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TR2/3/3A Mech. advance

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S

STeve 1958

Jedi Hopeful
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I replaced my vacuum advance and while I was at it I check the Mechanical advance a little closer. The weights and springs are all there but there is a lot of slop which allows the weight to move without spring tension. Is this normal or should the springs be tight to begin with?

I also cleaned the carb. with some Gum Out and then had to completely re-tune the carb. as everything changed. I guess it needed it.

I'm ready to go for a drive.

Steve
 
The big (thick wire) spring needs to have quite a bit of slop - it comes into play only as higher RPMs are reached.

The small (thin wire) spring will have little or no slop.

The real test is to figure out the advance curve. Can be done on a Sun Machine those most do not have access to one or with a timing light that has an advance dial (most do). There you just plot the advance at various RPMs and plot the curve.
 
"The small (thin wire) spring will have little or no slop."

OH, There's a small thin wire spring ??? Not on mine. Hope someone has a picture
 
The difference is perhaps less pronounced than Geo's description might imply. But he's quite right, one of the springs should be slightly smaller diameter wire, and slightly shorter, so it has some small tension with the engine not running. The other spring is wound with heavier wire, and is slightly longer so it doesn't come into play until the engine is turning 1200 or 1400 rpm (depending on which distributor you have).

Only photo I found is from an old article by Marcel Chichak, originally published (I believe) on the Jaguar Club of North America web site

LD8OoR4.jpg


FWIW, here's a plot I threw together long ago, to compare the centrifugal advance curve (including spec for initial timing) of several different TR distributors. Note that both rpm and advance are given in degrees at the distributor, so are 1/2 the values measured the normal way, at the crankshaft. The "knee" around 600-700 rpm (for the 3 TR2-4 distributors shown) represents when the secondary spring starts to take effect.

4yGQ6Hz.jpg
 
Thanks. I have a DK4A distributor that looks a little different but the theory should be the same. I have two identical springs, both loose.
This car was raced so maybe the previous owner altered it. It was also rebuilt by idiots..."non-british mechanics" who might have messed it up.

I'll look into buying new springs.

Steve
 
Not that I know everything, but I am always happy to learn new things, had no idea the one spring on a Lucas distributor was supposed to be a little loose and kick in later. I think I mucked around with one several years ago and tightened up that spring, live and learn (slowly).
 
It's not just Lucas; the curve with the knee is very common across all cars and the loose spring is how they used to do it (before the computers took over timing duties).

Eg (for a small block Chevy V8)
igvance1.jpg
 
If you are uncertain about the curve in your distributor, consider contacting Jeff Schlemmer at Advanced Distributors. He can recondition your distributor and suggest a curve based on your engine build.
 
Remember Engine advance is 2X distributor advance.
And Jeff is a great guy.
 
STeve 1958 said:
there is a lot of slop which allows the weight to move without spring tension.
Click to expand...

Check to make sure that the holes in the weights themselves are not worn. Mine were severely oversize. The weights are made of a soft metal (lead?) with a brass sleeve pressed in where they sit on the pin. Mine had worn through the brass long ago and the holes were about 3x bigger than they were supposed to be. Timing would return to a random point after advance. Jeff fixed me up though.
 
Jeff rebuilt my distributor with rebuilt vacuum advance module and new bushing,pointd and condenser, ,recurved ,cleaned to look like new for about 40 bucks more than a moss replacement vacuum module.Tom
 
I got the set of 5 springs from Moss Motors. One of them is longer and stronger, to use as the strong spring with some slack. The other 4 are shorter and still pretty strong.
My thought is to install the weakest of the four as my weak spring and check the timing advance. I was expecting that the weak spring would be much weaker so I was surprised at how strong they all are.

Any thought?

P.S. I had read that the weak spring was 15#/in and the strong was 205#/in. That was a typo and the weak is 150 #/in. which makes a lot more sense.
See I'm not as dumb as I look.
 
Last edited:
I replaced the weak spring and kept the existing strong spring since it seemed to fit the best. The car runs and advances with RPM. The vacuum advance also works now that it doesn't have to fight against two strong springs.
I'll get it out and drive it this weekend to see how good I did. Also getting a digital timing light to check the curve.
Thanks for all the help, I wouldn't have had the guts to do it without some guidance.
 
STeve 1958 said:
...The vacuum advance also works now that it doesn't have to fight against two strong springs...
Click to expand...

Glad it is working better, but FWIW I do not think the vacuum advance works against the springs. It merely rotates the plate so if the plate is binding there can be some resistance. There are (nylon?) nubs that help it slide -- I usually use a small amount of lube (e.g. Lubriplate) between the plate halves.
 
but FWIW I do not think the vacuum advance works against the springs.

Your right, I miss spoke. I also replaced the vacuum advance. Separate issue.
 
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