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600cc Comet Carbie?


Harry Hulstone

Website User
Non-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?
 

genedn

Well Known and Active Website User
VOC Member
Borrow an exhaust gas sniffer if you can or get the bike on a dyno.

I did this with the help of a fellow section member. It accurately tells you the air to fuel ratio throughout the range, allowing you to make changes to carb jetting etc as needed.

Gene.
 

davidd

Well Known and Active Website User
VOC Member
Harry,

I remember the conversations we had and it is nice to hear that you are doing well.

Are you trying to get it to start better or are you just trying for more performance?

The more you are able to retard the ignition, the better the power, but nobody makes an ignition that allows you to do this easily. All motorcycle electronic ignitions are made with automobile chips, as far as I know. Many retard the spark a huge amount (set by the supplier of the ignition when he programs the chip) and if you try to tune your engine, the retard can drop back below TDC. The supplier usually will not set the ignition where you want it, but they will often disable the retard as an option before you purchase the ignition. You may be on the edge of this phenomena, but there may be other reasons also.
Based on what you have mentioned, it sounds like you are on the right track. I know nothing of the Dellorto, but Terry's heads are a copy of the Factory big port heads which were designed for a 36.5 mm carb. So, 36 mm sounds fine and 38 mm might work well also, but the starting may suffer a little more as you continue to go bigger, because the flow tends to slow down as the size goes up. I am wondering if the carb is preventing a better start. In other words, are you getting the plug wet or is it quite dry after you have kicked it a few times and it has not started. It might be worth pulling the plug out some time and seeing if it is wet or dry.

A lot of owners like a 48T sprocket on the rear of a twin and a 50T on the rear of a Comet. This is usually a lot better in traffic, but you may feel you do not need that with the 600. You might try a 48T and see if you like it better, if you have not done so already.

I have forgotten what cam you are running, but the MK2 will make starting difficult also. Greg Brillus can attest to this.

All the best,

David
 

Albervin

Well Known and Active Website User
VOC Member
A common complaint with the new BT-H is the range from full retard to advance in not enough for bikes such as your's. Dave D would know all of this so maybe not the issue. What plug are you using?
Alyn
 

Bill Thomas

Well Known and Active Website User
VOC Member
Hello Harry, I use the same carb' on my Twin Special, Which I raced and used on the road, Now I have detuned it a bit, I found the pumper was too rich, I only ride on the road now, So I took the plastic lever out of the inside, To stop it pumping.
The other thing is, While riding, If I stop for 20 min' , To restart I have to use the Choke, Which works good, A proper choke !!. Good Luck, Bill.
 

greg brillus

Well Known and Active Website User
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.
 

chankly bore

Well Known and Active Website User
Non-VOC Member
The 650cc. Comet on Steroids performs perfectly well with a 34mm Mikuni. You only need a bigger carburettor if you are going to spend a lot of time at full throttle. Pazon ignition, twin plug head. Starts very easily once you get the throttle setting exactly right. Steve at Motocarb (U.K.) supplied the carb. and air cleaner. Started third kick from new once idiot owner had tightened the hose clips correctly. Idles like a tractor, never touched the settings. Terry Prince twin-plug head,100mm. stroke, 26 degrees of advance, 1 3/4" to 2" pipe, Supertrapp muffler. Mikuni advises choke below 20 degrees celsius only. On my 750 bevel Duck I never used choke, just one quick turn of the throttle before lighting up. NGK (real Japanese ones) BP7ES, never looked at them in 3000 miles. Pulls 19/45 sprockets. On the downside, it still breathes a little too much oil.
 

Harry Hulstone

Website User
Non-VOC Member
Harry,

I remember the conversations we had and it is nice to hear that you are doing well.

Are you trying to get it to start better or are you just trying for more performance?

The more you are able to retard the ignition, the better the power, but nobody makes an ignition that allows you to do this easily. All motorcycle electronic ignitions are made with automobile chips, as far as I know. Many retard the spark a huge amount (set by the supplier of the ignition when he programs the chip) and if you try to tune your engine, the retard can drop back below TDC. The supplier usually will not set the ignition where you want it, but they will often disable the retard as an option before you purchase the ignition. You may be on the edge of this phenomena, but there may be other reasons also.
Based on what you have mentioned, it sounds like you are on the right track. I know nothing of the Dellorto, but Terry's heads are a copy of the Factory big port heads which were designed for a 36.5 mm carb. So, 36 mm sounds fine and 38 mm might work well also, but the starting may suffer a little more as you continue to go bigger, because the flow tends to slow down as the size goes up. I am wondering if the carb is preventing a better start. In other words, are you getting the plug wet or is it quite dry after you have kicked it a few times and it has not started. It might be worth pulling the plug out some time and seeing if it is wet or dry.

