Ignition Timing & modern Fuel

[Martyn Goodwin]

I have been receiving conflicting advise recently in regard to the appropriate ignition timing for my Comet when using modern fuel - 91 RON. This was after an incident with a brocken top ring in my motor.

I had been setting it at 36 BTDC fully advanced but have been told that this may cause pre-detonation and possible destruction of the top ring leading to more dire consequences. It was suggested that it should be no more than 26 BTDC, fully advanced.

I am bemused.

What should the ignition be set to, fully advanced, on a comet with Mk1 cams, a 7.5 to 1 piston and using regular pump 91 RON unleaded fuel?

Thanks

Martyn

UPDATE December 7, 2013: with 36 BTDC with 91 RON fuel I was experiencing detonation that DID lead to ring/piston failure.

CURRENT RECOMMENDATION: Jan 14, 2014

Following a rebuild I have settled on 98 RON fuel (sans Ethanol) and have set the full advance to 31 BTDC which gives (in my case with "tweaked" ATD ears) 4 BTDC fully retarded. Ran over 1000 miles with this combination with no trace of detonation. FWIW the Carb is a 30mm Amal Mk1 Premier, 105 needle jet, needle in centre groove and a 220 main

M

 

[Howard]

There's lots of threads and even more opinions on here, have a root around. I think your 36 is closest, but I seem to remember twin plug ignitions are working at much smaller advance.

H

 

[A_HRD]

Martyn,

Everyone will have a different opinion!

Here is mine:
For a "standard" 500 Comet a max of 34.
For a 636cc bored/stroked Comet a good starting-point would be 26.
Both assume street-bikes with a single spark-plug.

Peter B
Bristol, UK.

 

[greg brillus]

Hi Marrtyn, I pretty much set all the Vins at 34 deg full advance with the retard at no less than 4 deg before TDC I generally run my Rapide on 98 premium fuel and 34 degrees full adv with the new type BT-H magneto this with stock low compression ( a bit like a model "A" Ford) with 2 up riding and up an incline on a warm day it will still ping at anything more than half throttle.....I would set yours at no more than 32 Deg full advance running on 91 fuel, and try it, you may need to bend the ears on the atd to make sure your retarded possition is no less than 4 BTDC otherwise your exhaust will get too hot at idle and the engine may seem a bit dull and hard to start. Generally if you have a keen ear you can hear an engine detonating, much like 2 hammer heads hitting one another.....this condition pretty much quadruples the cylinder pressures and temps as well, this is why the piston crown and rings/ lands are destroyed quite rapidly. In an engine that is supercharged or turbocharged running moderate to high levels of boost, detonation destroys pistons and combustion chambers almost instantly.........Cheers and good luck.......Greg.

 

[Monkeypants]

Since every engine is different, why not power time it?
Start with some known average setting, say Greg &Peter's number above. Find a big, high speed hill ( might have to search a bit in the UK) and approach it at a steady speed in fourth, say 70 mph. Lay into the throttle as you climb the hill and watch the speed. If it pings I would abandon the exercise and reset the timing with a couple of degrees less advance. If it does not ping, take note of the speed attained or held. Reset the timing a couple of degrees back and try again. Keep retarding the timing until power drops a little and speed is lost, perhaps only a couple of mph. Now advance the timing back to the point where the full amount of power is restored. You now have max power and the minimum amount of advance, ideal to prevent ping.

For cars and motorcycles I have always set ignition timing this way, and it was interesting to learn from Roy Robertson that he does the same with his Egli Racer. The only difference is he uses a dyno, I use the hill and the speedometer as a dyno.
We both are retarding the timing as far as possible without power loss.
Glen

 

[Mark Fisher]

Since every engine is different, why not power time it?
Start with some known average setting, say Greg &Peter's number above. Find a big, high speed hill ( might have to search a bit in the UK) and approach it at a steady speed in fourth, say 70 mph. Lay into the throttle as you climb the hill and watch the speed. If it pings I would abandon the exercise and reset the timing with a couple of degrees less advance. If it does not ping, take note of the speed attained or held. Reset the timing a couple of degrees back and try again. Keep retarding the timing until power drops a little and speed is lost, perhaps only a couple of mph. Now advance the timing back to the point where the full amount of power is restored. You now have max power and the minimum amount of advance, ideal to prevent ping.

For cars and motorcycles I have always set ignition timing this way, and it was interesting to learn from Roy Robertson that he does the same with his Egli Racer. The only difference is he uses a dyno, I use the hill and the speedometer as a dyno.
We both are retarding the timing as far as possible without power loss.
Glen

I have to admit that I set the ignition on my egli the same way (twin plug heads) as there isn't a book for a bike with terry prince heads and an unknown compression ratio! It runs a Kirby Rowbottom ignition (basically a boyer) so it is easy to adjust. I originally set it in the low 20's and just kept advancing it in small increments until I got it to ping under load, then went back by one increment (aprox 2 degrees) and it has had that setting ever since. The only problem is that I don't know what that number actually is! If at sometime I have to disturb the ignition system I'll have to strobe it first so I know where I'm at. I know all this sounds very unscientific but it works and the bike starts and runs perfectly, it gets used 2 or 3 times a week so it's no trailer queen....could do with a bit more padding in the seat though! mark

 

[Monkeypants]

Hi Mark
At one time I did the same as you, advance to ping then back off a bit. The thinking was get as much advance as possible without ping ,that is where the most power is. This method worked fine .
I now go about it a bit differently , instead it seems to work well to retard the engine as far as possible without power loss. This might only be a couple of degrees retarded from the "just before ping"setting, but it gives a little cushion for safety in the event that some low octane fuel is encountered.
BTW, when doing extensive dyno testing, Roy Robertson found his Egli race bike (modified Prince heads, high compression, two plug ignition) made identical power anywhere in a range between 28 degrees advance and just 24 degrees advance. Above twenty eight degrees power dropped off , as it did below 24 degrees.. With no advantage in running it at the advanced end of the range, for engine preservation he chose the setting with the least amount of advance, 24 degrees.

Glen

 

[nkt267]

Find a big, high speed hill ( might have to search a bit in the UK) and approach it at a steady speed in fourth, say 70 mph. Lay into the throttle as you climb the hill and watch the speed. If it pings I would abandon the exercise and reset the timing with a couple of degrees less advance.

70 Mph uphill on a Comet???.The best I could get uphill out of Waterworks Corner was 60 Mph.And my Comet was a little under geared then,it would only do 85 flat out 2 up on a level road.John

 

[Monkeypants]

Sorry, 70 was for the Rapide. 55 or 60 might be a better target for a Comet....although I have heard it said that a 600 Comet with Prince squish head, good cams and high compression will give the average twin a good run!

Glen

 

[Albervin]

Martyn, before fiddling with the timing why don't you run a tankful of BP Ultimate or Caltex Vortex 98? Most 91 RON petrol in Australia contains a minimum of 10% ethanol which is a waste of both petrol and ethanol. Shell 98 is also OK as long as you don't let it go "off". It takes less than a month for it to go off.