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E: Engine Ignition Advance

Martyn Goodwin

Well Known and Active Website User
VOC Member
In these long long days of Covid isolation I have been mentally reviewing the preparation of my Comet in preparedness for when (if?) the restrictions on movement are lifted. For months now we have be confined to within 5km of our home .

So over the last 10 years having melted 3 or 4 pistons with the bike used only for touring I revisited the situation - in all cases there was no sign of lean running , actually in every case there were signs of being over rich!

So I now turned my mind to the ignition.

The ATD fitted as standard to ALL VIncent B and C series machines is a Lucas ATD model 47505A/D that provides an advance range at the magneto of 16º to 18º. This translates to an advance range of 32º to 36º at the crank. Depending on the individual ATD the actual advance can be anywhere within that range. And as the ‘fingers’ of the ATD wear, all be it slowly, then the advance range will increase.

It is generally acknowledged that the ignition sweet spot for easy and reliable starting is 4º BTDC (before top dead centre).

Reference to the Vincent Riders Handbook 10th edition advises ignition timing of 38/40º BTDC at full advance for twins but only 37/38º BTDC for singles. For twins this fits in exactly with an original unworn ATD and is close to the lower limit for singles. So why the need to fiddle with something that’s within the original specification? Compression ratio and fuel volatility.

Fuel Volatility: Vincent motors were designed over 70 years ago and were intended to operate on ‘pool’ fuel that had a very low octane rating, but more importantly burnt relatively slowly. Modern fuels have much higher octane ratings and burn much much faster. This faster burning means that combustion will happen much faster and with the ‘original’ ignition timing this results in peak combustion pressure inside the cylinder happening well BEFORE the piston reaches the top of its upward stroke. This has 2, both undesirable, effects. First it tries to force the piston back down the cylinder, before it has reached TDC putting a massive destructive load on the bottom end of the motor, especially the big end bearings. Another consequence of this is excessive heat generation that can lead to piston overheating and failure. Second effect of this faster burning is that almost, if not all, of the charge has been expended before the piston gets to TDC resulting in output power being diminished.

The remedy to the effect of increased fuel volatility is to lower the ignition timing at full advance. How much? On the basis of feedback in MPH across the years and suggested by Irving in “Tuning For Speed” and endorsed on the VOC Forum back in 2018, a reduction of around 4o is a reasonable starting point.

Compression Ratio: There is a tendency of motor rebuilders to use higher compression ratios (CR) that originally fitted at the works. Original for all B and C series, other than Shadows, was a CR of 6.8 to 1, Shadows were 7.3 to 1. It is now more common to find 8 to 1 or even slightly higher CR being used. Again, looking at the advice from Irving as the CR is increased the ignition advance should be reduced. Why? The increased compression ratio can result in an improvement in combustion efficiency which in itself results in an increase in the speed of burning of the fuel in the cylinder. Irving tells us that an increase of 3 in the CR should be matched by a reduction in ignition timing of 5o. So as the increase of the CR from 6.8 to 8 to 1 is around half that, it should be accompanied by a decrease in ignition advance of around 2.5º.


Conclusion: Allowing for both modern fuel and compression changes it appears that sensible maximum full advance to use on Vincent motors today is: For twins 38 less 4 less 2.5 gives 31.5º BTDC; for singles the result is 1º less thus 30.5º BTDC. You will recall that for starting 4º BTDC is optimal and advance at the ATD is half that at the crank.

Twins: 31.5º less 4 = 27.5, divided by 2 = 13.75º advance at the magneto

Singles 30.5º less 4 = 26.5, divided by 2 = 13.25º advance at the magneto .

Remember – the original ATD advance range at the magneto is 16 to 18º

Twin Spark Heads: All of the above relates to generally standard Vincent motor fitted with a single spark plug. If your motor is fitted with twin spark plug heads then based on experience in the field, its desirable to retard the crank shaft full advance by a further 4 degrees – or if you prefer retard the ATD advance by 2 more degrees – it’s the same thing. Result at the magneto is 11.75º for twins and 11.25º for singles.

