E: Engine Ignition Advance

[Martyn Goodwin]

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

 

[Magnetoman]

‘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.

OK - what have I got wrong??

I'm not saying you have anything wrong, but more than once I've searched for studies of the rate of burn but have never been able to find anything. It's true that many people say modern fuel burns faster, but I've yet to find any actual evidence that either supports or refutes that.

Before anyone jumps in with their own anecdotal "evidence" on this subject, is anyone aware of a published study in some reputable journal like that of the SAE?

 

[Martyn Goodwin]

Charles,

There was two things got got me pondering the situation.

Up front - I have a standard Comet, Mk1 Carb, original (rebuilt) magneto, Amal Mk1 30mm concentric carb, single spark plug head, been using Carello 7.2 :1 pistons for years and 92 octane fuel

With the last piston replacement (piston had overheated resulting on the top ring groove opening up to almost twice its width) around 12 months back I decided to fit a decompression plate under the muff. In doing so I had expected a reduction in performance but was prepared to accept that if it meant saving the next piston. To my surprise there was actually a seat of the pants improvement in performance even though by lowering the CR I had reduced the combustion efficiency.

The second thing was that around 2 months back before taking my bike out for a run (there was just a short window when the covid restrictions were eased) I decided to check the points and timing. Well I needed to adjust the points and as a result needed to reset the timing. After doing so I found the bike reluctant to start - it has been a first kick starter for years. Anyway got it started and went on my ride. It was a revelation suddenly I had significantly more power (torque) on the hills and also under acceleration. Hills where I needed to drop down a gear I was sailing over in top. Overall the motor sounded and performed much sweeter. Now this is with the decomp plate still in place.

On returning home I resolved to recheck the timing and was surprised to find I had made a mess of it when I set it prior to my ride. Normally I had been using 4 BTDC at full retard (gives very easy starting) and 34 BTDC at full advance. What I found was I had set the timing at TDC full retard and 30 BDTC full advance.

I reset it - but based on that experience I set 2 BTDC retarded and 32 BTDC advanced. Bit easier to start but it took the 'edge' off performance.

As I said, it was this that got me thinking about timing and what was going on and why.

Anyway I have some old ATD parts that I am now in the process of modifying so that I can have 4 BTDC full retard for starting and 30 BTDC full advance.

Martyn

 

[greg brillus]

Several things to look at there...... so you have the initial advance point (or full retard) the max overall advance and the perhaps the one that is overlooked most, is the rate of change of the advance. It is this rate of change that is the dangerous one.......on a lot of old bikes and cars for that matter, the advance mechanisms are not only worn, but the spring rates that control this are all over the place. I had a late model Velo here that the owner basically cooked the engine........on closer inspection I found the original BTH advance unit was going to full advance way too soon. So even with the advance set to say 4 or 5 degrees BTDC for good starting and a max advance of say 32 to 34 degrees full advance, on a ride around the block, up and down some of my local hills......The engine would ping at anything approaching 1/2 throttle in say 3rd or top gear without even trying.........after some careful rework of the advance unit return spring which was very weak in it's action, this immediately transformed the bike and the detonation near stopped unless you really worked the engine hard on an incline in top gear.......In these instances you are better off to use a slightly higher octane fuel if this is available........ I never recommend using 98 octane fuel here unless the bike is used very regularly as i feel it is more suited to modern fully enclosed fuel systems where the top end volatile properties of the fuel are held........If that fuel is left for 2 to 3 months in an old bike, or car, the fuel quickly becomes reduced to near kerosene, or not much better.

 

[Albervin]

Martyn, here is an article I wrote for the Riley Club magazine. I do intend to have the timing of my Riley checked soon but in the meantime I seem to have found some power and removed the pinking.
After years of taking pot luck with fuels in the Riley I decided to try some additives this week. The engine
never seemed to be always happy with the fuel I fed it. Some tanks delivered good energy and others did not. A bit of a lottery
even though I try to use BP Ultimate or Caltex Vortex 98 in the RMB. Admittedly the engine is probably in a higher state
of tune than original but only by way of 8:1 pistons, K & N air filter and a stainless exhaust system.
I added some NULON Pro Strength Octane Booster at HALF the recommend dose. I was warned by my engine builder that
using full strength could be harmful to valves/guides and always best to be careful. The petrol tank was quite low according to the fuel gauge.
I know, these are as accurate as measuring a bore with a tape measure but on a flat surface with the car not moved for ten minutes the gauge was
just off empty. I added forty litres of Vortex 98 then half a bottle of the Nulon. A trip up Bulli Pass was a bit of an eye opener; the car not only
cruised up in third gear it would accelerate whenever I pressed the accelerator. Much better than previous but the real test came when I
turned into my street. A narrow, tight left turn up a steep incline normally required a gentle throttle or else there was some “pinking” or
pinging but this time the car sailed up and again was open to accelerate. Turning into my drive I have to stop and engage first gear,
again the car just roared up the drive. Impressed was I! Next was to see if the engine “ran on” when I turned it off: NO it did not.
Next day I carried out another test on a one hundred mile route and there was no doubt about it, more torque and much easier
acceleration. Of course there is a cost to all of this and at $23 a bottle that is about an extra $12 a tank. Next tank I may use less and compare the results
but as I don’t cover that many miles a year I am happy to have a bottle on the shelf for when needed.
The other additive I bought is a fuel stabiliser. It supposedly stops our modern excuse for petrol from going stale and is quite cheap compared to the
octane booster, about $2.50 for a 236ml bottle. Ideal for when you don’t use the car very often.
Both of these additives should be used with FRESH petrol so wait until you next fill up before considering using either or both.
I hope to give an update after a couple more tanks.

