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Comet Timing

Hugo Myatt

Well Known and Active Forum Website User
VOC Member
My Comet dynamo has packed up. I removed the timing cover intending to fit an Alton that I have had on the shelf. I discovered two of the ATD stops (ears) lying at the bottom of the timing chest and the remains of one spring. The other spring and pair of stops are still in place. Fortunately all the timing gears are steel and there is no apparent damage. I am amazed that the ATD continued to function in this state but the bike has shown no symptoms. It was starting easily and running well. I will re-time it with a replacement ATD. My question is what do I time it to at full advance? The engine is in standard spec so do I go by the book? I understand modern fuels require less advance but how much less? If I time it at say 35 degrees before TDC on full advance will the throw of the ATD put the fully retarded position way out?

Advice greatly appreciated.

Hugo
 

Ken Tidswell

Well Known and Active Forum Website User
VOC Member
As long as the spark occurs at 4-5degBTC.retarded You will be OK.It needs this to allow for the flame propagation down the bore , so the thing will start easily.Time it at 35 BTC and check the retarded position . The ATD gives about 17deg on the stops at the mag end, Ken
 

Hugo Myatt

Well Known and Active Forum Website User
VOC Member
Thanks Guys. I will check the retarded position. That sounds a bit like me at the moment. I seem to forget things that I found simple 50 years ago. This was a rebuilt engine too. When I took the ATD off there was more carnage than I realised. The pinion was loose on the rivets. I rummaged in my spares boxes and found 4 ATDs. They were probably all rejected at different times. Two had loose rivets. One didn't tighten on the taper but the fourth one was usable if worn. I should like a new one but the Spares Co. have none available at present.

Hugo
 

riptragle1953

Well Known and Active Forum Website User
Non-VOC Member
You will find some A.T.D.'s have more or less total travel than others.... but as long as your full retard is somewhere around T.D.C. you'll start fine.... actually, some small advance at starting doesn't hurt a bit. Personally, with a 7.5 to one piston
I'd run 38 degrees full advance.
 

nkt267

Well Known and Active Forum Website User
VOC Member
If your retarded timing is past TDC it will be a pig to start,trust me I know:mad::mad:..
I have coil ignition on my Comet and although I returned it for checking as I thought the advance was wrong it was a pig to start.
I was running Mk2 cams and standard compression and for some reason had set the timing to 42 deg BTDC fully advanced and all was fine. Unfortunately I listened to some 'good advice' and it became a right beggar to get going. I now set my timing at 4 deg BTDC retarded and all is fine..John
 

riptragle1953

Well Known and Active Forum Website User
Non-VOC Member
One has to think of their own Vincent as an individual.... there are so many variables due to the age of these machines and what's been modified, worn, or set however that I doubt there are any positive given settings that would
suit all, thus someone like yourself has to tinker a bit to discover what your individual Vincent likes best. I used to say that long ago.... give the machine what it wants. Even the spark plug you use can make a difference. But, the
main thing I guess is to have fun, tinker and ride. Good luck buddy!
 

davidd

Well Known and Active Forum Website User
VOC Member
The engine is in standard spec so do I go by the book? I understand modern fuels require less advance but how much less?

Advice greatly appreciated.

Hugo

Hugo,

I think the most direct answer is: if your engine is stock go by the book. The Factory most likely came to the listed timing figures by testing the engines on their dyno.

As to the second question, in the best of all possible worlds, one would test on the dyno to see what happens to the power and torque when the timing is altered. As Rip mentions, there are lots of machine variable that could affect the outcome. If you ask what does the timing number represents, it will help you make decisions about where you should time your bike. The ideal timing would occur near TDC. The spark would occur and instantaneously push the piston down ATD. This assumes that combustion is started and finished in a degree or two of crank rotation. That would be an incredibly efficient burn. If the Factory reccommended 40 degrees at full advance then they are saying the combustion chamber is quite inefficient. If you fire at 40 you are effectively lowing the compression ratio from 7 to 6 because you only get the pressure from the 7 to one piston at TDC. This lowers the combustion pressure and combustion pressure is power.

Additionally, the longer the combustion, namely the more advanced, the hotter your engine will run because the flame is hanging around so long. More heat means less power. This also happens when you have a high compression piston with a high dome and a deep combustion chamber. This set up could double the surface area exposed to the flame.

If modern fuel burns faster then you can inch your timing closer to TDC. I do not know if that is the case, but as you say, others are suggesting it. The use of dual plugs tends to speed up combustion by creating two flame fronts instead of one, thus, two plug ignition timings tend to be closer to TDC. When dyno timing I move the mag 2 degrees at a time. With dual plugs, I ended up a 19 degrees fully advanced. What I think this told me was that some of the combustion chamber mods worked and I made the burn much quicker.

The reason I mention the above is that Comets need all the power they can muster. It is worth trading some heat and compression loss for BHP, so as long as your jetting is good and the starting procedure still works, I would not hesitate to make a small change toward TDC.

David
 

riptragle1953

Well Known and Active Forum Website User
Non-VOC Member
As I was saying above generally there isn't really a "stock" Vincent so you have to experiment with the Vincent you have to find what it likes best. For instance.... how tight are your lines to your muffs? Could it be that your combustion chamber
runs hotter that others.... this will effect where the engine is best timed. So many variables to deal with.
Read this..... it's of interest to engine owners: http://www.contactmagazine.com/Issue54/EngineBasics.html
 
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