600cc Comet Carbie?

Dave Hulstone

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VOC Member
Sorry Bill, I was showing Martyn, aka Blacksheep, my ear plug holder.
image.jpeg
 

vibrac

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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

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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.
 

Monkeypants

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Non-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

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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

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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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