A lot of owners like a 48T sprocket on the rear of a twin and a 50T on the rear of a Comet. This is usually a lot better in traffic, but you may feel you do not need that with the 600. You might try a 48T and see if you like it better, if you have not done so already.

I have forgotten what cam you are running, but the MK2 will make starting difficult also. Greg Brillus can attest to this.

All the best,

David
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
 

Harry Hulstone

Website User
Non-VOC Member
Hello Harry, I use the same carb' on my Twin Special, Which I raced and used on the road, Now I have detuned it a bit, I found the pumper was too rich, I only ride on the road now, So I took the plastic lever out of the inside, To stop it pumping.
The other thing is, While riding, If I stop for 20 min' , To restart I have to use the Choke, Which works good, A proper choke !!. Good Luck, Bill.
Thanks Bill, I have tried taking the pumper device off, because I thought I was flooding it. It's back on now because like you say, everytime I go to start it, even warm , I have to give it full choke. I'm chasing my tail I think. It's looking like a cam change.

Harry
 

Harry Hulstone

Website User
Non-VOC Member
The 650cc. Comet on Steroids performs perfectly well with a 34mm Mikuni. You only need a bigger carburettor if you are going to spend a lot of time at full throttle. Pazon ignition, twin plug head. Starts very easily once you get the throttle setting exactly right. Steve at Motocarb (U.K.) supplied the carb. and air cleaner. Started third kick from new once idiot owner had tightened the hose clips correctly. Idles like a tractor, never touched the settings. Terry Prince twin-plug head,100mm. stroke, 26 degrees of advance, 1 3/4" to 2" pipe, Supertrapp muffler. Mikuni advises choke below 20 degrees celsius only. On my 750 bevel Duck I never used choke, just one quick turn of the throttle before lighting up. NGK (real Japanese ones) BP7ES, never looked at them in 3000 miles. Pulls 19/45 sprockets. On the downside, it still breathes a little too much oil.
Right, I certainly won't be going with a 38mm then. What cams are you running?
 

Harry Hulstone

Website User
Non-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.
Thankyou Greg,
I'm liking this forum, a lot.
A common complaint with the new BT-H is the range from full retard to advance in not enough for bikes such as your's. Dave D would know all of this so maybe not the issue. What plug are you using?
Alyn
Hi Alyn. I am using NGK IRIDIUM BPR6EIX. 2 of
I think I'm changing my cams back to standard tomorrow. I'm not brave enough to pull a ton anyway with the front suspension the way it is, ( I've been keenly reading your posts about the steering head mod) so I'm, for once, not chasing speed. Having confidence in starting is my priority. I ride in constant fear of stalling it in traffic. It's crazy.
 

Black Flash

Well Known and Active Website User
VOC Member
HArry
I don't know what kind of gearbox you are using on your machine but with a Norton AMC box the internal gearing of the K/s mechanism is different from a Burman. After wiggling the engine over TDC you should bring down the lever about 1/2 to 3/4 of a swing. Then bring the lever up again and give it a good kick.
If you try it the Burman way there is not enough inertia built up to bring it over the next compression stroke.
I am using the same kit with a 36 mm Mikuni twin sparks and a Boyer Branden ignition. The bike always started 1 or 2 kick and never needs a choke despite using his MK 4 race cam. Tick over was rock solid. Unfortunately I wrecked the engine by filling the wrong fully synthetic oil after an oil change. At the same time I felt the urge to raise the gearing so that the engine wasn't revving so high on longer motorway journeys..
A fast ride on a very hot day up a long steep hill with tall gearing and the engine seized.
The fully synthetic oil had loosened all the old crud in the oil tank and this stuff ruined my pump and oilsupply within 10 miles.
The engine is rebuild and I am waiting for the JE headset and spring to come.
Bernd
 

Bill Thomas

Well Known and Active Website User
VOC Member
Hello Harry, I wonder if the ign' is the wrong side of TDC when starting, Do you know how much advance the mag' gives, Remembering it' s half speed.
I had to do a special job on my "D" distributor bob weights, When I ran 24 degrees. With the carbs, I thought the plastic sleeves between the carb' and manifold may make it want choke ?. Cheers Bill.
 

vin998

Well Known and Active Website User
VOC Member
VOC Forum Moderator
What I found a few years ago is that for easy starting you need you ignition to be around 4 - 6 degrees before TDC fully retarded. This is the same single plugs or two plugs per cylinder. On a road going bike with a kickstart anything greater will kick back and anything less gets harder to start. On racing machines it doesn't matter as rollers are usually used to start them.
The problem is with specials, twin plugs etc you usually need to retard the fully advanced figure from the factory 36 - 38 degrees to what you are running which is 26 degrees fully advanced. With ignition systems the advance range is usually fixed to a set range and so when you reduce the fully advanced figure you also reduce the fully retarded figure.
On a new BTH you can only select and set the fully advanced position. You then need to measure the fully retard position when running which is not easy to do but there was a timing disc been made a few years ago that allows a strobe to be used which fitted in the big end quill and still provided an oil flow. My bet is you are trying to start the bike with a fully retarded ignition timed at tdc or even worst after tdc and ideally need an ignition system with a reduced advance range. On coil ignition or magneto it's easy as you just add weld to the ears of the mechanical advance device to reduce the range. On anything electronic you need the manufacturer to make a special unit to suit your requirement.
 

