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600cc Comet Carbie?


BigEd

Well Known and Active Website User
VOC Member
VOC Forum Moderator
I have found a couple of instances where starting became hard with the new BTH mags, the one I struck was on a customers Black Shadow that has always been a good starter. Though I do know he keeps the bike in a shed where moisture is not uncommon. When he brought the bike to me last, he complained of hard starting which surprised me a little, so I gave it a go, and true enough it would not start. After a small amount of brain scratching, I decided to remove the end cover off the BTH (the instructions tell you not to do this, but that never stopped me before) anyway what I found was the two pole pieces which form the primary part of what is basically like a CDI unit where lightly covered in rust .......!!!....Out with the Dremel and a small wire brush, I polished the ends of the pole pieces, wiped them with a clean rag, and refitted the end cover. The bike started first kick, and again, and again, no problem. So the slight build up caused enough resistance to create a descent spark at kicking speed.................. Now there's a lesson for you all............;)
The instructions say this because some people can't leave well alone. You looked and did some gentle cleaning, all that was required in your case. People who venture further will start taking bits off which is when they are likely to get more problems. A simple instance being that if they loosen or take off the rotor arm it is unlikely that they get it back on in the correct position. This can cause the angle between the two firing points to change amongst other things.
 

greg brillus

Well Known and Active Website User
VOC Member
Most all people with bikes and anything electrical scares them a bit especially magnetos.........But I spent several months working with probably the best magneto/generator repairman here in Australia. The previous one of these BTH mags I had seen with the same problem, was a member down in Victoria with a very nice rapide who rides rain hail or shine, who had removed the cover off his and emailed me some pictures, the pole pieces and the arm/shaft were very rusty, more than just a light coat too........I don't know what his outcome was as he had been in several discussions with the original owner/manufacturer of these mags. I contacted the chap from BTH and explained the issue, and I suggested that perhaps better sealing of the units might help, but that was some time ago now. I think it is quite safe to carry out what I described, so long as you don't disturb anything else. I have never had a bad word to say about these mags and I have installed quite a few of them, with good success, but occasionally something like this can crop up.
 

chankly bore

Well Known and Active Website User
Non-VOC Member
Thanks Dave. Yes you helped me a lot with your ignition timing advice for a twin spark head. To this day it's the biggest power gain I've had, and the cheapest.

My plugs are always wet when I pull them out when it won't start. It seems to me that I aren't able to kick it over fast enough. I have to give it one almighty swift kick with everything I've got behind it if I have any hope of starting it. I have seen seasoned pros start my bike easier than I do so I do think it's a lot to do with my technique, or lack of. I'm getting better, but my bike seems crowd shy.

When I used to run a 48 rear sprocket I found 1st gear a bit pointless and the bike was very vibey? I could never pull top with 46, 4400rpm was about it.

I am running Terry's mk ll cams. I still have my standard ones somewhere, I don't know what profile they are, nothing too sporty I suspect because my dad was a plodder.

Back to the shed.

Thanks again David
A thought. Have you checked that the choke plunger on the Dellorto is bottoming and sealing correctly?
 

Dave Hulstone

Website User
VOC Member
Crikey, was all that 3yrs ago? I ended up fitting a Grosset ignition designed for a twin spark, set at 24/2 and an Alton electric start. I Still have the Dellorto 36mm fitted. I have recently put back on the original, loud, straight through silencer from Armors? while my VOC spiral one is being welded and have had to increase my main jet size from a 145 to a 160 and my pumper jet up a size to keep it happy. It is noticably quicker but anoyingly loud.
Life is good in Vincent world.
 

MartynG

Well Known and Active Website User
VOC Member
Crikey, was all that 3yrs ago? I ended up fitting a Grosset ignition designed for a twin spark, set at 24/2 and an Alton electric start. I Still have the Dellorto 36mm fitted. I have recently put back on the original, loud, straight through silencer from Armors? while my VOC spiral one is being welded and have had to increase my main jet size from a 145 to a 160 and my pumper jet up a size to keep it happy. It is noticably quicker but anoyingly loud.
Life is good in Vincent world.
Ear Plugs ??????????
 

