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Gordon
United Kingdom
692 Posts |
 Posted - 03/04/2008 : 16:15:37
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Help! I have carried out a trial fit to the engine and the drive dog is not engaging. Looking at the Lucas distributor what I notice is that the drive peg is slightly off-centre so that the distributor will only engage in one orientation ie you can't assemble it 180 degrees out! This implies that the drive peg for the Lucas distributor is different to my scitilla vertex drive dog where the drive dog is central with no offset. Is this offset the standard arrangement? Yes the magneto is marked "L" so is designed to rotate the right way!
The mag is also 8" from the bottom of the casing to the top of the red cap. As such it will foul the bonnet side so is this the correct mag for the PB or do others modify their bonnet side to clear a scintilla vertex mag?
All comments will be very welcome.
Gordon
Gordon |
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Blue M
United Kingdom
1474 Posts |
Posted - 04/04/2008 : 08:57:26
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I'm afraid I can't help with this problem, but while we are on the subject, can anyone tell me what advantage a magneto has over coil ignition? Is it worth altering the bonnet for?
Thanks
Ian |
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Bob Stringfield
United Kingdom
854 Posts |
Posted - 04/04/2008 : 10:20:32
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A magneto needs no electrical input, therefore will provide sparks for an engine with no electricity generator, normally a stationary engine, early vehicle or racing unit. It has a reputation for giving a big spark at high revs, though I have no justification for this statement other than received opinion.
Coil ignition is cheap, easy to build and rebuild, efficient and easily specified to suit your engine and fuel. When a magneto dies, dust off your wallet.
MG used magnetos on some road and racing cars, turning the drive through 90 degrees to avoid the unsightly bulge, which needs to be bigger than the dynamo one on a pushrod car.
'Magneto trouble' was common amongst old cars and was often given as a reason for racing retirements because the public could relate to it.This avoided telling them that there had been an engine failure which would reflect on the car's maker. |
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David Allison
United Kingdom
665 Posts |
Posted - 04/04/2008 : 15:18:13
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I have a Scintilla Vertex magneto fitted to the NA since 1935.
Although the unit has been rebuilt a couple of times - it has on the whole been very reliable.
The current unit is over 20 years old (since its last re-build) and I replaced the points only last year.
Each time the unit was rebuilt (and this was only 3 times in the 46 years since Dad bought the car) it was because of wear rather than any curious hoo-doo "magneto trouble".
Magneto's are neither more or less reliable than coil ignition.
Magneto's used to give a stronger spark than many coil systems available pre-war - but the passage of time and developments in electrics mean that the coil ignition is probably equal to the magneto in this regard too nowadays.
Most "magneto problems" are to do with people trying to save money - and trying to run a pretty well worn out magneto.
There are a number of companies offering re-builds at varying prices and quality of service (the bigger the price does not always offer a better service either).
The majority of problems are wear in the shaft causing slack in the mechanism and ignition timing issues (these issues also occur on worn out coil ignition distributors too).
There is sometimes wear in the inner charge output assembly which can cause misfires and a lack of sparks - but this is also normally caused by old age, damp or a combination of both.
If you have a magneto and it is a good one - stick with it.
A good mag is always better than a bad coil!
However the same is true of a coil too!
MG normally fitted magnetos to racing cars because as Bob points out they need no electrical input, the unit is self generating.
The racing cars normally had a pressurised fuel tank and could therefore drop the battery, starter motor, dynamo etc.. to save weight.
The engine could be started with a push start - later they fitted the battery and starter motor - but the battery would be saved purely to start the engine and as usefull ballast over the rear wheels.
The K3 was fitted with a BTH magneto in 1933 - this is a horizontal mounted magneto - hence the need for a right angle drive.
These magnetos fit snug under the line of the sparking plugs and inlet manifold - I can remember that a couple of the plugs are awkward to get at (paticularly when hot).
The NA is fitted with a Scintilla (Vertex means vertical) and on the NA this fits under the bonnet without modification.
On a J or P type you will need a slight alteration to the bonnet - but nothing like as large as suggested.
The NB special raced by my sister (also fitted with a Scintilla) has a narrower bonnet line and we have a slightly raised area on the bonnet side over the magneto approx. 20 mm deep.
On a J or P this dimple might end up rather deeper than this - but certainly nothng as large as the T-type power bulge around its dynamo (although I might be mistaken of course).
There are a number of J and P types around with a vertical magneto and I dont recall any significant power bulges on those cars.
