Bob weight springs

[p5andrew]

I need to replace the (badly stretched) bob weight springs in my P5B distributor.  I am struggling to source these.  Can anyone recommend a supplier?  Wadhams do not seem to list them.  

[Sam Bee]

Try 'Distributor Doctor Ltd', excellent website. Email  [e-mail].

[enigmas]

If the engine is not detonating or pinging at any point then that might be counter productive to engine performance...stretched springs(?) or not. To get the maximum performance potential from your engine the advance needs to come on as quickly as possible and be limited just prior to detonating. So if it doesn't detonate under load, then why not leave it alone. If it detonates slightly move the static timing back a few degrees. This will also take the load off the starter motor during cranking in cold weather. Emission engine's or truck engine's pulling heavy loads (old tech) have "slow" timing advance. Modern ECU controlled engines run at the peak of ignition timing efficiency just prior to detonation for optimal engine power. You can tweak the distributor by careful fettling if you have an "ear" for engine timing.

PS.
Most 1950's - 70s BMC car distributors can provide a range of differing rate spring sets. Motor wreckers/dismantlers are a good source. Similarly period Chrysler and GM (V8) distributor springs can be fitted.
Depending on the distributor type some spring posts can be tweaked for more or less tension. Generally one heavy and one light spring is fitted.

[p5andrew]

Many thanks Sam and Vince.  

I shall try Distributor Doctor and see if they can supply the springs.  Not a company I had come across before, despite being less than 30 miles from where I am sitting!

I take everything on board you say Vince.  However, the springs are definitely stretched - both are loose when the bob weights are fully retracted and the cam can be moved free back and forth through several degrees.  Predictably, the engine runs as one would expect - unevenly.  Sometime it pinks at relatively low throttle openings, at other times it does not pink at all.  I feel that for the modest outlay of a couple of springs it is worth replacing them before going any further.  Incidentally, the distributor is in otherwise very good order - the bushes are tight, the vacuum advance is nearly new, Lumenition electronic ignition is fitted and I have recently fitted a new genuine Lucas distributor cap and rotor arm.  I should add that although not relevant to the pinking issue, the plugs (NGK BP6ES - the engine is fitted with SD1 heads requiring long reach plugs) leads (copper) and suppressor plug caps (NGK) have all been replaced within the last year as has the coil (also NGK).

[Ken Nelson]

Vince and others may have a better knowledge of this than I do, but I believe it may be normal for the distributor springs to look somewhat 'stretched'. Here's a picture of my distributor, and the large spring is definitely loose on it attachment points. The smaller spring (initial one to stretch for advance) is likewise very light and easy to expand (ie advance). My car runs fine, and this seems similar to the distributors on my other British cars.

[Ken Nelson]

Another try at picture.

Attachments:

[enigmas]

Fair enough Andrew. In certain distributors the spring posts aren't solid posts or at least one post for each spring is adjustable. It can be bent forward or backward to increase or reduce tension on the specific spring. This affects both the rate and rpm point at which the centrifugal weights throw out. It's how most mechanical advance mechanisms are setup on distributor machines.

Changes to advance can also be effected by changing the centrifugal weights (making them lighter by grinding material from the side or leading edge of a weight with care. A digital scale that measures in fractions of a gram is useful here.)
The weights can also be pinned for less advance.

A Quick Check.
If you can get hold of a cheap clear plastic schoolboy protractor it's quite easy to check how much advance the distributor is providing.

Method:
* Remove the distributor cap. Leave the rotor button in place.

* Mark the edge of the distributor body where the rotor button is pointing to with a dot using a whiteout pen or similar marker.

* Now turn the rotor button (using your thumb and index finger) gently to it's maximum advance point. It will turn one way and not the other. Mark where the rotor button is pointing.

* The distance between the 2 points measured in degrees using the plastic protractor is distributor advance. The center point for the protractor is the rotor button spindle.

* Distributor advance is half of that shown on the harmonic balancer(front engine pulley) when using a timing light. On average it's about 12° - 15°
For example, 15° of distributor timing yields 30° degrees of engine timing as indicated on the front pulley's timing marks.

Ignition Timing for Petrol.
An example:
15° distributor = 30° engine (as shown on the harmonic balancer/front engine pulley) + 5°BTDC static or initial timing = 35° total ignition timing.

35° total timing is about the average for petrol.

At cruise speeds the vacuum advance capsule may pull in another 5° - 8° due to the slower flame front of combustion. This added vacuum advance just improves fuel economy.

The distributor weights, spring posts, springs and stops can all be tweaked. If you have a "feel" for an engine and a methodical approach it can all be done quite easily with each slight modification noted. If you have a spare second distributor you can use it as a test bed.

Good luck with however you sort the pinging issue.

[Ken Nelson]

Let us know the outcome Andrew.

[p5andrew]

Many thanks.  Springs now on order.  It will be interesting to see how much (if any!) effect they have.  Will report back.

[p5andrew]

Reporting back as promised.

The replacement advance/retard bob weight springs duly arrived this week.  Today I stripped down the distributor.  Comparison of the new springs with the old was interesting; the old lighter-gauge spring was around 2mm longer than the new so had become stretched quite significantly.  Examination indicated that this was more likely due to ham-fisted attention by someone in the past rather than wear-and-tear.  The old heavier-gauge spring had also gained length by approx 1.5mm.  Not very surprisingly, as a result of the consequent looseness of the springs, the distributor cam was able to rattle around back and forth by several degrees before any spring tension took effect.  

After reassembly of the distributor and refitting, I initially set the timing to 4 degrees BTDC, this having proved to be the optimum in the past.  During the timing process it was very noticeable how much more stable the readings were with the new springs fitted - hardly surprising really given the looseness of the previous springs.  My usual road test route takes in a steep, long hill which is perfect for showing up any liability to pinking.  With 4 degrees BTDC there was no pinking.  I therefore advanced the timing to 8 degrees BTDC and there was just a little pinking and I finally settled on 7 degrees BTDC. I should add that I run the car on 99 RON super unleaded.

The engine is running noticeably more happily now so it was certainly worth the effort to change the springs!

[djm16]

My reading of the centrifugal advance function is that it has an initial rapid advance controlled by the lighter spring, then a slower advance from the heavier spring.

Since within the limits of elasticity of the springs, their length v tension is linear, to have a two component advance curve, the heavier spring would need to have some free play to allow for initial rapid advance.

[Ken Nelson]

Thanks for the feedback Andrew. As DJM posted, and as seen in the picture I posted, my heavier spring does have a slight amount of slack to allow the lighter spring to advance before the hearvier is engaged. So far my car seems to be running fine now at 3º BTDC but it is a 3-litre P5 with manual/od.