!!). I think I might have solved the mystery of the 13 or 25 degree max advance stops. The 13 or 25 degrees refers to the MAXIMUM in distributor degrees which can be set in the factory. The actual max advance appears to be further restricted by fitting an end stop collar of chosen size over a pin, which is then swaged over. In my example a DMBZ6 Part No 40640A unit from an XK150 3.8 uses the 25 degree “end”, but with a collar of about 0.25 inch diameter reducing the distributor centrifugal advance range to 18 degrees, i.e. a 36 degree range at the crank.