OK - what have I got wrong??

Martyn
 
Last edited by a moderator:

RobGorLin

New Website User
VOC Member
In these long long days of Covid isolation I have been mentally reviewing the preparation of my Comet in preparedness for when (if?) the restrictions on movement are lifted. For months now we have be confined to within 5km of our home .

So over the last 10 years having melted 3 or 4 pistons with the bike used only for touring I revisited the situation - in all cases there was no sign of lean running , actually in every case there were signs of being over rich!

So I now turned my mind to the ignition.

The ATD fitted as standard to ALL VIncent B and C series machines is a Lucas ATD model 47505A/D that provides an advance range at the magneto of 16º to 18º. This translates to an advance range of 32º to 36º at the crank. Depending on the individual ATD the actual advance can be anywhere within that range. And as the ‘fingers’ of the ATD wear, all be it slowly, then the advance range will increase.

It is generally acknowledged that the ignition sweet spot for easy and reliable starting is 4º BTDC (before top dead centre).

Reference to the Vincent Riders Handbook 10th edition advises ignition timing of 38/40º BTDC at full advance for twins but only 37/38º BTDC for singles. For twins this fits in exactly with an original unworn ATD and is close to the lower limit for singles. So why the need to fiddle with something that’s within the original specification? Compression ratio and fuel volatility.

Fuel Volatility: Vincent motors were designed over 70 years ago and were intended to operate on ‘pool’ fuel that had a very low octane rating, but more importantly burnt relatively slowly. Modern fuels have much higher octane ratings and burn much much faster. This faster burning means that combustion will happen much faster and with the ‘original’ ignition timing this results in peak combustion pressure inside the cylinder happening well BEFORE the piston reaches the top of its upward stroke. This has 2, both undesirable, effects. First it tries to force the piston back down the cylinder, before it has reached TDC putting a massive destructive load on the bottom end of the motor, especially the big end bearings. Another consequence of this is excessive heat generation that can lead to piston overheating and failure. Second effect of this faster burning is that almost, if not all, of the charge has been expended before the piston gets to TDC resulting in output power being diminished.

The remedy to the effect of increased fuel volatility is to lower the ignition timing at full advance. How much? On the basis of feedback in MPH across the years and suggested by Irving in “Tuning For Speed” and endorsed on the VOC Forum back in 2018, a reduction of around 4o is a reasonable starting point.

Compression Ratio: There is a tendency of motor rebuilders to use higher compression ratios (CR) that originally fitted at the works. Original for all B and C series, other than Shadows, was a CR of 6.8 to 1, Shadows were 7.3 to 1. It is now more common to find 8 to 1 or even slightly higher CR being used. Again, looking at the advice from Irving as the CR is increased the ignition advance should be reduced. Why? The increased compression ratio can result in an improvement in combustion efficiency which in itself results in an increase in the speed of burning of the fuel in the cylinder. Irving tells us that an increase of 3 in the CR should be matched by a reduction in ignition timing of 5o. So as the increase of the CR from 6.8 to 8 to 1 is around half that, it should be accompanied by a decrease in ignition advance of around 2.5º.


Conclusion: Allowing for both modern fuel and compression changes it appears that sensible maximum full advance to use on Vincent motors today is: For twins 38 less 4 less 2.5 gives 31.5º BTDC; for singles the result is 1º less thus 30.5º BTDC. You will recall that for starting 4º BTDC is optimal and advance at the ATD is half that at the crank.

Twins: 31.5º less 4 = 27.5, divided by 2 = 13.75º advance at the magneto

Singles 30.5º less 4 = 26.5, divided by 2 = 13.25º advance at the magneto .

