Comet Timing & BTH Advance Curve

[redbloke1956]

Let's see if this works..
I downloaded the curve to post here..john
View attachment 2366

Thanks John, oddly enough the graph doesn't show up on my computer but it does on my ipad?
That is what sh#ts me about computers as opposed to bikes........on a computer I can't clean/tighten/grease/adjust/loosen/drop a nut into an inaccessible spot/remove a shim/replace a shim/add some Loctite/add some Silicone to fix something

Thanks again and regards
Kevin

 

[redbloke1956]

First; you need to read the graph "backwards". At idle of 400 rpm its about 38 degrees retarded, and at 2,300 rpm is around 7 degrees retarded, so that means at 2,300 rpm the ignition is firing around 31 degrees BTDC, not 7! Once the revs reach 3,250 then the motor is firing at 38 BTDC, assuming the graph is correct.
Hi Martyn, On the BTH site, I cannot get the curve but this is mentioned directly above the invisible diagram "Below is a typical automatic electronic ignition advance curve supplied with your BT-H magneto." I am not sure if the diagram is therefore a Generic curve picked up from somewhere or the actual BTH Curve?

Does that mean that the effective ATD/ARD range on the BTH is around 38 degrees? And the data on the Lucas unit suggests its range is 35 degrees.

The Lucas info looks about spot on when one looks at the ignition settings suggested in the Riders handbook.

BUT I wonder about the BTH range of 38 degrees. Generally, if you set the fully retarded point to be after TDC then starting will be difficult. and my experience is if you set the fully advanced to 38 degrees or more you face a real risk of detonation which causes piston and bearing damage. I'm not saying that the BTH will do this - all I am questioning is the accuracy of the curve on the BTH web site.

IF The range is 38degrees and I have set my timing at 33 degrees BTDC and that would mean I am starting at 5 degrees ATDC , yet the Comet starts first kick every single time?

 

[Martyn Goodwin]

IF The range is 38degrees and I have set my timing at 33 degrees BTDC and that would mean I am starting at 5 degrees ATDC , yet the Comet starts first kick every single time?

As I said in my earlier post - I question the accuracy of the advance/retard graph on the BTH web site.

 

[redbloke1956]

Blimey. I would prefer the ATD curve personally. But a combination of the two would be better. I'd like to think that at 50mph/2,300rpm on my Twin the timing was pretty much at full advance - not 7 degrees retarded. Motorway cruising at 70mph, and not yet fully advanced, would worry me.

Peter B
Bristol, UK.
WANTED: Series 'A' Front Frame

Hi Peter I just looked at about 20 advance Curves and they were predominantly showing full advance at an average of 3000rpm, the attached graph is from a Smart-Fire Vincent V-Twin 12 Volt :: Smart-Fire Ignitions (pazon) which shows full advance at 2000rpm? Yep I am still learnin'

Kevin

 

[A_HRD]

Hi Peter I just looked at about 20 advance Curves and they were predominantly showing full advance at an average of 3000rpm, the attached graph is from a Smart-Fire Vincent V-Twin 12 Volt :: Smart-Fire Ignitions (pazon) which shows full advance at 2000rpm? Yep I am still learnin'

Kevin

Ah, that's much better Kevin, I like this Pazon one! 5 to 6 degrees of advance at Kickstarter speeds, 20 at and just above tick over, then a fast rise to max advance at 2000 engine rpm. But the full range is still too wide for a street Vincent on today's excuse for petrol. As had been said before, around 28 to 30 degrees full range is enough….

Peter B
Bristol, UK.
WANTED: Series 'A' Front Frame

 

[A_HRD]

First; you need to read the graph "backwards". At idle of 400 rpm its about 38 degrees retarded, and at 2,300 rpm is around 7 degrees retarded, so that means at 2,300 rpm the ignition is firing around 31 degrees BTDC, not 7!

Martyn,
Yes I realise this. What I meant was that at 2,300rpm the timing is still 7 degrees off from full advance. If, as you say, the graph is truly representative…… I think I'll stick to ATDs and Magnetos. They have served me well for the last 45 years of Vincenteering.
Peter B
Bristol, UK.
WANTED: Series 'A' Front Frame

 

[redbloke1956]

G'day Red,
I think you are getting too technical for the 1950s. The the ATD should really be call an automatic retarding device. I once carried out some tests to ascertain what was happening with the ARD. All it really does is retard the ignition for starting and idle and goes to full advance at about 12-1500 RPM if I recall. Nothing like the extended advance curves of todays engines.

Phelps.

Thanks Ken, I am playing with/designing a device that I hope will quickly and simply allow me to check my timing (accurately) with or without a timing light at the side of the road, My new VOC spares Electronic Tacho just arrived Wooo Hoooo!!

Red

 

[Martyn Goodwin]

This is getting confusing. Here is the ignition curve on the Pazon system I purchased for my Comet. And I have checked with Pazon and they have confirmed that it is what the system they supplied to me should actually be doing.

Now the rub is when I installed the Pazon late last week , in place of the rebuilt magneto I experienced a "seat of the pants" reduction in bike performance. So I tweaked the distributor position , moving all points towards further advance, yielding a start at 6 BTDC and full advance at 32 BTDC. But I must say that even with that change, I thought the magneto gave superior performance. So the original Lucas magneto is back in the bike with the ATD modified so I have 4 BTDC at zero engine speed and 32 BTDC fully advanced. FWIW both the Pazon and the magneto give first kick starting.

M

 

[BigEd]

IF The range is 38degrees and I have set my timing at 33 degrees BTDC and that would mean I am starting at 5 degrees ATDC , yet the Comet starts first kick every single time?

A few points that may be taken as my observations or views but not necessarily absolute fact.

• It is quite difficult (not impossible) to get accurate dynamic (i.e. engine actually running) ignition timing figures with a stroboscope on a Vincent.
• Whatever the theoretical timing figures it is what happens when the engine is running that matters.
  
• The graph posted is "an approximate graph for the later BT-H twin magnetos electronics".
• The later BT-H twin magnetos electronics were "tweaked" a little so that it started to spark at a slightly lower rpm. A side effect was that this slightly extended the effective advance range.
  
• The theoretical range of say, an electronically controlled advance curve may not be the same as the actual range. The actual advance range cannot start until the magneto until the magneto is turning fast enough to generate sufficient energy to initiate a spark. (A battery/coil/ and points system doesn't have a starting to spark threshold.) Most electronic systems will stop advancing at a fixed rpm and some, at high revs, may reduce advance by a degree or two.
  
• The lines on a graph may be extrapolated at the ends so are not really relevant because of what was said in the previous point.
• The ignition timing should be set at full advance position rather than an assumed fully retarded figure. If the actual advance range is greater than the theoretical you could end up with too much advance.
• An example of this applied to a mechanical advance retard mechanism might be a worn unit or one that has had the "arms" adjusted to limit/extend the travel of the bob-weights. (I think this is one of the things that MartynG found when doggedly sorting problems on his Comet.)
• My Rapide has the later BT-H electronic advance curve. The ignition is set, as accurately as I can measure, to full advance around 34° before TDC. It almost invariably (touch wood - fingers crossed) starts first or second kick and has performed well with this setting for 25-30,000 miles. We often look to theory and printed figures when making our initial settings From this point we can adjust to find what works for our own machine.

Discuss. That should keep this thread going for a while longer.

 

[redbloke1956]

Kevin
Just had another response from BTH :
Hi Kevin
by all means share it, yes the twin does have a slightly longer curve, (it goes up to 4000 revs )if you cant get it from website let me know and i will scan it and send it on to you
Regards
peter