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Triumph TR6 - Mechanical advance and Vacuum retard

Question #1: One of my mechanical advance springs seems a bit stretched, will any set of advance springs with the same dimensions work?

Question #2: My retard capsule is shot. When it works the car idled nicely and when the vacuum hose was removed and plugged the idle speeb would increase by about 400rpm. Now without the retard capsule to keep the timing correct for the rest of the rpm range I have to lower my idle speed, but I can't get it idle nicely and evenly now.
I know some peolpe have purposely disconnected the vacuum retard as a vicious emissions devise. How do they make the car idle nicely?
Chris W

Chris-TRF shows the advance springs in the green cataog and price list.
Berry

TRF did have some springs listed a while back but as far as I know they only had 3sets and Fred of British Autosport ordered the remaining ones. He became aware of this when I needed a set for my car.
While you could match up the dimensions of your current springs, the spring rates could bevery different b/c of the difference materials and besides I am sure your springs are stretched.
I disconnected the vac retard but the spring lever that originates from the unit must stay attached to the contact breaker plate so that it does not move (not to change the timing). Other alternative is to carefully adjust the sliding to where it connects to the vac retard rod normally and fix the 2 plates together with a screw or tack weld.

Perhaps the rod originating from the vac is loose.
Steven

Only the early cars had a retard unit, so why not use the specs from a later model. Seems to me that should work - and maybe you'd need the advance curve of a later mode also ?? Peter G
Peter G

My '73 has a retard unit. To get a good idle without retard I had to

• use the "early" gasket on the air by-pass valve, &
• replace the worn carb "throttle shaft"
• replace the throttle shaft seals

Basically, air leaks kept me from getting a good idle.
Brent B

The very early carb cars had both an advance and a retard unit. The later CC and CF carb cars had a retard unit only. As far as I know, all PI cars had advance units, somebody that knows the PI cars chime in and correct this if wrong. One way to tell the difference between an advance unit and a retard unit is to see where the vacuum capsule is located. If it is the side where application of vacuum would pull the breaker plate into an advanced timing mode, it is an advance unit, if it would pull the breaker plate to an retarded position, it is a retard unit. If you look at the cover plate for the tach drive gear and the tach drive housing, a vacuum capsule above that would be an advance unit. A capsule on the other side of the vacuum unit boss would be a retard unit.

Unless you plan on running some electronicly controlled ignition system with adjustable advance (such as a MSD 6AL ignition box with a MSD 8951 programable timing controller) , do not fix the distributor plates together as this would defeat the mechanical advance feature built into the distributor.
SteveP

I would like to add in clarification:

if the car is only fitted with a retard, then fixing the 2 plates together will NOT have an effect on the mechanical advance . The centrifugal force for the advance works on the weights and springs and moves the cam and the shaft/action plate in relation to each other (in TRF catalogue DU14 and DU17). This movement moves the lobbed cam to advancement and allows the points to open and close earlier. Hence the need for the correct springs.

On earlier cars (69-71) with a vacuum advance then the plate needs to move for this advance in addition to the centrifugal advance.

Vacuum Retard was only added as part of a pollution control device.
Steven

re question 1:
when looking for an exact replacement spring you need to match:
number of turns,
diameter of coil,
length of coil,
diameter of the wire
temper of the wire

Its not easy!!

I have a suspicion that the two springs sre designed different in order to achieve the required advance curve. First thing to do would be to check the advance curve with a timing light.
P H Cobbold

Steven,

I stand corrected on the advance mechanism operation on the "retard only" cars. I should have remembered this as I have had a number of these apart. Must have been brain fade.

Peter,

What is the scoop on the PI distributors? I have seen a few PI cars that had vacuum units on the advance side, but is it like the advance like that on the dual unit fitted to most 1971 US spec cars and not hooked up or did it indeed supply advance over and above the mechanical advance? It's been a while since I have seen one, don't remember and am curious.

Back to the Topic

I broke out the catalogs and what little of my old Lucas print data that I still have. TRF shows 3 sets of carb model advance springs available according to the price list:
LU54420215 for the 41202 and 41306 dual vacuum unit distributors
LU54424202 for the 41385 retard only distributor
LU54426281 for the 41558 retard only distributor
The 54423078 springs for the 41352 distributor are shown to be superceded to the 54424202 springs. Since the 41352 and the 41385 use the same advance cam and the advance curve on the 41385 comes in a little slower and not quite to the same peak as on the 41352, the 54424202 must be a little stiffer spring set.