Remember – the original ATD advance range at the magneto is 16 to 18º

Twin Spark Heads: All of the above relates to generally standard Vincent motor fitted with a single spark plug. If your motor is fitted with twin spark plug heads then based on experience in the field, its desirable to retard the crank shaft full advance by a further 4 degrees – or if you prefer retard the ATD advance by 2 more degrees – it’s the same thing. Result at the magneto is 11.75º for twins and 11.25º for singles.

OK - what have I got wrong??

Martyn
Have just fitted a forged piston to Comet and noticed much improved power - but what a lumpy beast.
I have a ATD with a welder to fit some metal to reduce the ATD range fron 16 Deg to 13.5 Deg - will be interested to see how it performs - a bit of fettling required to set the final ATD - but no issue
 

Gordon Ryley

Active Website User
VOC Member
In these long long days of Covid isolation I have been mentally reviewing the preparation of my Comet in preparedness for when (if?) the restrictions on movement are lifted. For months now we have be confined to within 5km of our home .

So over the last 10 years having melted 3 or 4 pistons with the bike used only for touring I revisited the situation - in all cases there was no sign of lean running , actually in every case there were signs of being over rich!

So I now turned my mind to the ignition.

The ATD fitted as standard to ALL VIncent B and C series machines is a Lucas ATD model 47505A/D that provides an advance range at the magneto of 16º to 18º. This translates to an advance range of 32º to 36º at the crank. Depending on the individual ATD the actual advance can be anywhere within that range. And as the ‘fingers’ of the ATD wear, all be it slowly, then the advance range will increase.

It is generally acknowledged that the ignition sweet spot for easy and reliable starting is 4º BTDC (before top dead centre).

Reference to the Vincent Riders Handbook 10th edition advises ignition timing of 38/40º BTDC at full advance for twins but only 37/38º BTDC for singles. For twins this fits in exactly with an original unworn ATD and is close to the lower limit for singles. So why the need to fiddle with something that’s within the original specification? Compression ratio and fuel volatility.

Fuel Volatility: Vincent motors were designed over 70 years ago and were intended to operate on ‘pool’ fuel that had a very low octane rating, but more importantly burnt relatively slowly. Modern fuels have much higher octane ratings and burn much much faster. This faster burning means that combustion will happen much faster and with the ‘original’ ignition timing this results in peak combustion pressure inside the cylinder happening well BEFORE the piston reaches the top of its upward stroke. This has 2, both undesirable, effects. First it tries to force the piston back down the cylinder, before it has reached TDC putting a massive destructive load on the bottom end of the motor, especially the big end bearings. Another consequence of this is excessive heat generation that can lead to piston overheating and failure. Second effect of this faster burning is that almost, if not all, of the charge has been expended before the piston gets to TDC resulting in output power being diminished.

The remedy to the effect of increased fuel volatility is to lower the ignition timing at full advance. How much? On the basis of feedback in MPH across the years and suggested by Irving in “Tuning For Speed” and endorsed on the VOC Forum back in 2018, a reduction of around 4o is a reasonable starting point.

Compression Ratio: There is a tendency of motor rebuilders to use higher compression ratios (CR) that originally fitted at the works. Original for all B and C series, other than Shadows, was a CR of 6.8 to 1, Shadows were 7.3 to 1. It is now more common to find 8 to 1 or even slightly higher CR being used. Again, looking at the advice from Irving as the CR is increased the ignition advance should be reduced. Why? The increased compression ratio can result in an improvement in combustion efficiency which in itself results in an increase in the speed of burning of the fuel in the cylinder. Irving tells us that an increase of 3 in the CR should be matched by a reduction in ignition timing of 5o. So as the increase of the CR from 6.8 to 8 to 1 is around half that, it should be accompanied by a decrease in ignition advance of around 2.5º.


Conclusion: Allowing for both modern fuel and compression changes it appears that sensible maximum full advance to use on Vincent motors today is: For twins 38 less 4 less 2.5 gives 31.5º BTDC; for singles the result is 1º less thus 30.5º BTDC. You will recall that for starting 4º BTDC is optimal and advance at the ATD is half that at the crank.

