E: Engine Ignition Advance

Gordon Ryley

Well Known and Active Forum 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.
 

Monkeypants

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Non-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 Forum 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.
 

Martyn Goodwin

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Non-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

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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.
 
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