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Comet Timing & BTH Advance Curve

redbloke1956

Well Known and Active Forum Website User
Non-VOC Member
Hi all from Hot and Sunny Queensland (43 c at the moment) I am carrying out a few timing experiments on my Comet, fitted with a BTH Electronic Mag, can anyone tell me where I can find the advance curve for a BTH?
In case I have the wrong terminology: I am looking for a graph that shows changes in Timing Advance against engine RPM.
My other question is, I don't have my new Tacho yet (and can't find my dwell tach meter) so can anyone tell me an approximate value of RPM, at idle, for a well tuned standard Comet, before anyone jumps on my head, I do realise that this is a variable but am only after approximation...i.e. 500 - 700, 600-800, 1200-1400 etc I will get down to fine details once my tacho arrives.

Thanks and regards

Kevin Dennis
 

ogrilp400

Well Known and Active Forum Website User
VOC Member
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.
 

BigEd

Well Known and Active Forum Website User
VOC Member
VOC Forum Website Moderator
....I am carrying out a few timing experiments on my Comet, fitted with a BTH Electronic Mag, can anyone tell me where I can find the advance curve for a BTH? ..........
Thanks and regards
Kevin Dennis

There is an approximate graph for the later twin magnetos electronics at the following link. The earlier mags and most singles had similar advance to the mechanical ATD being fully advanced at around 2000 - 2500 rpm. http://www.bt-h.biz/advance_curve.htm
The simplest way to experiment with ignition timing is to fit the BT-H mag with the studs midway in the slotted fixing holes, set full advance using the timing peg and then use the adjustment in the slots to advance or retard the ignition until you find the sweet spot. If you run out of adjustment just move the mag gear around on the taper in the required direction and run it again.
 
Last edited:

greg brillus

Well Known and Active Forum Website User
VOC Member
Hi there Kev, I think you will find somewhere in the range of 700 to 800 rpm will give you a nice idle on a Comet, the cam can make a difference but my Comet used to idle around that figure with a new Mk 2 cam in it. Any higher and it can make gear selection a bit crunchy, like a bowl of cornflakes in the morning....Didn't see you at the swap meet..! It wasn't a bad one either.....Cheers. Greg.
 

vibrac

Well Known and Active Forum Website User
VOC Member
yes the slots are another great bit about the BT-H (note the dash I have a BTH on the Grey Flash) I set up the comet racer to 34 and swing it to the back of the slot I have no doubt I shall be fiddling with my long allen key wearing ear defenders when we get her on the rolling road. while the intrepid rider watches the computer . He loves to do my Comet he says its such a nice change from the plastic crutch rockets and he is always enthralled by the way she rises up on the girdralics under power "Straining on the leash" he calls it
 

nkt267

Well Known and Active Forum Website User
VOC Member
Let's see if this works..
I downloaded the curve to post here..john
mk2adv.jpg
 

A_HRD

Well Known and Active Forum Website User
VOC Member
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
 

Martyn Goodwin

Well Known and Active Forum Website User
VOC Member
Let's see if this works..
I downloaded the curve to post here..john
View attachment 2366
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.

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.
 
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