Bill Thomas

Well Known and Active Website User
VOC Member
I wonder what a 2" down pipe with a one off Big Armours Silencer would sound like,
Would it be louder or more Mellow ? :) , I haven't seen my old Race Pipe for many years,
It might be rusted through, I bent it myself in 1980, And it had a few wrinkles, But I liked that it looked home made.
 

vibrac

Well Known and Active Website User
VOC Member
And very sixties breathers I see. nice to see tradition alive in the midst of the latest (and no that is not a facetious remark) v nice bike
 

Dave Hulstone

Website User
VOC Member
Thanks lads. I know the purists would hate it but it's become a very user friendly bike over the past few years. I've had a lot of help and learnt a lot in the process. It's by far my favourite bike in the shed. Call it a 'one lung' a 'Vomet' 'half a Vincent'
whatever, I love it.
 

Glenliman

Well Known and Active Website User
VOC Member
The 2 plug head ignition timing info in this thread
is much the same as the info I got from Terry Prince, Bob Dunn and Roy Robertson.
Andy at Pazon made up an ignition map based for my T Prince head Special. He used the info kindly provided by the above three Vincent men.
That map gives 26 degrees total advance and 4 degrees advance for starting.
Total advance occurs right around 2 k rpm. Bob Dunn said it would be OK to have total advance even earlier at 1600 rpm.
Andy thought 2 k would be better, but he did listen to Bob's advice on getting that full advance in much earlier than is typical for other EI ignition maps (5000 rpm for Boyer on a Norton!)
There is no real advance curve, just a steep straight slope from 4 degrees to 26 at 2000 rpm.
Seems way to simple, especially when you look at some of the complex curves that are used for ignition on some other bikes.
It works really well, there is no preignition and the CR is 10.8 to one.

Roy , with all of his dyno tuning experience, reversed my thinking on ignition timing.
In the past it had always been " advance ignition until it pings ( pinks, UK) then back off slightly. This was called " Power Timing" and was used by many, including Smokey Yunick, a man who won a lot of races.

On the dyno, Roy found that there was a range of ignition timing where power stayed much the same. Toward the top of that range ( most advanced), power dropped off markedly. It also dropped off a cliff if too far retarded , but did rise slightly as advance was reduced, as long as one stayed a bit back from the cliff. So he went with the least advanced position that still gives full power. That's the safest place to be in terms of engine destruction due to detonation. Safer and also slightly higher in total power output than at the more advanced end of this range.
I believe this agrees with David's experience.

Made sense to me. The earlier you have to start that flame the greater the power loss as you are starting to push back on a piston that is still rising.
If one goes far enough back it can knock a hole in the piston or bend a rod. If just a little bit too far advanced then it's just power loss and some extra heat to contend with.

But you do need about 4 degrees of advance to get good starting, so the map has to be made that way for a non programmable type EI.
Each new map at Pazon costs $500 so it was nice to get it right in one take!

One question- David mentions that Terry Prince heads are a copy of factory big port heads. I've also heard that they are a copy of Manx heads. Another source said they were modelled after the Australian V8 Supercar heads. Yet another claimed that Terry made some changes after Steve Hamel greatly modified Terry's early heads for Steve's Bonneville racer. Someone else who seemed to know all about the heads said they were designed by the late John Trease. I asked Terry about the John Trease connection and he got a little animated.
The heads were not designed by John Trease, although he has great respect for John Trease's accomplishments.
Terry just said the heads are his own design, period.

Maybe at this point they are an amalgamation of the above plus Terry's own ideas?
Ive not seen photos of the factory big port heads. Did they have they kidney shaped combustion chambres and D shaped anti-reversion exhaust ports?

Glen
 
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b'knighted

Well Known and Active Website User
VOC Member
Hi Dave,

Can you give us any feedback on the starter system, please? Is it the same as the starter now stocked by the spares company?
 

timetraveller

Well Known and Active Website User
VOC Member
Big port heads are definitely not the same as Terry Prince's heads. The works big port head were similar to the standard heads with a smaller radius for the hemisphere and no lock rings on the lower valve guides. They did not have squish (quench) bands and they did not have 'D' shaped exhaust ports. One wonders who makes up these stories.
 

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