 

[Martyn Goodwin]

Several things to look at there...... so you have the initial advance point (or full retard) the max overall advance and the perhaps the one that is overlooked most, is the rate of change of the advance. It is this rate of change that is the dangerous one.......on a lot of old bikes and cars for that matter, the advance mechanisms are not only worn, but the spring rates that control this are all over the place. I had a late model Velo here that the owner basically cooked the engine........on closer inspection I found the original BTH advance unit was going to full advance way too soon. So even with the advance set to say 4 or 5 degrees BTDC for good starting and a max advance of say 32 to 34 degrees full advance, on a ride around the block, up and down some of my local hills......The engine would ping at anything approaching 1/2 throttle in say 3rd or top gear without even trying.........after some careful rework of the advance unit return spring which was very weak in it's action, this immediately transformed the bike and the detonation near stopped unless you really worked the engine hard on an incline in top gear.......In these instances you are better off to use a slightly higher octane fuel if this is available........ I never recommend using 98 octane fuel here unless the bike is used very regularly as i feel it is more suited to modern fully enclosed fuel systems where the top end volatile properties of the fuel are held........If that fuel is left for 2 to 3 months in an old bike, or car, the fuel quickly becomes reduced to near kerosene, or not much better.

Hi Greg,

After my last piston replacement and the fitting of th decomp plate I checked out the action of my ATD and found as you suggested that the springs in it were rather weak. I also took a look at the ATD curves available on the brightspark web site that show the impact of weak springs on the rate of advance - the two lines of interest are the Pink - far left - where there are NO SPRINGS and the Orange on the right with two preloaded springs

I obtained a set of new ATD springs and in the hope of flattening the advance curve - that is needing more revs to get it to move to full advance, I modified the 'bob' weights, reducing their mass. this means (in theory) the need for higher revs before full advance is reached.

In this photo the original 'bob' weight is on the left and my lightened version on the right.

So far I , as a result on newly imposed covid restrictions, I have not been able to road test the impact of the change to the 'bob' weights but with the inspection cover removed I have visually confirmed that the magneto still functions correctly, moving to full advance, as I increase revs with the bike on its stand.

With the weights lighter, basic physics dictate that the advance must be reached at higher revs than before.

I am trying to locate a calibrated strobe light/tacho so I can measure the RPM at the point of advance commencing and also at the point where full advance is reached - any ideas?

Martyn

 

[Martyn Goodwin]

Found one! Under $25 on evil bay

 

[Martyn Goodwin]

I'm not saying you have anything wrong, but more than once I've searched for studies of the rate of burn but have never been able to find anything. It's true that many people say modern fuel burns faster, but I've yet to find any actual evidence that either supports or refutes that.

Before anyone jumps in with their own anecdotal "evidence" on this subject, is anyone aware of a published study in some reputable journal like that of the SAE?

Hi Charles,

Found the attached - covers fuel volatility in the 1920's.

Current SAE studies are all about the conflict of reducing volatility in order to reduce emissions and increasing volatility in order to improve 'drivability'.

Come to think of it anyone wanting to perform empirical studies today comparing pool fuel to modern fuel would be thwarted by their inability to source 60 year old pool fuel .

 

[vibrac]

I do know directly that Mr price was talking about reducing the range of his atd by 1 or 2 degree before his demise .
Because of ethanol using a low octain petrol is a no no for me anyway. So I keep to Esso supreme at least it gives a stable base for comparison
Perhaps asking an analog device like the atd to work in a digital world is too much and if you keep to a traditional mag then your choice may be a manual lever at least in most cases that has a sophisticated computer on the end

 

[LoneStar]

Richardson provides advance recommendations for various compression ratios, not specific to singles vs. twins:

7.3 - 38-39
8.0 - 36-37
9.0 - 36
11.0 - 35
12.5 - 34

He also states "With Premier-grade fuels, it may be possible to advance one or two degrees for road work, in the interest of petrol economy."

In this discussion, it seems to be implied that burn speed of the compressed mixture is proportional to the liquid fuel's volatility - is this an accepted principle?