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E: Engine Ignition Advance

MartynG

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
 
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greg brillus

Well Known and Active Website User
VOC Member
Detonation can cause many different kinds of problems, not just a big vent in the piston crown........If you remember Ken and Barry Horner had issues with detonation causing a high frequency through the piston and connecting rod........This ultimately caused the rod to fail........ I have heard that running methanol or E 85 type fuels can cause detonation high frequency problems that relate to the fuel used. There are many images of pistons on the net showing damage from detonation that show clearly the effects of destructive shock waves that break either the rings and/or the ring lands.........This is what has happened to your piston..........so on part throttle how do you know what the mixture is.......This is where plug checks can be helpful........Plugs that look absolutely clean after a run are a sure sign the mixture is definitely on the lean side for an air cooled engine........Even if you run the bike up an incline in say 3rd gear and the throttle held in a certain position........ Then pull over quit the engine and remove the plug and have a look. I don't recommend you use 98 octane fuel, use 91.......91 remains constant and more stable for longer.......98 goes off after several weeks in the tank and leaves a fuel not much different to kerosene.
 

greg brillus

Well Known and Active Website User
VOC Member
That sounds good.......What plug are you running........I am generally using Champion N5C's in all the bikes now. They handle the sooty mixtures better without fowling up. I remember Ken Phelps prefers them as well.
 

MartynG

Well Known and Active Website User
VOC Member
On advice of the late Big Sid I had been using Nippon Denso W22EP-U plugs for years - and have plenty of new old stock on hand.

Just lately I switched to NGK BPR7EIX iridium, gaped to 0.018" as a trial, mainly to avoid plug fouling.

Here is a photo of the NGK plug after a 10 km run yesterday - no extended idling. Would rather run rich than lean!

1602382733112.png
1602383154767.png
 

greg brillus

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.
 

oexing

Well Known and Active Website User
VOC Member
All that talk about detonation - and then using 91 octane fuel - now I try to see the logic ?? So why have they come upon 98 or 100 octane fuel or avgas that had still higher octane rating in the past ?
There could be some truth about detonation in this engine which wrecked the piston ring. But then why on earth keep using 91 octane fuel at all ?
Also I´d like to see the advance curve of the ATD in this case. No use stating advanced and retarded degrees when not telling at which speed you have full advance. I´d like to have full advance NOT before 3000 rpm crank speed, maybe 3500 better. And mixture seems very rich, could lead to thinned oil from that, no great lubrication then. Some more photos of piston crowns would be good for drawing conclusions and to learn from. For knowing about mixture you´d better look at the piston or spark plug thread face, not the ceramic. Black sooty deposits are very rich , so go for the carby. When that is fixed you pick any spark plug that keeps the ceramic nice and bright - and this depends on your riding style, pottering or high speed motorway burns - very different conditions for plugs.
As long as you see a sooty piston crown I cannot believe in overheating effects , you´d have non deposits in that case but melted edges or worse. I remember one story decades ago with an all iron HD Sportster of my neighbour , got a holed piston from using "regular fuel" then because the guy from the filling station suggested to save the money by not having high octane fuel.

Vic
 

MartynG

Well Known and Active Website User
VOC Member
All that talk about detonation - and then using 91 octane fuel - now I try to see the logic ?? So why have they come upon 98 or 100 octane fuel or avgas that had still higher octane rating in the past ?
There could be some truth about detonation in this engine which wrecked the piston ring. But then why on earth keep using 91 octane fuel at all ?
Also I´d like to see the advance curve of the ATD in this case. No use stating advanced and retarded degrees when not telling at which speed you have full advance. I´d like to have full advance NOT before 3000 rpm crank speed, maybe 3500 better. And mixture seems very rich, could lead to thinned oil from that, no great lubrication then. Some more photos of piston crowns would be good for drawing conclusions and to learn from. For knowing about mixture you´d better look at the piston or spark plug thread face, not the ceramic. Black sooty deposits are very rich , so go for the carby. When that is fixed you pick any spark plug that keeps the ceramic nice and bright - and this depends on your riding style, pottering or high speed motorway burns - very different conditions for plugs.
As long as you see a sooty piston crown I cannot believe in overheating effects , you´d have non deposits in that case but melted edges or worse. I remember one story decades ago with an all iron HD Sportster of my neighbour , got a holed piston from using "regular fuel" then because the guy from the filling station suggested to save the money by not having high octane fuel.

