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MG MGB Technical - 48D4 Distributor timing, L.C. engine.

1973 MGBGT : My DBL 230 (etc) plated block is thought to be a low compression Leyland ring-in, fitted from new in 1973. I understand these blocks were sourced from stocks intended for Sherpa, Austin Princess, etc. at the time. I therefore have what I believe is the equivalent of the 18VV engine, unleaded head, HS4 carbs.
What I don't know, is whether the Lucas 48D4 distributor was also fitted in 1973 - the car came to me a few years ago with the 48D4 and Lumenition ignition fitted, which recently died.
I'm not sure whether to stick with the 48D4, as I can't find any info on basic timing settings or advance curves for the 48D4 - can anyone help? Alternatively, would I be better advised to source a more "correct" distributor, possibly a 25D4 to suit the age of the car, or a more common distributor that is suited to the LC engine?
I did find, incidentally, that the distributor drive gear was way out from WSM position, so corrected that so that in future it'll be easier to check everything against Spec.
All suggestions appreciated as usual.
John.(Warm North Queensland).

J P Hall

The timing will be down to the engine characteristics, the centrifugal and advance curve are designed for that as indicated by the reference number usually on the body, so it would be more a case of what timing did that engine have.

I've not come across an 18VV before - 18S, 18SV and 18SGB but all the others have been 18V plus three digits. Neil Cairns in his extensive 'MG Engine History' doesn't mention them.

You would probably be better off looking up the specs for the DBL230, or at least LC Marina/Princess/Sherpa engines. The 48D4 was used in a number of engines and I do see reference to Sherpa.
paulh4

Sorry, the second V was fat fingers.
I take your point about looking up specs for DBL 230 engines, but I wonder where? I will certainly search for 48D info, in case I'm tempted to keep the Lucas item and put in points and condenser instead of electronic/ Hall effect module. I admit I find it easier to be able to set initial timing via points and light, followed by dizzy tweaking.
Any suggestions where to find 48D4 distributor info would be appreciated.
John.
J P Hall

Based on the foregoing i.e. where DBL230 was reputed to have been used try

https://www.leylandprincess.co.uk/

and

https://morrismarina.org.uk/

There are several references for Leyland Sherpa as well.
paulh4

Paul, I remembered that the MMM Register had reproduced a detailed list of Lucas distributors and their applications. There on page 10 I found the following reference to 1973 MGB, 18V Low Compression motor, and "672Z, 673Z" (not sure what that refers to) -

Service # 41491
Model 25D4

RPM 2400 ... Advance 19.5
RPM 1900 ... Advance 16
RPM 1000 ... Advance 8.
(I assume to these must be added the 10 degrees static advance).

Vacuum Code 10/15/5.

My 48D4 distributor is # 41775A - the MMM list refers # 41775 (no "A") as -

1978-81 British Leyland -
Austin Princess 2 1700, Morris 1.7 HC, Ital 1.7HC, and Naylor TF Replica.

This suggests that my 48D4, and the DBL230 block, <might> have been taken from Austin or Morris stock, as a replacement engine arrangement?

My compression figures are 130psi on all cylinders (dry test) - does this qualify as "High" or "Low" Compression, and - how critical is this to distributor choice? - distributor model # 25D4 is listed as LC, while my 48D4 is listed as HC!
Thanks for any and all feedback.
John.

J P Hall

672z and 673z are both us pollution spec engines, the only difference is that 673z has an anti runon valve
Both run the vac. adv. direct to the manifold and there's a weird timing spec for them like---and don't quote me but it's something like 18 deg @ 1500rpm or some weird spec like that
If your car hasn't got a cat. on it just set it up like an early b and be done with it, with the vac to the carby port
10 deg static
12-13 with a strobe at idle and make sure it doesn't go over 32 full advance----with the vac. line OFF while checking
William Revit

Great advice, Willy. Shall do - probably with new points & condenser tomorrow. So 130 psi is Low Compression, but perfectly good readings? I haven't gone to the length of looking at the head number, piston crown depth etc, as the car ran beautifully with the Lumenition until it died. The cause of death was: module inside dizzy is only secured by 1 screw; if the screw loosens, the module can pivot inwards and be hit by the spinning plastic chopper. The module fries as a result.
The (cheap) new Accuspark module is secured by 2 screws, but so far I'm having trouble setting the timing up nicely, so tempted to try points for comoarison.
Here's hoping the perfectly good 48D4, with points, can be made to run as it did before.
John.
J P Hall