Bill Thomas

Well Known and Active Website User
VOC Member
That's funny Simon, We have been playing with a few standard bikes with standard mag's, And we were thinking what timing to use with this petrol we have now, And with the wear in the ATD's, But we found it more easy to set them at a few degrees before TDC fully retarded, To save messing with holding the weights open. Cheers Bill.
 

Harry Hulstone

Website User
Non-VOC Member
I'm using the very leaky Burman box and using the starting method that you describe to a T. A friend of mine who is a very experienced engineer and Vincent nut taught me that same method. He has watched me struggle many times and has even made me a smaller engine sprocket in a bid to get it turning over faster. I have never fitted it.
Very unfortunate what happened with the old crud in the oil tank episode. I have never heard of that before. Did you manage to stay aboard? I've just had my tank acid dipped as it sprung a leak. There must have been 66yrs of crud in there, I don't remember my dad ever washing it out.
 

vin998

Well Known and Active Website User
VOC Member
VOC Forum Moderator
That's funny Simon, We have been playing with a few standard bikes with standard mag's, And we were thinking what timing to use with this petrol we have now, And with the wear in the ATD's, But we found it more easy to set them at a few degrees before TDC fully retarded, To save messing with holding the weights open. Cheers Bill.
Bill
A few years ago my brother in law bought a Rapide that had a modified Lucas ATD. The owner had made a new atd body / backplate that had an outer and inner continuous ring and the weights been a curved shape on their outer and inner face just moved into contact with the rings and acted as the stops. No ears to break off. Now my idea was this could be modified so say you had a removable sleeve on the inside of the outer ring. With a range of different thickness sleeves you could very quickly swap them over and thus alter the fully advanced timing figure without altering the fully retarded figure and also without disturbing the mounting tapers etc.
It was just an idea and never went any further. I will ask my brother in law if he has a photo of his atd and if so post it here.
Simon
 

Bill Thomas

Well Known and Active Website User
VOC Member
I used a bit of plastic tube on the shaft of "D" distributor, To stop the weights closing, Same thing !. Cheers Bill.
 

greg brillus

Well Known and Active Website User
VOC Member
HArry
I don't know what kind of gearbox you are using on your machine but with a Norton AMC box the internal gearing of the K/s mechanism is different from a Burman. After wiggling the engine over TDC you should bring down the lever about 1/2 to 3/4 of a swing. Then bring the lever up again and give it a good kick.
If you try it the Burman way there is not enough inertia built up to bring it over the next compression stroke.
I am using the same kit with a 36 mm Mikuni twin sparks and a Boyer Branden ignition. The bike always started 1 or 2 kick and never needs a choke despite using his MK 4 race cam. Tick over was rock solid. Unfortunately I wrecked the engine by filling the wrong fully synthetic oil after an oil change. At the same time I felt the urge to raise the gearing so that the engine wasn't revving so high on longer motorway journeys..
A fast ride on a very hot day up a long steep hill with tall gearing and the engine seized.
The fully synthetic oil had loosened all the old crud in the oil tank and this stuff ruined my pump and oilsupply within 10 miles.
The engine is rebuild and I am waiting for the JE headset and spring to come.
Bernd
Bernd Jus wondering........What crank assembly are you running..........?........Cheers........Greg.
 

greg brillus

Well Known and Active Website User
VOC Member
Yes everyone's comments about electronic ignition, and not knowing what the retarded point is, very... very... valid. As good as these units are for reliability, they inherently have almost nil adjustability when it comes to their total range. This is quite a king size mistake from their manufacturers as far as I am concerned. Not too many of them still run the original ATD. You would actually be better off running a good ATD and have and electronic unit with fixed timing. This can be done with the new BTH mags but no one ever bothers, as they can be ordered with fixed ignition timing. The people running twin plugs are the ones affected most, because they need less range than a single plug ignition set up. I had the task the other day of setting up the ignition timing on a 1952 MK 1 alloy head Ariel Square Four, this with its points replaced with a simple electronic trigger..........How do you recon I got on with that one.........???
 

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