Vic
Hi Vic, see my post #7. I have a non contact tacho on order so I can measure the rpm where the advance starts and also the rpm where full advance is reached.

In my post #6 it seems that full advance is reached at around 2,500 rpm with the standard ATD.

As part of the recent changes I have lightened the ATD bob weights in order to raise the RPM point where full advance is reached. photo in my post #6

Once I get the tacho all should be revealed regarding the action of my atd.

Martyn

PS my 'revised' signature says it all.
 

vibrac

Well Known and Active Website User
VOC Member
Hi Martyn,
Try putting a socket over the bolt head and giving it a gentle tap, this should set the tapers and prevent the armature moving. Cheers, Stu.
It took a real motorcyclist who had suffered like all of us with the prince of darkness silly tapered mag to atd connection to come up with the BTH electronic mag through peg
It really goes to show what a passive customer the pre and post war motorcyclist was no wonder Vincent's went bust pearls before swine comes to mind.
 

Chris Launders

Well Known and Active Website User
VOC Member
Martyn.
A little trick I have learnt with Lucas magneto's when timing them on your own.

Remove the earth brush and clean out the threads in the body (5/16 BSF on K type), get a 5/16 BSF bolt with about 1" of thread, put a smooth radius on the end and holding the magneto in the advanced position screw the bolt in FINGER TIGHT ONLY. Any tighter and you will risk damaging the armature.

This will lock the armature enough for you to be able to tap the ATD on and start the nut without needing 3 hands.

Do NOT use this to hold the armature while you tighten the nut and remember to remove the bolt and fit the earth brush after.
 

greg brillus

Well Known and Active Website User
VOC Member
Oh don't worry the newer BTH mag installation is not always spot on. Ok, not necessarily a fault of the mag more the pinion nut not machined quite correct.......In short, they become shank bound before the taper pulls up tight........This causes the timing to shift either sooner or later causing starting problems and so on. So it's not always the main item that's the culprit more some other part of the puzzle.
 

BigEd

Well Known and Active Website User
VOC Member
VOC Forum Moderator
Once I get the tacho all should be revealed regarding the action of my atd.

Martyn

PS my 'revised' signature says it all.
When we talk about the advance/retard range it depends on what rpm we start our calculation from. I remember some years ago doing some test on one of the late Don Alexander's twins that had an electric start fitted. I think that the figures were around 280 - 300 rpm mark. I will check mine next time I'm in the garage. I don't remember what the kick starting speed was.
 

Glenliman

Well Known and Active Website User
VOC Member
This is the Pazon Smartfire ignition curve for 2 plug head high compression Vincent twins.
The curve made up for the 1360 is similar but only goes to 26 degrees total advance. With the big squish areas on the TP heads, it would probably work at 31 but 26 is a lot safer. From Roy Robertson's dyno findings, on that type of engine there is no power loss by dropping back to 26.
 

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MartynG

Well Known and Active Website User
VOC Member
Does Martyn use Mk2 Amal ??, If He is some people use a 105 needle jet, I have found 106 for me, U.K.
I know He says use a bigger main jet, But the petrol still has to go though the needle jet.
And being hotter out there, I would have thought it needs to be rich.
Pulled the jet block today. Main jet is 220 and needle jet is 106. Needle positioned midway between middle and bottom groove.

Amal Mk1 Concentric Premier 30mm
 

Bill Thomas

Well Known and Active Website User
VOC Member
All you need now is to use the best, Highest cost petrol you can buy, Can't see the point in you
using 91 octane.
All my Bikes are race spec' I don't have pinking, But it's not so hot over here !.
Also use a single grade oil 40.
Cheers Bill.
 