130 is LC so not comparable with the HC versions used in MGBs at the time. As Willy says the 672 and 673 were USA and Canada LC for 1973 with all sorts of emissions controls, so also not comparable with the LC DBL230 used in the UK. The 672 and 673 had a 25D4 41491 with

Static 6
Strobe 11 at 1500
Centrifugal
16 at 2025
32 at 3825
39 at 4800
Vacuum - manifold Starts 10, Ends 15, Max 10 degrees

On the first MGB distributors if you added centrifugal to static then you did get the strobe figure but that wasn't the case on later ones for some reason. In the above case 6 static and a strobe of 11 at 1500 degrees implies a centrifugal of 5 at 1500 but as you can see a graph indicates it was 12 at 1500.

The only significant difference between carb and manifold vacuum is that the former gives zero vacuum advance at idle whereas manifold gives significant additional advance, probably close to 10 degrees in this case. As soon as you go off idle carb vacuum rises rapidly to meet manifold and thereafter they are the same for all practical throttle positions. The benefit is that more advance at idle gives a higher idle speed, so you reduce the idle screws to get that back down, which means you are using less fuel and hence reducing emissions. It's why strobe readings have to be taken with the manifold port disconnected and plugged. UK continued with carb vacuum until we had to meet emissions limits as well.


paulh4

Ripper thanks Paul - more news tomorrow when I try both setups and Willy's figures. John.
J P Hall

John. Whether an engine is "low compression" or "high compression" cannot be accurately determined merely by taking a compression reading. I have seen compression readings of 160-165 PSI from the 8.0 (low compression) engines and readings of 120 PSI from the 8.8 (high compression) engines. Much depends on the condition of the engine and other factors such as the camshaft being used. Unless you know that the engine, after some 40+ years, is still all original and in near new condition, all a compression check can tell you is what the current compression readings are--nothing more.

The same thing applies to the distributor. The model number is, most often, indicative of what the distributor is based on. The Lucas 45D4 was a points type of distributor to be used in a four cylinder automobile. The Lucas 45DE4 and 45DM4 were similar, but designed for two different types of "electronic ignition" devices. But, to the best of my knowledge, all of the internal parts are interchangable within the same body. Again, unless you know that the distributor is completely original, all the model number will tell you is what parts you might expect to find inside it. The specification number is even more of a trap because it only tells you how the distributor started out, but nothing abour what it is today.

As to the ignition advance curve that your car actually needs, it can only be determined on a chassis dynamometer (rolling road). That can determine the approximate degree of mechanical advance you will need at various engine speeds under various load states. With a required curve determined, it is possible to determine what advance curve is best suited to your engine and how well the advance curve of your current distributor meets that need.

The only method of determining what your distributor's actual, current, advance is would be to test the distributor at various engine speeds and plot the curve. This can be done on either a distributor test machine (not often found today), or on the engine using an assistant, a good tachometer, and a dial back timing light.

As a side note, one of the distributors that I took apart and checked out (a brand new "rebuilt" unit) was so far off of any MGB specification that it had a high probability of causing engine damage with even limited use. You need to find out exactly what you have, determine how worn it might be, and determine how well it is actually capable of doing its job if you are to make any progress in a realistic, intelligent manner.

Les
Les Bengtson

John
Adding to Les' comments-
I doubt very much that your engine is low compression unless it's a US import.

With the Accuspark you have---what's the issue you're having setting the timing---I take it that it's in your old distributor so timing should be same as you had before, the module change wouldn't/shouldn't effect timing and why did you have to move the drive gear position---unless it got mixed up by someone setting it in the MGA position--all MGB's use the other position contrary to what some info says.

willy
William Revit

Les - wow, interesting, thank you for chipping in. Your last comment is exactly why, if anything, I lean towards keeping the genuine Lucas (but 48D4) distributor and running it with either points/condenser OR an electronic gizmo, whichever I can get to run better. Unfortunately after getting close with the timing yesterday with the Accuspark module installed, I suddenly lost all spark! Checking connections, coil output etc. today.
Willy - the drive gear (& rotor position) was way out, so I felt like setting it up according to the Book, rotor pointing to about 1.00 so that future owners' lives are made easier. There was nothing wrong with the running of the motor with the previous alignment - just my peace of mind that it's now aligned "correctly". John.
J P Hall

Drive gear and hence rotor can be in as many positions as there are teeth on the gear. On one is 'correct' but if the leads are positioned in the cap appropriately they will all run the engine. If you follow Haynes you will end up with the rotor 180 degrees out, with the WSM it will be correct. This is because the WSM tells us to rotate the crank 360 degrees after fitting the chain but Hayne doesn't.

