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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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Gordon Ryley

Website User
VOC Member
Hi no wonder we have exhaust pipe discolouring even with a drop of Millers Sub
plugs the right colour manuel magneto if the engine sounds right it must be ok ?
Even with expensive recroming waste of money pipes still discolour,so it is petrol.
 

Glenliman

Well Known and Active Website User
VOC Member
These are strange times indeed Bill. People getting about under the speed limit due to lack of practice. Not sure a drastic measure like fitting a 26mm carb to your Comet is the answer though. We will get through this, I hope.
Dave, this one made me chuckle.
We will get through Covid without changing to small carbs.:)
While the rest of the world is suffering with their petty problems, respiratory and otherwise, we are facing possible carb shrinkage and lost performance!
But we will soldier through.

I'm roughly the same as the others with two plugs - 4 /26 on ignition with 11 to ones and big squish area.
Those numbers came courtesy Roy Robertson and Bob Dunn.
Roy has done lots of dyno work with his twin plug racer. He rolled the timing back until he saw power loss and then forward to same.
He then set the ignition as far retarded as possible, just above the point where power loss occurs.
Bob Dunn made the suggestion that advance should be fully in by 2000 rpm , or even lower.
No fancy curve needed or wanted for a Vincent, just a straight steep ramp from 4 degrees to 26.
Andy at Pazon tailor made the ignition that way and its very good. Andy's suggestion was to use two ignition drivers instead of the usual one. That way there is twice the spark energy of his usual twin plug setup. It also draws twice the current so a good charging system is needed.
Some say a strong or weak spark, single or twin spark makes no difference but I feel that two hot sparks are preferable to one weak spark, or even one hot spark. Especially for kick starting an 11 to 1cr 1360cc engine.
If one manages to roll it through compression it's nice to have a couple of big fat sparks cardiovert the thing to life.
Otherwise I'm the one requiring cardioversion!


Glen
 
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Nigel Spaxman

Well Known and Active Website User
VOC Member
Dave, this one made me chuckle.
We will get through Covid without changing to small carbs.:)
While the rest of the world is suffering with their petty problems, respiratory and otherwise, we are facing possible carb shrinkage and lost performance!
But we will soldier through.

I'm roughly the same as the others with two plugs - 4 /26 on ignition with 11 to ones and big squish area.
Those numbers came courtesy Roy Robertson and Bob Dunn.
Roy has done lots of dyno work with his twin plug racer. He rolled the timing back until he saw power loss and then forward to same.
He then set the ignition as far retarded as possible, just above the point where power loss occurs.
Bob Dunn made the suggestion that advance should be fully in by 2000 rpm , or even lower.
No fancy curve needed or wanted for a Vincent, just a straight steep ramp from 4 degrees to 26.
Andy at Pazon tailor made the ignition that way and its very good. Andy's suggestion was to use two ignition drivers instead of the usual one. That way there is twice the spark energy of his usual twin plug setup. It also draws twice the current so a good charging system is needed.
Some say a strong or weak spark, single or twin spark makes no difference but I feel that two hot sparks are preferable to one weak spark, or even one hot spark. Especially for kick starting an 11 to 1cr 1360cc engine.
If one manages to roll it through compression it's nice to have a couple of big fat sparks cardiovert the thing to life.
Otherwise I'm the one requiring cardioversion!


Glen
Glen it is interesting that Bob Dunn rolled back the timing until he saw power loss and set it just ahead of that. I think the old way would have been to advance it as much as possible. I wonder how far he would have had to advance it before he would have seen power loss again? I think that with many of these bikes they spend most of their time cruising along with the throttles barely cracked open. I think when the engine is running that way, with barely any mixture in the combustion chambers it might be that quite a bit more ignition advance would be required to get a good burn and good economy. That might be the reason why the vacuum advance can help with fuel consumption. I am just guessing though. I have a vacuum operated switch on my ignition and I experimented with having it shorted to ground all the time and also having it open all the the time. With the ignition curve I had that was varing the full advance timing from 34 to 38 degrees. My seat of the pants impression was it didn't make any difference at all and the fuel consumption was pretty much the same as well. I don't think a Vincent with 8:1 compression is all that fussy about the timing really. Usually I get at least 50 MPG riding on short trips. I get 60 MPG on most longer journeys but I have had as high as 70MPG on long downhill runs.
 

MartynG

Well Known and Active Website User
VOC Member
Here is a photo or 2 taken years back - piston came out of my Comet. 98 octane fuel, 30mm AMAL Mk1 concentric, Mk1 cam, 7.2:1 piston and 36 degrees BTDC full advance - top ring 'popped up' after around 130 miles of highway running in the 70 to 75 mph range. Made it home on just the oil ring! Bit sedate on the hills!

Helps explain my fixation on getting the ignition timing right for reliability.
 

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Bill Thomas

Well Known and Active Website User
VOC Member
Anybody else had trouble with those pistons, Can't say I have heard of them,
I would go 106 needle jet, Can't think what the heat is like in AUS,
They say you should roll the throttle off every now and again, To get oil up on the piston etc,
I am worried about the piston I have for my next build, May have to dig out an OLD Specialloid.
 

van drenth

Well Known and Active Website User
VOC Member
Got the same problem, noted at rebuild that the holes in de cilinder muff where to "small"for
the cilinder studs,
Drilled them bigger , AND problem solved.
Muff could expand better when really hot.
Jan van Drenth
 

MartynG

Well Known and Active Website User
VOC Member
Got the same problem, noted at rebuild that the holes in de cilinder muff where to "small"for
the cilinder studs,
Drilled them bigger , AND problem solved.
Muff could expand better when really hot.
Jan van Drenth
Thanks for the tip Jan. Next time I have the muff off I will check carefully.

