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600cc Comet Carbie?


Normski

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VOC Member
600cc Comet Carby?

Hello fellow enthusiasts. I'm new to this so go easy on me.

I converted my Comet to a 600 using one of 'our Terry's ' kits a couple of years ago. It goes well, although its a pig to start with a BTH ignition. I'm using a 36mm Dellorto pumper carby at the minute, although Terry did recommend a 38mm. After seeking advice from Dave D I'm at 24degree ignition timing. I had a 47 tooth rear sprocket made by a legendary Vincent owner Brendan ONeal in Geelong. Am I on the right track?
David's advice(and his help and advice has been enormously helpful to me too) of 24degree timing has given you good running, but this lower figure for advance may be giving you your starting problems, would it be worthwhile timing the ignition to say 30 degrees and see if the starting improves? If it did you'd know you need to find a means of having less advance/retard in order to keep the 24 degrees for running and still have decent starting.
 

Robert Watson

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VOC Member
I might ask what plus you run with the BT-H ignition.
I run NGK 7's on my Harley inspired ignition and it starts on half a prod. With the BT-H on another bike it was a bearcat to start on 7's, and on 6's became a one prod starter.
 

Black Flash

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VOC Member
Hello Greg
I'm using a standard rebuilt crank. I had loads of problems and went through 5 different silencers destroying most of them trying to get the noise down to be able to pass the MOT in Germany.
I finally built one myself. The engine easily spins to 6200 then slows down to 6400.
The Comet is geared to 110 mph at 6500 and feels rockets hip fast. Boy she really flies compared to a standard one.
I had loads of problems with the silencer as any change affected the carburation massively, I completely underestimated the effect of a race cam in a 4 stroke single
. With open pipe the engine spun to 6700 with earbleeding noise.
Bernd
 

vibrac

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VOC Member
Silencers and tests:
I was once volunteered as a noise tester at a race meeting, during testing a very very noisy NSU sports max came to be tested, we checked the stroke and found the correct revs the reading was to be taken at. The rider increased the revs the noise increased then as the correct measuring rev band came near all went quiet and at the measuring point it was legal at 105 Db. I of course asked the question and the rider showed me that under the crankcase at right angles to the main exhaust was a branch pipe sealed off with a brazed on disc. he had found by experiment just the length that cancelled out the sound waves. I belive helmholtz chamber is the key to further reading. If you cant join em beat them.
 

Black Flash

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VOC Member
Hello David,
Thank you for posting the pictures. I have no laptop at the moment so I can not get hold of my pictures.
I was on my last test ride before the MOT when I ruined my engine.
I got the noise down from 110 db with a Gold star silencer to 86 db with my self made silencer. Measured at 4000 rpm static. That is how the test goes in Germany depending on the standard rpm for the original bike.
When the engine is under load its a different story though.
But I think we are digressing .
I wish Harry all the best with his Vincent.
From my personal experience I would completely disassemble the engine and check everything. An easily solvable problem can bust your bank account if you break the engine.
Also with not so many specialists left, it could take years to get the engine back.
 

ericg

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VOC Member
Hello David,
Thank you for posting the pictures. I have no laptop at the moment so I can not get hold of my pictures.
I was on my last test ride before the MOT when I ruined my engine.
I got the noise down from 110 db with a Gold star silencer to 86 db with my self made silencer. Measured at 4000 rpm static. That is how the test goes in Germany depending on the standard rpm for the original bike.
When the engine is under load its a different story though.
But I think we are digressing .
I wish Harry all the best with his Vincent.
From my personal experience I would completely disassemble the engine and check everything. An easily solvable problem can bust your bank account if you break the engine.
Also with not so many specialists left, it could take years to get the engine back.
Talking about specialists left, Is Bob Dunn still running its business?
 

davidd

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VOC Member
I think the factory spec MK 2 cams are ok for starting, but the Mk 2 cams that Terry sells can make starting very difficult, because the inlet valve closes very late. Try about 85 degrees BTDC too late, ok for a race engine, not street.
Greg,

Yes, I did remember that you had some problems, but the Factory Mk2 does the same, although not to the same extent. When comparing the Factory MK1 and Mk2, the Mk1 makes more cranking or starting pressure than the Mk2, due to the similar late closing of the intake on the Mk2. With the Factory Mk2 (usually no more than 75°) I would not be too concerned about bad starting, but I would be concerned about the cumulative effect of several mods that makes starting difficult...and using a cam that doesn't start as easily as the Mk 1. After all, the Phil's did not feel that the Mk2 was a suitable street cam although most owners today will put up with the problems.