Twins: 31.5º less 4 = 27.5, divided by 2 = 13.75º advance at the magneto

Singles 30.5º less 4 = 26.5, divided by 2 = 13.25º advance at the magneto .

Remember – the original ATD advance range at the magneto is 16 to 18º

Twin Spark Heads: All of the above relates to generally standard Vincent motor fitted with a single spark plug. If your motor is fitted with twin spark plug heads then based on experience in the field, its desirable to retard the crank shaft full advance by a further 4 degrees – or if you prefer retard the ATD advance by 2 more degrees – it’s the same thing. Result at the magneto is 11.75º for twins and 11.25º for singles.

OK - what have I got wrong??

Martyn

Martyn, Very informative article something else to worry about, having a manual
mag the old adage of if it sounds right & feels right it with small movements of
the ign lever depending how fast I am going it will perhaps be ok. Shall I stop
putting Millers Sub in the tank?
 
Last edited by a moderator:

Marcus Bowden

VOC Hon. Overseas Representative
VOC Member
Even when I had a BTH ATD, 5 roller caged assembly much more robust unit than a Lucas I also had a manual leaver to play with and on long climbs it got worked quite a bit. this was 1970 on wards when the Scintilla was fitted, one can get the feel of it very easily.
bananaman.
 

Montlhery

Well Known and Active Website User
VOC Member
A number 7 plug is a bit cold, better off with 5 or 6.......All those fancy fine electrode plugs are a waste of time......Even for racing you are better off with normal full width electrode plugs........Much bigger spark. It does look a little on the rich side, but that is better than too lean.......A Champion plug would look better than that.
Thanks Greg ,I tried the champions after being a long time NGK user . First kick starting and runs well !(good coffee colour on the electrode too) I tried them years ago (many) and was disappointed. They must have changed there manufacturing or quality control?
 

vibrac

Well Known and Active Website User
VOC Member
I guess Champions are as bad as they ever were its just that NGK have slowly lapsed into lower standards as they know the dam electronics in modern tin boxes will sort things out to give a perfect combustion environment so they dont have to make magic plugs anymore. I guess Champion have not caught up with that trick yet, unfortunately they will at the next factory upgrade (if indeed they bother).
 

oexing

Well Known and Active Website User
VOC Member
My guess, most troubles motorists have with spark plugs are just a consequence of picking the unsuitable grade of plugs. When deciding on grades you don´t want to go for racing types as is state of the art in Indianapolis but instead you really look at YOUR style of riding. This will be the factor to get the sutable grade and the brand will not be essential. Otherwise the "bad" brand would have been in real troubles for selling "bad" products for ages. A Vincent engine is not different from other gasoline types in this respect - apart from having crap magnetos for ignition often enough.

Vic
 

Simon Dinsdale

VOC Machine Registrar
VOC Member
VOC Forum Moderator
Knowing something about high voltage gas and vacuum switches and ceramic insulators having worked on them for years I think it's down to the ceramic insulator material used today. Now taking that knowledge and looking at a spark plug I have a theory about what's happening but I no longer have access to stuff like electron microscopes to look for contamination of a cross-section of the ceramic surface etc.

Basically spark plug manufacturers today make plug down to a cost and so have a manufacturing process that produces plugs that suit modern computer-controlled injection electronic ignition where they can start an engine cold without using a rich mixture and so reduce the emissions. The upshot of this is the spark plug never sees anything like the rich mixture seen in our old engines and so the spark plug manufacturer can use a lower cost ceramic which I suspect has a coarser grain structure. I don't know if spark plugs 50 years ago had glazed ceramic insulators but looking at new plugs today they don't appear to be glazed. Now these modern plugs appear to fowl easier with a rich mixture and once that occurs the plug is scrap and this is what we are seeing with modern plugs in our old engines fed via carbs. I have taken such a fouled spark plug and tried to even bead blast them clean and they still won't work so the contamination has penetrated the grain structure of the ceramic surface and is not simply sat on the top of the surface.

