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David C 01-10-21 07:24 PM

Fitting 123 ignition
I finally got around to fitting a 123 ignition to my 403.

When I first fitted it the distributor did not go in far enough for the dog to engage properly in the drive. I then machined about 1/8 in off the base of the distributor to make the shaft longer. This then meant that the base of the distributor fouled on the bolt with spring that holds the timing adjustment plate. I removed the bolt and spring and replaced with a shallow headed bolt with spring washer and used a locknut underneath.

Anyone else had any issues? I spoke to a couple of people with the conversion before I bought mine and neither mentioned installation issues.

Geoff Dowdle 02-10-21 06:45 AM

David ,
I have never fitted a 123 Distributor so unfortunately can not help with your problem.

But I'm interested in finding details about the ignition advance curve you have decided on . I understand when you purchase one you have a choice of about 8 advance curves .
Could you please advise what Static/Idle timing BTDC and what advance at say 3000rpm and again at 5000rpm (or Maximum )

Have you noticed any difference in the way the engine runs ?
Was it previously fitted with the original Lucas distributor ?
Most of them are well worn and need replacing.
Is the 403 still positive to Earth ?


David C 02-10-21 08:02 AM

Hello Geoff.

The car is still positive earth. The same unit will do either simply by swapping over the red and black leads.

The car was indeed still on the original Lucas distributor with worn weight bearings and tired springs. It seemed a good plan to get rid of mechanical variations. Also I like being able to add vacuum advance.

As to the advance curve - I only fitted the unit yesterday so haven't got that far yet. I have a hand drawn graph for a 100A distributor that came with an instruction manual I bought years ago. Unfortunately I can't find it at the moment so if anyone can help me with a starter for 10 I will be much obliged.

The plan is to get the car on the rolling road as soon as practical. The iphone Bluetooth app has ability to make live incremental adjustment + / - 10 degrees which can be noted down and used to edit the curve.

Geoff Dowdle 02-10-21 11:39 AM

David ,
Your plan of putting the 403 on a rolling road dyno is really the only way to accurately design the best adance curve for a Bristol. That plus checking the mixture through the rev range is also very important .
Over the years I have had three of my Bristol engines in 400's on a rolling road. Two had similar curves(plus a 406) and the last 400 engine needed 8 deg less at 4500 rpm.
The engine of an Arnolt Bristol of a friend needed even less again, this is a high compression sports engine running on 98 octane fuel.

I suggest you might start with a curve of
Static/Idle 12 deg BTDC
3000 rpm 36
5000 42

This is what my first two 400 engines (85A & 100) were graphed at.

The information I have on the original 403 Lucas Distributors are....
Part# 40345A , Advance Curve 520
Advance Starts 650 - 900 Engine RPM & Deg
16 - 20 deg @ 2900 Engine RPM & Deg
34 - 38 deg @ 4000 Engine RPM & Deg

So with a 12 deg Static would give
28 - 32 deg @ 2900
46 - 50 deg @ 4000
That is assuming the distributor is in excellent working order and using the low octane poor quality fuel of the mid 50's .

I look forward to hearing what curve you decide on .

David C 02-10-21 12:43 PM

Thank you, Geoff. That should get me to the rolling road without too much incident.

I will be fascinated to discover how similar the curves are between different engines. Even with the same camshaft profile there is likely to be significant variation in compression ratios due to different amounts of head skimming.

I'm no expert but I would guess that as compression ratio increases, less advance is required.

I really don't know where to start with the vacuum advance since the Bristol didn't have the feature originally. I have great hopes that this will offer quite an improvement.

David C 02-10-21 07:43 PM

Confirmation, should we need it comes from Andy Gibbs in the latest BODA newsletter. He reports that manufacturers instruction (presumably 123) had him start with a setup that gave about 60 degrees of advance at 2500 RPM. He has now reduced this to 32 degrees and reports significant improvement with hot running eliminated. This would still be slightly high according to your figures but not very far off.

Geoff Dowdle 02-10-21 11:38 PM

I'm certainly not an ignition expert but have had experience with Bristol engines and distributors over the past 40 plus years.
All Lucas advance curves are quoted in Distributor degrees and Distributor revs .
These figures have to be doubled to correlate with engine RPM & Deg advance .
There could have been a misunderstanding between Andy and the 123 literature as to whether Distributor or engine revs & degs were quoted.

Ask yourself "what would the manufacturers of 123 distributors know about what advance curves Bristol 6 cylinder engines require in their current mostly modified condition, without even considering the various types of Bristol engines produced".

I suggest you don't think too much about the curve until the rolling road tells you what advance curve is required for your engine.
Yes the camshaft , compression , fuel octane and other factors can change the advance curve required.

A vacuum advance is probably not necessary with any good quality distributor.


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