I wonder if any one else has had this issue?

Martyn
 

oexing

Well Known and Active Website User
VOC Member
Hi MartynG,
looking at your photos I wouldn´t blame the fuel or ignition or some such matter for doing that sort of disaster. My guess would be a substandard piston ring material that broke at higher revs and eating into the grooves. Your engine does not seem to be that highly stressed from installed components to have this sort of effects I´d think.

Vic
 

greg brillus

Well Known and Active Website User
VOC Member
The piston shown is a classic case of detonation........And very much yes a combination of fuel and ignition. I'm surprised that you cannot hear the engine pinking away.......I've ridden plenty of Comets and twins that are pinking and it is easy to hear........it is most likely a case of too lean a fuel mixture for the throttle position held and too much advance for that given engine speed.......Nothing wrong with the piston or rings........Just the set up.........Also running a Comet at 75 MPH continuously is probably asking a bit much of it........If you are going to do this, you need to know that the tune is correct, just like i am doing at present on the racer........I don't have any spare pistons, so I can't take any chances. if you just guess at what you are doing, then you are taking a risk.
 

oexing

Well Known and Active Website User
VOC Member
Hmmm, at 7.2 c.r. and 98 fuel detonation ?? I can´t see any traces of detonation or melting on the piston top, just pretty black so it was well below critical temperatures I´d think. And lean on fuel - does not look like it but maybe an effect of limping home . 75 mph too much for a Comet - really ? Our 1935 Guzzi, all cast iron, does 70 all day long, on motorways if it had to. So my guess, there were several bits of piston ring broken which wrecked the top groove ??

Vic
 

Nigel Spaxman

Well Known and Active Website User
VOC Member
Those look to be really nicely made pistons. It does look like it was running rich and I wouldn't have thought running rich would ever cause any harm to pistons. I have had some very well used Triumph 650 hepolite pistons do that. One time I had a piston ruined because one of my cylinder liners moved down. It was on a Triumph with an aluminum big bore kit. The kit wasn't very well engineered, the problem was the liners didn't have much of a lip a the top and the aluminum used for the casting seemed pretty soft. The head gasket pushed the liner down slightly. This caused it to pinch in which pushed the rings down ruining the piston. I fixed it by making liners with a much larger lip. Vincents don't have that problem. Make sure your bore is straight and there is no lip at the top. Make sure also that your liners are not to tight a fit where they fit into the recess in the head.
 

greg brillus

Well Known and Active Website User
VOC Member
What kind of normal loading could possibly go through a piston and it's rings to cause them to break up........I cannot imagine a manufacture with the experience an knowledge of CP or any other top quality piston having that kind of issue.......Not all symptoms of detonation melt holes though piston crowns, the shock load from the flame fronts hitting each other is what has overloaded and caused the rings to break........In many cases the actual ring lands break off. This is also a symptom of detonation at lower engine speed and loading thus causing eventual damage rather than something more catastrophic like a hole melted in the piston crown from a lean condition whilst the throttle is held wide open.
 

Gerry Clarke

Active Website User
VOC Member
In relation to the cylinder stud holes issue mentioned above.

I had cause to rebore two years ago. In the process I ordered a new muff and found on reassembly that I couldn't seat the cylinder head onto the spigot - the head just wouldn't travel all the way down the cylinder studs. Rather than the holes being too small, it seemed to me that the studs were being drawn slightly together and the pitch between the stud holes in the cylinder muff was .020" less than on my old, believed original, cylinder muff. I then did what I should have done in the first place, which was to have my old cylinder bored out. As expected, the head went on perfectly well then and we were back on the road.

The Spares Co. were very good about this and refunded me, but said they couldn't see any issue with the cylinder holes on whatever slave system they used to check. I was surprised that it only seemed to happen to me.

Gerry
 

MartynG

Well Known and Active Website User
VOC Member
It seems to me that the piston crown , especially in the area near the exhaust valve, became overheated and 'plastic' allowing the ring groves to distort and eventually for the end of the top ring to break through into the combustion space right under the exhaust. At the time my ignition at full advance was 36 BTDC.

The replacement setup, same make of piston, new muff n liner with 0.0045" piston/bore clearance was rebuilt but this time with ignition at full advance 34 BTDC. After around 8,000 miles noticed compression was a tad low so pulled the barrel only to find the same thing was happening - distortion of the ring groves - but not as severe as earlier.

Again new piston, this time 0.020" oversize and rebore with 0.005" clearance and fitted a decompression plate under the muff. Still with 34 BTDC - result was a surprise seat-of-the-pants slight but noticeable improvement in overall performance.

It was only recently that as I explained in post #3 that I have now changed the ignition to give full advance at 30 BTDC, full retard at 4 BTDC. Went for a short run yesterday (COVID rules mean I cannot travel more than 5 km from my home) and was very pleased with the performance, definitely improved torque on the hills.

I am running the original magneto. I stripped the ATD down, added metal to the fixed ears then filed the added metal back to give the advance range I wanted being 13 at the magneto that yields 26 at the crank. The hardest bit of all is setting the timing as the ATD tends to move as you tighten the retaining bolt. Patience and swearing helps!

Martyn
 

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