Please note this is all just a theory, but after suffering problems with Champion plugs in my Vincent, but not in modern cars, I have found Denso Spark plugs to be one of the better ones for older designed engines and have used in my Vincent the Denso W20EP-U for the last 15 years and the last one I changed was as 20,000 miles and still working fine. I have stocked up with enough so I will never have to buy a spark plug for one of my bikes ever again, but beware and always buy from a known source as yes there are fake plugs out there, probably made in China.

Just realised I probably started a panic buy on Denso plugs, but at least it different from panic buying toilet rolls"

Simon
 
Last edited by a moderator:

greg brillus

Well Known and Active Website User
VOC Member
Simon I would say you are most likely correct........Plugs can be a pain.......I would say the main thing is to have an engine that is in a well enough state of tune that the plugs run well enough to not give trouble........And that should be the case. Of course that is easy for me to say, given I am a mechanic, and play around with these things every day. So basically if your engine is burning excess amounts of oil or the carburetion is too rich, then yes plug failures will be a common problem........Like our modern fuels, we have to live with it all.........An engine old or new that is tuned up well enough will run happily and not fowl plugs regularly.......
 

highbury731

Well Known and Active Website User
VOC Member
Is the Green Spark Plug Co a safe supplier?

I tried their site for the Denso W20EP-U - they list it for £1.70 inc VAT. Seems cheap to me

Also, I read that they are factory set to a plug gap of around 1mm / .040" ish, and are delicate wires. Can they be set to small clearances for our poxy magnetos?

Paul
 
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oexing

Well Known and Active Website User
VOC Member
These days many spark plugs have an internal resistor at 5 kOhm to fight radio emissions. In case of crappy mags that type may not be the best idea so go for plugs with no resistor, possibly this being the cause for some troubles today. So not really a question of brands, more like wrong grade or resistor. On some plugs an R in the code may show the resistor kind.

Vic
 

Simon Dinsdale

VOC Machine Registrar
VOC Member
VOC Forum Moderator
Is the Green Spark Plug Co a safe supplier?

I tried their site for the Denso W20EP-U - they list it for £1.70 inc VAT. Seems cheap to me

Also, I read that they are factory set to a plug gap of around 1mm / .040" ish, and are delicate wires. Can they be set to small clearances for our poxy magnetos?

Paul
Thats where I get them from. Yes they are cheap as they are the Denso std non fancy range and contain no exotic electrodes.

They are a std copper / nickel plug with no resistor and don't have any fancy narrow electrodes made of unobtanium etc so don't I know what you mean by delicate wires. Yes you can easily alter the gap to suit you ignition and they run fine on coil ignition and Lucas magneto as I have used them with both ignition types.
 

LoneStar

Well Known and Active Website User
VOC Member
I've used Denso plugs with success - but on the other hand, never had any issues with NGK even though I flood the bike sometimes due to over-tickling.

Denso (and others) made non-resistor plugs with platinum fine-wire electrodes. These fire at lower voltage than plugs with larger, standard electrodes - helpful with a magneto that may be less powerful than when new. In any case, magneto voltage is proportional to RPM - a disadvantage compared to coil ignition when kick-starting.

Denso platinum plugs that will fit the Vincent are WxxEP-ZU, where xx is the heat range. 20, 22 & 24 correspond with NGK 6, 7 & 8 heat ranges.
 

erik

Well Known and Active Website User
VOC Member
Maybe it is a Little bit interesting what I did whith the atd unit : I drilled a 5mm hole in the tufnol idler so when I insert a 5mm pinn the atd unit is blocked in full advanced Position .No flimsy wedges necessary which could Escape into the Timing chest.To my mind this makes life easier to find the correct firing Point. Regards Erik
 

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