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'Destructive' Lucas 25D4 Distributor


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Hi Gents,

These comments have been brought about as a result of the recent thread initiated by Lee TR4 in the hope that they may prove a useful warning!

 

By virtue of 'figure of eight' problems (and subsequently a cracked head), I'm having a 'ground-up'engine re-build and as my dizzy hasn't been looked at (? for ever!), I took it to Martin Jay (Distributor Doctor) last week. As do many others, I know Martin quite well from his Revington days and he gladly spent what was his lunch break explaining his facilities and modus operandii.

 

He also immediately confirmed that mine was not a TR4 Dizzy (although a 25D4) and had come off something like an Austin A40 and had a 16degree cam. He then proved that the dizzy produced an extortionate amount of 'advance' (18 degrees) above approx 2.5K revs which of course could be disastrous for a TR4 engine. He went on to say I was lucky that not being a particularly heavy footed driver and not investigating 4.5K revs too often - I was fortunate not to have wrecked the pistons or engine. Apparently he has another client with a TR6 who has done just that by virtue of an incorrect and highly 'advanced' cam.

 

I might add that the car has been performing very well and has never 'pinked' - but would one hear 'pinking' above 3K revs in a TR4!

 

I am now awaiting the return of a correct and no doubt beautifully turned out Dizzy from Martin. However, there is a sequel - when my engine was stripped, the pistons were worn and the rings cracked (and they were quite new), so it seems my incorrect Dizzy was already doing its evil work!

 

Now, from the point of view of my re-build, the loss of the pistons and rings etc is negligible as they were to be replaced during the work. The moral is - do check you have absolutely the right Dizzy in all respects!

 

Regards

Geoff

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  • 2 months later...

Hi all,

 

BTW, does anybody knows the right advance curve for a TR4, possibly under various camshafts?

As far as I always thought, a correct 25D4 should give 10° dynamic advance (stamped on the small masses), plus the 4° static, plus the depression.

I am quite puzzled by the behaviour of my 123-Ignition: I'm down to curve "0" with a sprint 88. Pinging was still slightly present at curve "1".

 

Badfrog

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Hi Geoff,

 

I recently had my distributor rebuilt by Martin, he did an excellent job! Found out that mine was advancing to much due to the wrong springs (plus a load of other stuff that was wrong) Came back looking sparkling new, and starts first time :)

 

Ed

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So is there any way of telling whether our dizzys are for a TR4 without stripping down looking at curves, etc...

 

Also, how much does Martin charge for a rebuild.

 

On a slightly different tack, I bought a New dizzy from a highly respected TR specialist, which was, i think , imported from the far East, and the points lasted about 500 miles !!

Once taken out, they looked very cheap and nasty.

 

David

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Badfrog, I had Martin Jay build me a DM2 with the correct build date for my car, and the centrifugal advance curve is as follows according to his calibration test: 3 at 0; 5.5 at 550; 8.5 at 850; 10 at 1050; 10.5 at 1300; then a steady 10.5.

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So is there any way of telling whether our dizzys are for a TR4 without stripping down looking at curves, etc...

 

Also, how much does Martin charge for a rebuild.

 

On a slightly different tack, I bought a New dizzy from a highly respected TR specialist, which was, i think , imported from the far East, and the points lasted about 500 miles !!

Once taken out, they looked very cheap and nasty.

 

David

 

Hi David,

 

It rather depends how much Martin has to do to the Dizzy, but you are probably looking at £150 plus. He does a great job though - both inside and out!

 

Geoff

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Hi all,

 

BTW, does anybody knows the right advance curve for a TR4, possibly under various camshafts?

As far as I always thought, a correct 25D4 should give 10° dynamic advance (stamped on the small masses), plus the 4° static, plus the depression.

I am quite puzzled by the behaviour of my 123-Ignition: I'm down to curve "0" with a sprint 88. Pinging was still slightly present at curve "1".

 

Badfrog

 

'Morning Badfrog,

 

I can't offhand remember the spec of your 'Beast' - mine is virtually standard with 87mm pistons and a 4A cam. The plot on my Dizzy after Martin's re-build was - 1K RPM 3.5 Degrees - 1.5K RPM 7 Degrees - 2K RPM 9 Degrees - 2.5K RPM 10 Degrees - 3K RPM 10.5 Degrees and 10.5 Degrees thereafter.

 

Hope this helps.

 

Geoff

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Hi Geoff, I must have misread my distributor advance graph, confusing the ordinate and the abscissa. Apologies to all, and I'll correct it when I have access to the graph again.

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My DM2 reaches its maximum total centrifugal advance of 10.5 degrees at 1300 rpm, as shown in the attached calibration from Martin Jay. This includes the static timing of 3.5.

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