MarBl

Website User
VOC Member
on that type of engine there is no power loss by dropping back to 26
It should be pointed out, that on most engines advancing the ignition is done not only because of power gain, but of heat reduction. In general, the more the ignition is retarded the hotter the engine runs.
If you have an ignition time intervall that delivers more or less constant power, you choose the most advanved ignition point to get the thermal optimum.
Now of course high speed pinging/knocking has to be avoided at all cost, so that sets an upper limit. But you want to run the engine close to that point.
That effect has not to be confused with the retarding of the ignition in a twin spark system. Because of the two ignitions on opposite sides of the combustion chamber the mixture is burnt much faster overall (but still in a controlled way).
One other advantage of a twin spark system is the elimination of ignition induced pinging. A weak spark (and/or slow burning mixture) in a big chamber (500cc and above) can lead to the effect, that the (pressure) shock wave of the ignition starts an uncontrolled, compression induced detonation somewhere else. You dont want that. You want the whole mixture being ignited by the flame front, not by pressure increase.
 

Marcus Bowden

Well Known and Active Website User
VOC Member
I've been twin sparked since 1990, retarding ignition to 24 / 25 degrees with the addition of a manual advance/retard as conditions do vairy with fuel and terrain, giving less negative power as pressure build-up comes later. It also reduces the blueing of exhaust pipes, a start-up with pipes off the blue fame is much shorter on two plugs than a single plug firing. I have switches to isolate each double coil so periodically I can check that each coil is working. 2010 a coil failed, dropping an exhaust seat in front and hole in the rear piston, the reason for installing switches. (All the time modifying.)
Question:-
Careful inspection of a head I think it possible to fit smaller spark plugs either side of the valves, if the addition of one plug retards ten degrees what would another four do??? Oh, also a central plug as that's been done too.
1990 I stayed with Herve' Hammond for a month doing an engine strip for him to sleave my main bearing housings and saw an engine he had made for record-breaking attempts on low profile three-wheelers for fuel economy, cylinder head consisted of four inlets & four exhausts also four plugs the engine size was 33cc valves operated by solenoids. I think it was shelved because the electrics had to be made by the fuel a not a battery sauce. But it was a magnificent bit of engineering machining.
going bananas !
 

Glenliman

Well Known and Active Website User
VOC Member
Roy's bike, with ignition set up with timing as late as possible,
It should be pointed out, that on most engines advancing the ignition is done not only because of power gain, but of heat reduction. In general, the more the ignition is retarded the hotter the engine runs.
If you have an ignition time intervall that delivers more or less constant power, you choose the most advanved ignition point to get the thermal optimum.
Now of course high speed pinging/knocking has to be avoided at all cost, so that sets an upper limit. But you want to run the engine close to that point.
That effect has not to be confused with the retarding of the ignition in a twin spark system. Because of the two ignitions on opposite sides of the combustion chamber the mixture is burnt much faster overall (but still in a controlled way).
One other advantage of a twin spark system is the elimination of ignition induced pinging. A weak spark (and/or slow burning mixture) in a big chamber (500cc and above) can lead to the effect, that the (pressure) shock wave of the ignition starts an uncontrolled, compression induced detonation somewhere else. You dont want that. You want the whole mixture being ignited by the flame front, not by pressure increase.
In the past, this was always my method too, advance as much as possible but stop just short of the pre-ignition point. Back in the 1960s, Popular Mechanic's Smokey Yunick referred to this method as " Power Timing."

Roy set his racer's timing in just the opposite manner, retard as far as possible without power loss. Knowing how successful and reliable that engine has been when used for years of Sprinting, Road racing and Land speed racing ( holds a UK record), I followed his advice on the ignition timing.
So far its been fine for about 5 k road miles with occasional abuse, although nothing like the beating his 1272 will have endured.

Glen
 
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van drenth

Well Known and Active Website User
VOC Member
On advice of the late Big Sid I had been using Nippon Denso W22EP-U plugs for years - and have plenty of new old stock on hand.

Just lately I switched to NGK BPR7EIX iridium, gaped to 0.018" as a trial, mainly to avoid plug fouling.

Here is a photo of the NGK plug after a 10 km run yesterday - no extended idling. Would rather run rich than lean!

View attachment 37877
View attachment 37879
Check your choke valve.
Got the same problem with my Dellorto carbs due to ethanol.
Jan van Drenth
 

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