E: Engine Comet Ignition Timing 2018

davidd

Well Known and Active Forum User
VOC Member
The thread about Comets running hot caused me to wonder if the recommended timing might be too advanced. I would note that Richardson says the Flash should be timed at 41 degrees, while the Comet is 38 degrees. Both of these figures seem to be advanced. I would note that 3407 was timed at 38 degrees, not the higher figure. I don't know how they came to this number, but if it were done by noting the maximum torque that would have been more advanced than what a modern tuner would have selected. Most modern tuners will not advance ignition to get additional power. They are prone to saying that just because you can advance an engine and get more power that does not mean you should.

Recently, David Tompkins adjusted the timing on his Comet on the dyno. This was a reasonably stock machine, 32 mm Concentric, 9:1 piston, stock cylinder, stock head and Mk2 cam. The timing came out to 31 degrees. I don't think anyone else has done dyno work to set the timing, so I think that is a good number.

This does not mean that 34 degrees is not fine, but it may indicate that the Comet does generate some heat from advanced timing.

David
 

greg brillus

Well Known and Active Forum User
VOC Member
There are infinite variables to this though. The two main issues with a dyno, is the limited time you are brave enough to hold it on full throttle, which does not simulate road or track conditions, and second is the cost. Some on this forum have concluded that is is fine to run at or near the factory settings at full advance. That may be so, but not from my experience (although I do live in a hot climate) Perhaps we need to separate the topic into either full advance/retard amounts verses the actual advance "Rate" and the actual advance curve itself for a given ignition system. It seems well enough known now that a lot of aftermarket ignition systems do not have adjustable advance curves, and/or some are fine for single plug whereas most are really not suitable at all for twin plug heads................The ignition set ups for twin plug engines is very sketchy at best, and I feel there are a lot of bikes using twin plug heads, and not necessarily Vincent's where the engines ignition is far from correct and if pushed hard the engine will suffer one way or another.
 

Robert Watson

Well Known and Active Forum User
VOC Member
My Take on timing is that
1) The ATD is a pretty primitive device and I think the factory recommendations were a compromise to deal with what they had.
2) Many modern devices have a fixed advance curve that is a "best guess" at what the engine needs.

I have been running a 2 stage Harley setup for some 20+ years. I set it when I first installed it by an initial advance of about 34 degrees although really can't recall how I did that. I then took it for a ride. Tried to make it detonate and couldn't
Advanced it a little on the adjustable trigger plate. Still no issues.
Repeat
Repeat - Ahh funny noises when lugged hard in higher gears.
Turn back a little.

leave alone for more than 20 years...... Change the plugs once a decade, if I remember.

When it was installed I think we surmised that at higher RPM it was reaching as much as 50 deg advance... with not too much throttle, and when the throttle was wicked open it switches to a more retarded curve.

I suppose it would be more useful if I put my Herve Hamon timing disk on and actually checked to see where it all was.

As for the Flash calling for 42 I wonder if that was an adjustment in the compromise knowing that it would be revving higher generally than a street Comet at 38??

How's that for comments providing no real world use to anyone!
 

greg brillus

Well Known and Active Forum User
VOC Member
It has been a while since I played with mag's and genny's on a test bench, but it is possible with The Flash's and indeed Lightnings, that their manual advance magneto's might have had more range of advance travel from retard to full advance than a more commonly used ATD.
 

vibrac

Well Known and Active Forum User
VOC Member
I run my Comet at 34 deg but I also have a Tony Cooper ATD that has reduced advance travel and Full retard is about 4 deg.
I think a reduction from 17 degrees on his ATD's had been started by Roy Price before he passed away, so its worth checking
Using 34 degrees on a modern BTH on a comet ( I had to smile at what is "reasonably stock" in USA David:)) should give about 4 degrees at full
retard shouldn't it?
the reason i ask because my recently fitted BTH pinched off the grounded racer is proving a bas**rd to start
 

Bill Thomas

Well Known and Active Forum User
VOC Member
Is it the decompressor, Ron can't get on with it. I will go down there some time and have a go, He still has the valve lifter , So no problems.
But as I have said, If you just roll it back on compression in 1 st, You don't need anything.
Funny thing is, I think all the workings are in the T/case, On mine, Just need a cable !!.
Cheers Bill.
 

Robert Watson

Well Known and Active Forum User
VOC Member
I think a reduction from 17 degrees on his ATD's had been started by Roy Price before he passed away, so its worth checking
Using 34 degrees on a modern BTH on a comet ( I had to smile at what is "reasonably stock" in USA David:)) should give about 4 degrees at full
retard shouldn't it?
the reason i ask because my recently fitted BTH pinched off the grounded racer is proving a bas**rd to start

What plugs are you running with the BTH. I had B7 series in the Black B and it had the same parentage as you are experiencing. Swapped them out for 6's and it became a one kick start almost every time.. Had the exact same issue on Tony's 636 Comet. The BTH does not like cold plugs
 

BigEd

Well Known and Active Forum User
VOC Member
VOC Forum Moderator
What plugs are you running with the BTH. I had B7 series in the Black B and it had the same parentage as you are experiencing. Swapped them out for 6's and it became a one kick start almost every time.. Had the exact same issue on Tony's 636 Comet. The BTH does not like cold plugs
Not disputing what you have found but my"B" Rapide with a new type BT-H starts and runs fine with B7's. These old ladies can be fickle.;)
 
Top