But sudden loss of spark will be something else.

HC should give a clear indication of excessive advance by oinking under load, especially on lower grade fuel. But I've read that you can stall the starter on an LC by putting in too much advance but it still won't pink when eventually started.

If it _is_ HC, then the simplest way of setting the timing is just short of pinking at any combination of revs, throttle opening and load (Haynes says to aim for light pinking but I feel that's too aggressive), and that will vary with the fuel used. Other than that you would need a lot of time tinkering with weights and springs either on a static or a rolling road. Programable distributors like the 123 Tune enable you to do this with a laptop but are expensive, and really only worth it if you are looking for every last horsepower.
paulh4

up till it pings and back a bit---a bit old school
You wouldn't do that on rollers--not for long anyway
William Revit

Just a quick update : sent the 48D4 to a distributor specialist in Victoria; he confirmed that the Lucas unit, springs, weights, shaft etc are all perfect. The Accuspark module apparently had too large a gap between it and the magnetic ring; a new unit was fitted and bench tested perfect. SO - I'll refit distributor, new coil and leads and set up the timing along the lines of Willy (above), and then tweak to see what's the best combination. The last time I road tested the B, a kindly gentleman took a photo and sent it to me in the mail together with a bill for $1100!
John.
J P Hall

Mrs.John would have been impressed with that mate.
Did the distributor repairer fella give you a readout of the advance curve-

willy
William Revit

Ha! Yes Willy, Joanne was not amused. I tried to float the argument that if they'd pinged me a minute earlier, on a quiet straight half-mile, it would've been 156k, whatever that is in miles. I was sent packing.

Re. the advance curve, you wouldn't believe it - When I received the goodies, I rang them back (Retro Race & Track, Melbourne) to ask for precisely that info, because it's a non-MGB distributor of unknown provenance - to be told the distributor guy (Mark) had not recorded the progressive numbers at,say, 600, 1000, 2000 and 3000 rpm. I was surprised to say the least. In fairness, the distributor guy and his machines are outside subbies, not R R & T staff, otherwise I think they'd have been for the highjump.
How do you suggest we proceed - tune up from idle upwards using the fancy digital dial-up strobe, and see what we get? I've scoured the internet and cannot find, anywhere, typical advance curve numbers for a 1980 Lucas 48D4.
Frustrated, Atherton
J P Hall

Short of putting it back on a machine just use a dial-back strobe and plot the advance at several rev points - vacuum source disconnected and plugged if fitted. And ears probably at the higher revs ...
paulh4

OK
You have to put out of your mind that your distributor came out of a boat or a milk truck or whatever-
Concentrate on the fact your car is an MGB.and set it up as that. If I had your car and were setting your dist.timing It'd go like this--- Vac hose OFF--
Set your static to spec or near as to get it running-
Start it up and warm it up---set the idle to 750, seems to be the number to go to,--NO more than 800 or it'll probably run on.
Now--the thing with these dist's if working properly is that they start advancing up from around 500rpm onwards so setting the idle at 750 the dist has already advanced a bit so should show--at 750rpm--12-13deg including the 10 static -so thats now idling 12-13deg total@750--- If you're adjusting the timing and the idle speed changes, adjust to 750 again and recheck until you've got the combination right.
Then check at 2000 you should have around 25/26 deg. inc.
and at full advance you should have 30/32 deg all in by 2500rpm.
A well set up dist. for an MGB should definately be all in by 3000 at the latest.
Now---my pet hat is adjustable timing lights, I've checked mine against a timing tape and had to adjust the scroll on the roller 4deg. to get an accurate read and that's on an expensive machine-----I've checked several against it and I'd say 2/3 of them are out of whack, so I'd suggest you check your light against the marks on your timing cover---
And don't get confused with your light--When the roller is right back to zero the flash will be exactly what's happening---eg -at idle 13 deg but as soon as you touch that roller you are now comparing to the tdc mark so by rights if you roll up 13 deg on your screen then the flash should be at tdc.
Have lost count of how many cars I've had set up off the 10 mark
Just make sure your roller is right back if you're reading off the timing cover scale at idle, it's real easy to bump the roller and not realise and be a few deg. out

willy
William Revit

Perfect thank you Willy o'the South and Paul of the north of the North.
I shall attack with renewed vigour shortly - just need to finish re-skinning the outside rear of the motorhome. Long story.
John.
J P Hall

This thread was discussed between 11/03/2024 and 23/05/2024

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