It is vital that you have clearance between the bonnet side and the magneto cap (minimum 5-10 mm), the magneto is very good at finding an earth for stray sparks and quite a few misfires have been caused by bonnet sides being too close the cap and leads (on coil ignition cars too).
Hope this helps - but dont dismiss the Magneto - they are like pre-selector gearboxes, much maligned and misunderstood.
That said if I were building a car for racing now and I did not have a magneto - I would not bother.
Coil ignition is lighter and cheaper to maintain - the components are easy to get hold of and not so fussy to maintain.
Also as an aside the latest electronic ignition systems work in a very simlar way to the magneto - a pulse operated spark generator firing a capacitor in time with a sender unit mounted on the drive unit.
OK the electrical geniuses have worked their majic on the timing of the pulse and varying the ignition timing to suit the way in which the car is being driven - but the principle is the same.
Nothing new is there?
Regards David |
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Richard Hardy
United Kingdom
2162 Posts |
Posted - 05/04/2008 : 23:14:16
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I run an NV4 Scintilla mag on my J type and I did not need to alter the bonnet. It is within about 1/8 inch of it though!
Yorvik |
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Gordon
United Kingdom
692 Posts |
Posted - 06/04/2008 : 10:59:48
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Having now done a trial fit of the Scintilla Vertex I can confirm that David is correct in that a bulge of at least 20-25mm will be needed to accommodate the mag. This bulge as far as I can determine will enroach into the bottom of the louvres which is a pity!
Gordon
Gordon |
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talbot
United Kingdom
718 Posts |
Posted - 06/04/2008 : 17:34:17
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Wouldn't a 009 distribitor, modern coli and a Pertronix electronic ignition module be a better option?
Jan T |
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Richard Hardy
United Kingdom
2162 Posts |
Posted - 06/04/2008 : 22:41:07
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Must be different for the P then. As I highlighted, you can just get away with it on the J type.
Rich |
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David Allison
United Kingdom
665 Posts |
Posted - 07/04/2008 : 08:30:27
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Gordon
I think you might have a problem - either the mag is fitted with a longer than standard shaft or it has a longer than standard main body.
If the mag was originally destined for another function it is possible it was customised to suit.
Mags were commonly fitted to stationery engines (pumps and generators) and some of these might have required a specialist installation.
Check that the drive dog is fully engauged (this can win you another 15 mm) sometimes they are a tight fit.
it is also possible that the mag you have has the wrong drive dog for a MMM car - the Peugeot (Simca) stationery pump engine has a concentric drive dog.
I warn anyone competing in the VSCC NOT to fit the electronic ignition option - it will get you in serious trouble with the eligability scrutineers.
Make sure the installation is correct before altering the bonnet - also look for another car fitted with a mag and compare notes.
Rgerads David
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Gordon
United Kingdom
692 Posts |
Posted - 07/04/2008 : 14:17:10
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David,
Thanks for yhour post. The unit did in fact have a symmetrical drive dog which I have removed and replaced by one from an MGB! the shaft length and position of new dog and offset are identical to the original Lucas distributor so that is OK. Your suggestion that the body length might be different could be a possibility. My lenghth from top of cap to face that contacts the engine housing is 8 cm.. Has anyone got any measurements to compare this to, please. I am also aware that some of the stationary units had an impulse starter fitted and I believe that this was internal and so could give a longer body? However mine is not that type.
I have contacted one person who specialises in overhaul ofntilla vertex magnetos and he believes the body is the correct length. He also made some interseting points. The use of yellow paint on the various screws inside the cap is notfactory standard at original build and the presence indicates an overhaul has been done. If the HT coil has been replaced then the later ones are obvious as the HT connector will be black like nan HT plug lead and have a cap soldered on. It doesn't guarantee that the coil insulation is not shellac, which is a pity! I also understand thatif you nprovide an earth near the HT lead (with the cap off) then turning over by hand quite slowly will give a strong spark that should jump1/2" and be blue in colour. Mine has all the right attributes but the engine will let me know!!
I am not positive of my figure that I quoted for the bulge to clear the mag. I am making a test panel with a hole to let me accurately size the cut out and will report back when I have completed this test.
Gordon
Gordon |
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David Allison
United Kingdom
665 Posts |
Posted - 07/04/2008 : 15:26:12
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Sounds like things are going the right way - the cut out might well be a lot less severe than you thought I think.
Regards David |
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