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Advance Weight Springs

coolbop
Posts: 8
Joined: Sat Jul 13, 2019 10:10 am
Location: Woodbridge, Suffolk

Advance Weight Springs

Post by coolbop » Sat Jul 13, 2019 11:16 am

Hello,

I'm the new owner of a V8 saloon. I've been going through giving it a thorough service in the hope of fixing a few running issues. Stripping the distributor showed it pretty gummed up so I've given it a good clean. However even when clean and free moving there doesn't seem to be enough tension in the springs to pull the weights fully closed when static.

The attached photo shows the arrangement before I took it apart. Does anyone know if this is assembled correctly? The shorter stronger spring has some free play so wouldn't pull the weights fully closed and the longer weaker spring isn't mounted 'between' the weights and only acts on that small swivel.

I'm assuming the weights should be pulled fully closed when static so if it's assembled right perhaps the springs have relaxed in length.

Many thanks in advance (excuse the pun!).

Ian
Attachments
IMG_20190712_181958.jpg

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Vortex O'Plinth
Posts: 820
Joined: Fri Feb 12, 2016 8:15 pm
Location: Gloucestershire

Re: Advance Weight Springs

Post by Vortex O'Plinth » Sat Jul 13, 2019 2:30 pm

Your springs are assembled correctly.
Nick

"Open the pod bay door Hal".

migray
Posts: 123
Joined: Fri Feb 26, 2016 10:31 pm
Location: Bakewell

Re: Advance Weight Springs

Post by migray » Sat Jul 13, 2019 3:16 pm

A photo in the Wilf Stephens technical articles shows the springs in a little more detail. You can see that the fat spring is not stretched when the distributor is stationary.
WilfStephens/useful-photos/Distributor% ... abrook.JPG

coolbop
Posts: 8
Joined: Sat Jul 13, 2019 10:10 am
Location: Woodbridge, Suffolk

Re: Advance Weight Springs

Post by coolbop » Sat Jul 13, 2019 10:40 pm

Excellent, thanks guys. Seems like I need to give the longer spring a tweak to increase the tension a little.

Ian

Ian Hastings
Posts: 100
Joined: Tue Mar 15, 2016 3:00 pm

Re: Advance Weight Springs

Post by Ian Hastings » Sun Jul 14, 2019 10:41 am

No. The long spring is OK. It does not pull the weights together at rest, it controls how far the rotor arm twists when the engine is running.

Christopher Storey
Posts: 253
Joined: Wed Mar 16, 2016 6:40 pm
Location: Cheshire

Re: Advance Weight Springs

Post by Christopher Storey » Sun Jul 14, 2019 11:16 am

Just to be clear about this, the amount of free play governs the initial mechanical advance at idle . Assuming your distributor is a 40676A, there should be no advance below 460 crankshaft rpm, 3.5 degrees at 800 crank rpm , 16 degs at 1800 rpm and 23 degrees at 5000 rpm, all these figures to be added to whatever your static advance is . If your settings correspond within a degree or two of these, then your springs are in good order

coolbop
Posts: 8
Joined: Sat Jul 13, 2019 10:10 am
Location: Woodbridge, Suffolk

Re: Advance Weight Springs

Post by coolbop » Sun Jul 14, 2019 4:40 pm

Thanks again for the replies.

If there's no tension on the springs when the shaft is at zero advance then I don't see how you can maintain zero advance up until 460 crank rpm?

I'll see how much advance I get on a running engine before I change anything. I realise that in the real world these mechanisms are somewhat of a compromise.

Thanks
Ian

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JohnM
Posts: 86
Joined: Thu Feb 11, 2016 1:31 pm
Location: N W London

Re: Advance Weight Springs

Post by JohnM » Sun Jul 14, 2019 8:48 pm

As far as I remember, Russ Carpenter explained that the reason you can't "static time" these engines with a timing light is precisely because they do have what he called "cranking advance" - the timing does advance even at starter motor speed. Never really understood why........
regards,
John
John M in Middlesex, NW London
1962 SP250 "B" Spec

Christopher Storey
Posts: 253
Joined: Wed Mar 16, 2016 6:40 pm
Location: Cheshire

Re: Advance Weight Springs

Post by Christopher Storey » Mon Jul 15, 2019 10:05 am

Coolbop : there is not instantaneous advance because the weights themselves have both inertia and friction which has to be overcome before they slide outwards under the reduced centripetal force. The force varies as the square of the rotational speed where the circular motion is uniform ( I think ! ) but whether that holds true for the motion of the weights - which is eccentric - is beyond me, my A level physics now being some 60 years ago !

coolbop
Posts: 8
Joined: Sat Jul 13, 2019 10:10 am
Location: Woodbridge, Suffolk

Re: Advance Weight Springs

Post by coolbop » Mon Jul 15, 2019 10:52 am

Makes my brain itch the more I think about this!

Ok, in that case what happens when the engine slows down to an idling speed. Friction would tend to retain the weights in an advanced state if there was no tension in the springs to pull them further in?

Maybe the friction from the heels of the points on the cam would tend to drag the shaft back to the unadvanced state although that wouldn't happen if the car has electronic ignition.

What I'm realising is that when static timing it's really important to approach the mark by turning the crank in a clockwise direction and if rotating the distributor body to ensure the rotor arm is as clockwise as it will go, in case drag from the points has pulled the arm from its unadvanced position.

Thanks

Ian

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