600cc Comet Carbie?

Harry Hulstone

Forum 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?
 

Gene Nehring

Well Known and Active Forum 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 Forum 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 Forum 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 Forum 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 Forum 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 Forum 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

Forum 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

Forum 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

Forum 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?
 
Top