Speaking strictly for carb cars distributors, if I am remembering correctly, the drive post and the advance weights are the same on all of them (Lucas info confirms this for 41202, 306, 352 and 385). The advance cam itself varies by specific application and the total amount of advance that it will allow. I remember different numbers being stamped on the cams and it corresponded to the maximum mechanical advance for that distributor. On the springs, I never counted the number of coils, measured length, checked the diameter or calculated the spring constant. It should be easy enough for someone to do if they had a complement of new springs to work with, used ones would likely introduce too many variables for accurate determination.

The distributor you have will impact the timing and the timing will impact the idle, but I have to agree with those saying to check for any reapair any potential leaks. After that, tweak the timing while running to get best balance. If you seach the archive there was a thread on tweaking the timing some time back that may be helpful.
SteveP

Steve,
UK PI cars had a single capsule fitted which would pull the moving plate clockwise (i.e. advance the timing) when the inlet manifold was under vacuum. To my mind that would be called a vacuum advance device. But its really immaterial because they were never connected to the manifold. I guess some other model of car did use it and Lucas never bothered to remove it for the UK TR6s.
Can't comment on exported cars- I've caused enough confusion already!
It is very difficult here to get new distributor springs without buying a full exchange reconditioned disy. If you know a US source of springs with known characteristics do let us know! I need a set that will extend centrifugal advance out to 5000 rpm rather than being all-in at around 2200 (IIRC).
Peter
P H Cobbold

Peter,
So it looks like they pulled the same stunt as on some of the US spec cars. Yes, there is an advance unit there. No it is not hooked up, it's only on there for looks. You gotta love it.

The 54414202 looks like the best of the bunch in that regard for the US spec distributors but even then all of the US spec stuff made it to at least 4600 before being all in. I based my speculation above about it being a stiffer spring than the 54423078 ones on the max advance and engine speed max advance occurs. For the 41352 distirbutor with 54423078 springs here are the numbers, based on crank speed in rpm, and min, then max degrees advance at crank:

1300 2 7
1800 13 18
2200 17 21
4400 24 26
5600 26 30

Here is the curve for the 41385 distirbutor with the 54414202 springs.
1200 2 6
1800 8 12
2400 12 16
4400 21 25
5600 24 28

Since the center shaft, the advance cam and weights are the same, the difference is purely related to the springs. Part numbers are Lucas numbers both here and above. Perhaps if you were to contact some suppliers of Lucas stuff and just have them look for the 54414202 springs or any of the others for that matter, you might could get them there.

All that being said, I don't have any idea how this compares with the PI distributors, but if you static time the car at somewhere in the 10 to 12 degrees BTDC with no retard, I bet that there is maximum of 20 to 25 degrees of mechanical advance available, less than that range would not surprise me. How these springs would work with your distributor would be a function of how the cam and weights were relative to the US spec units. The only way to know the curve for sure is to find someone with a distributor machine, find some springs, fit them and have the distributor checked out on the machine. Do bear in mind that all of these numbers would be cut in half when measured at the distributor. From a US source standpoint, I pulled The Roadster Factory's on-line price list and it showed three Lucas spring sets as being available for the TR6. Seeing that the owner has a TR5 (saw it several years ago, nice car, but it was wadded up a couple of months later and turned into a rebuild project, haven't seen after the rebuild), they might have some information on the PI models that isn't normally out in circulation in this country.
SteveP

Steve,
Many thanks for that info - I've archived it for when I get round to tuning the ignition timing.
Peter
P H Cobbold

Every supplier I contact says springs not available and vacuum retard capsule either not available or on back order for months.
Chris W

In between rounds with the stuff we were working on last night, I pulled out my box of dismantled TR6 distributors to try and confirm things. Everything was either a 41352, 42385 or a 41558. The 41352 and 42385 had the same weights and advance cams, both were stamped 13 degrees, which ties in with the advance figures above. The one that got me was the 41558. Its advance cam was marked with a 17, which would provide a potential 34 degrees of crank advance. Unfortunately I have no information on that distributor regarding the mechanical advance curve.