There are programmable ignitions that will deal with the timing retardation issue, but that would take extensive dyno tuning, which is unlikely. The MSD 4217 comes to mind for single cylinders. It can be programmed to start at 4°, for example. I think it will go down to 0°, but it won't go negative for Bill. I think it will hold two timing curves which can be selected with a switch on the handlebars.

Once the Ariel Club gets wind of your tuning skills applied to the Square Four you will be inundated with the bikes.

David
 

Bill Thomas

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VOC Member
Talking of starting a Comet, We built a Comet out of a load of bits in 2015 for a friend, Bog standard, Even old carb', And trying for 1 kick starting, We found getting it over compression with the lifter, Then moving the kicker down a small bit !!, Then let the kicker back to the stop ,Then give it a good kick, Would bring us to a position that would get the flywheels moving in a better place. Just like Black Flash said.
But I have never had to do that with my Comet, Which has Mk2 cams, Big piston etc !!.
These are Funny Bikes, No 2 seem the same !, But lots of Fun. Cheers Bill.
 

Black Flash

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VOC Member
I have no experience with late timing on starting on a motorcycle.
In fact I have no idea when my ignition will fire while starting the bike.
I do know from my programmable ECU of my 5 litre TVR with a very racy cam that we start the engine actually at 2 degrees ATDC. Also at idle the engine is well retarded which eases off the extremely lumpy running with the semi racecam.
I doubt that the timing Harry is using will make his bike a non starter. If you look at the actual piston motion at - 5 to +5 degrees around TDC in my belief the firing point is not very important to start the engine. After the first few revolutions the engine will speed up and the ignition advance will start.
I tend to think that the spark may not be strong enough. Or it could be the plugs, the carb settings, obstruction in the idle circuit or a problem with the BTH.
From all the starting problems I had till now it was never the timing 80% of the problems were carb related 20% plugs.
But maybe I was just lucky
Bernd
 

MartynG

Well Known and Active Website User
VOC Member
Here is the BTH advance curve, from their web site. If you have set FULL advance to be 24 BTDC then when u are trying to start it it at a horrible 8 degrees ATDC - out by almosy 12 degrees from the easy starting position.

So. reset your ignition to 34 BTDC fully advanced which is close to 4 BTDC, the ideal start point. If the you find it is easy to start you are 50% on the way to the solution. For just now do not worry about power at speed. - that can be addressed AFTER you fix the starting problem.

Tell us how it goes.
upload_2017-1-8_19-51-20.png
 

greg brillus

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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............;)
 

Bill Thomas

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VOC Member
It could also be something as simple as Carb' Settings ?, Those Delorto's are funny, I love them, But the instructions for me came in Italian !!, My next door man was Italian, But could not give me the info' I wanted.
I have had to grind weak slides, To stop plug wetting, And even when I pulled 6500 revs on my Twin, I only had 190 main jets, That was a race engine with a Super Special Exhaust, I have been told people try to use much bigger main jets, Just like I did !!. Cheers Bill.
 

vibrac

Well Known and Active Website User
VOC Member
Interesting about the pole rust my 2 bth have been in not heated sheds for many winters between riding and racing seasons I haven't noticed any great changes and any I put down to advancing age maybe it's not me it could be rust! perhaps I should take a peek?
 

greg brillus

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VOC Member
There are no parts in there to worry about other than the shaft which looks similar to a rotor arm with a magnet on the end of it, simply rotate the engine over if it is in the way. I would not disturb this arm at all for obvious reasons, and the only slight tricky bit is where the small grommet at the rear where the wires come out for the HT Coil/s and the kill wire ....This sits in a groove in the cap which is held on by two metric cap screws, not too hard to do. The mags are well made but I don't think they have ever been completely water proof. The issue we have here is moisture from our hot humid climate in Summer, but any engine part that gets hot and has cavities of some form in it, will attract some form of moisture as it cools down.......Like our oil tanks...........:)
 

vin998

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............;)
I've got a BTH that I removed from my Rapide because it got harder to start to the point it wouldn't start at all. I put the point coil ignition back on and it reverted to 1 kick starting. I will get the BTH out and have a look inside.
 

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