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MG TD TF 1500 - Distributor cam - what is the difference?

I got a distributor DKY4A from a friend to check against the distributor I own.

In the WSM there is a picture of the cam which shows a symmetric and a asymmetric one. What is the difference? If all distributors have the same fixing plate, same cap and the same points it seems that there is a difference in timing (looking at the slot for the rotor arm). I guess I'm wrong - but what is the fault?

W_Mueller

The cam shape influences the cam angle.
Because the cam angle begins at aperture of contact (front side of the cam)and ends at the contact closure (back side of the cam).
OK but the cam angle can also be set adjusting the points gap. correct

for a givern cam angle, the cam design allows to control the rise time and fall time of the points. It may have an influence on the electrical bouncing of the contact and also the wear out of the fiber part of the moving contact.

Laurent.
LC Laurent31

Thank you Laurent for your explanation.

The cam shape is one thing. I guess symmetric means the front side is like the back side whilst the assymetric cam sides are different. This is (for me) not visible in the picture of the WSM.

But the other thing is the rotor arm. In the picture the slot in the cam for the rotor arm differs. In my opinion the result is an other angle and therefor another timing for the spark.
W_Mueller

The position of the slot on the cam is related to the direction of rotation of the distributor, nothing else.

Lots of information on this subject in the archives.

for example, one entitled, "Dizzy CAm ...again'

John

J Scragg


DISTRIBUTOR CAMS

The early distributors were fitted with "symmetric" or "asymmetric" cams, these use a point gap setting of 0.010" - 0.012". Dimension across high points of cam lobes is 0.748" and across cam flats is 0.701"

The later "High Lift" cams use a point gap setting of 0.014" - 0.016". Dimensions across the high points of cam lobes is 0.748" and across cam flats is 0.643". These dimensions are my own measurements and not factory numbers. The high lift cam was fitted to later distributors to improve ignition system performance and increase contact life.
From Bob Gruneau.
Len Fanelli

I never noticed that the slot was shown on the WSM diagram. And I wonder why it's not on the same place for both profiles.
John may be right. Symmetric is represented for a clockwise and the asymmetric for an anticlockwise or something like that.

I agree that the diagram of the assymetric really shows a totaly asymetric cam ! only one cylinder could be tuned with such a cam.

Laurent.

LC Laurent31

The early dky4a (symmetric) runs anticlockwise.

Does highlift cam and asymmetric run clockwise?
W_Mueller

Laurent,

Your comment "John MAY be right" is slightly offensive, it implies that you doubt my statement. Information contained in this is based on documents published by the manufacturer. It is not just a wild guess nor a theory. If you take the time and trouble to check it out yourself, you see will that there is no ambiguity.

John
J Scragg

John

now I've read the archive the question is answered. Thanks for your hint.

1. Because the direction is appointed by viewing from the engine side the direction of rotation is clockwise. (if you look at the camshaft after removing the cap it is anticlockwise ;-))

2. The different camshafts (symmetric, asymmetric, high lift) have the same direction.

2. The illustration in the WSM is not correct with regard to the slot for the rotor arm.
W_Mueller

Apologize John,
My Englich level is not good enough to feel that sort of meaning in my sentences.


Laurent.

LC Laurent31

One question is remaining:

What is the dwell for symmetric, asymmetric and high lift?
W_Mueller

Normally dwell is 2/3 of the available angle, 4 cylinder has 90deg so 60deg would be dwell I think. 6 cylinder has 60deg so dwell of about 40deg.

But, I learned that a long long time ago, my TF is about 58 or 59 right now.

Peter
P G Gilvarry

Symetric; 41 - 49 deg.
Asymetric; 45 - 53 deg. (I do not think ever fitted to T or Y distributors).
High Lift; 57 - 63 deg.
As can be seen in my attachment, the asymetric and high lift cams had the slots in the same place (or orientation).
On later rotor arms, and I think the only ones you can now buy, (for anticlockwise rotation from above), the brass insert is extended to the left at the outer end. If you fit a high lift cam to a distributor that originally had a symetric cam, with the combination of different slot position and left extension of the rotor brass insert, I think the spark will be 'seeking' the next peg (in the distributor cap) around. Thus you will have to move the HT leads around one place, and possibly adjust the distributor body position (clamp plate or cotter bolt) to get correct ignition. If you changed complete distributors, you would clamp the new distributor to fire at the correct time, probably without noticing any major change in position from the old distributor - well, that's what I think.


R A WILSON

If you download from healey6.com/bulletin.htm section 3 of the Lucas maintenance manual will give you all the info on cams and rotation.
This manual covers all aspects of Lucas electrics
Ray TF 2884

Ray Lee

This thread was discussed between 05/11/2017 and 11/11/2017

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