I know a guy that has a vast selection of old catalog and technical information that he collected in the process of buying out dealer inventories. I will try to get by there today and see if I can fish up anymore information on the mechanical advance curves for the various TR6 distributors.
SteveP

Steve,
Has the shape of that 17 degree cam weight simply been machined to give more swing? Or is it lighter in weight too?
Peter
P H Cobbold

I have a real good PDF file on distributor weight/spring workings that may be of some use - not mine, something scavanged. I don't have a site to post it on, but will gladly send it out. Let me know.
Brent B

Brent,
I'd appreciate a copy. I was going to go back to school physics- MVV/r - to get an approximation, but no point in reinventing the wheel.
TIA
Peter
P H Cobbold

Brent,

Please sens me a copy also.

I am afraid that I was tied up for longer than I thought the other day at the paint shop and the trim/glass shop and didn't get a chance to go by to check out the old information. I still plan on going by and digging through the stuff, but it may be a couple of weeks off.
SteveP

About this advance, I'm building a 69 race motor and I'm trying to figure where I should set the timing. I've put in an advance of 13 degrees and not hooking up the vacuum to the advance unit. What is the total advance I want to try to get? The cam is a D9 grind fairlt similar to a PI cam, a little hotter. I'm running tripple Webers now and will be switching to PI later. Any ideas?
Charly Mitchel

Hi Chris
Sorry this answer is a bit late I havnt been on the site of recent. I think the heavier of the two springs should be slack on its post as it doesn't start to work until the engine is above cranking speed. The light spring quickly advances the timing until the heavier one takes over.
r algie

Easy things out of the way first, yes the heavy spring should be somewhat slack (but not excessively so) on the posts as Lucus uses a two stage mechanical advance curve. The lighter spring allows a higher advance ramp rate before being overcome by the heavier spring which then slows down the advance ramp rate.

The total amount of advance is hard to say as there are many variables, but the wedge type combustion chamber used in the TR6 takes along the lines of 35 degrees (+/- 3 degrees or so) total advance. However, that does not mean that your particular engine can take that much. The higher the compression, the lower the octane level of available fuel, etc will cause you to have to back off on the total of advance. Assuming that you have a properly operating, box stock 41202B or 41306 (both are dual vacuum units) as would have been fitted to the earliest TR6 cars, there are 22 degrees of mechanical advance at the crankshaft available. So 13 degrees static timing plus the 22 degrees puts it at 35 degrees. Changes made to the distributor or lack of proper function can vary what you get, so verify. I guess my bottom line questions are: 1) Are you getting the total advance you want when checked dynamically and 2) Are you getting engine knock. If the answer to 1 is yes and to 2 is no, then no problem. If the answer to both is no, then you can advance the timing, but I would not recommend going beyond say 40 degrees of total advance at the crank. If the answer to 2 is yes under any circumstances, you must back off on the timing even if it isn't what you want. As a reference point, we used 38 degrees of total advance on a Nissan A14 engine in a GT4 car.

Now for the "I need everyone's help" part. I am trying to gather up info in the Lucas 22D6 and 25D6 family of distributors. Where I am falling down is on information related to car particulars. So if you have friends with things like MGCs, big Healeys, other 6 cylinder Triumphs (including salloons) and various 6 cylinder engined Jaguars (saloons and sports cars) try to find the 5 digit Lucas distributor number and if there is shop manual information, gather up as much mechanical advance curve information as you can, then send it to me. If they have 6 cylinder Lucas distributors sitting around they could look into to find the stamp indicating maximum potential advance, so much the better. If the advance cam can be verified in one fitted to the car, even more stuff to try and cross reference. Best of all, if you can find technical information on the spring constants Lucas used, that would be perhaps the most key bit out of the bunch.

So far I have determined the following on the 22D6/25D6 front. There are two different potential weight sets used in the 22D6/25D6 family. For the TR5/250/6 applications, one weight type is used in all but a single distributor (41542) used in some of the later PI cars, but that weight is also used for some Jaguar and GT6 (Lucas replacement for OE distributor). There are five different advance cams used across the TR5/250/6 range, but I can only pin down two advance limitations, a 13 and a 17 degree advance cam. Again, in some cases these cams are also common to Jaguar applications. Nine cams in total were found across the 22D6/25D6 family. The springs are all over the place application wise and impact the curves on distributors that are otherwise fitted with the same parts, kind of an obvious conclusion, but I can't seem to dig up info on the spring constants. Plates and action shafts seemed to pretty much be tied to a specific Lucas customer's engines and no cross over was noted between different manufacturers engine families.

So if you can get information to me on application, advance curves from manuals and max advance indicated by a given distributor, spring constants, we just might be able to pull a cohesive set of tuning information out of the bag and set it loose on the board.
SteveP

This thread was discussed between 07/02/2003 and 16/03/2003

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