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Super rich Comet

paulthom1199

New Forum User
VOC Member
#1
I have just purchased a series C Comet and having issues tuning it to run correctly. It came with a 930/71 Mk1 concentric carb, which was worn, so I replaced it with a new premium one from Burlen services. I used the same settings as the one that I took off but it ran very rich. Wet sooty plug and white smoke from exhaust. I've reduced the main jet from the 270 that was fitted to the previous one and gone down to a 250. I've replaced the throttle valve to one with a higher cutaway and moved the clip on the needle up to the top notch. None of this seems to have made a difference. The bike runs ok for short runs but the plug and clouds of white clearly indicate it's far from right. Anyone using the same carb and can let me know their settings or suggest other changes I can try. Many thanks.
 

vin998

Well Known and Active Forum User
VOC Member
#2
White smoke usually means burning oil. So does a shiny wet oily looking spark plug.
Black smoke means too rich. Usually with a rich mixture the plug is black and sooty, but not oily in appearance.

Simon
 

vibrac

Well Known and Active Forum User
VOC Member
#3
Remote diagnosis is fraught with pit falls but if I said 'clouds of white' I would be looking a few inches past a new Burlens carb in the direction of the engine.
Sorry I cant help you with the settings concentrics are not on my favored list but plenty of people here can, and if you find that thir settings and yours (originally or now) are in the right ball park its time for the spanners
 

paulthom1199

New Forum User
VOC Member
#4
I did wonder about the engine being an oil burner, rather than rich running, but the compression is really good which makes me think too rich. The plug smells of petrol rather than oil. I could just be kidding myself but would like to eliminate the carb before digging deeper.
 

oexing

Well Known and Active Forum User
VOC Member
#5
Seems your needle jet is far too rich as setting the needle to its lowest position does not cure the richness. Look at a diagram at AMAL pages downloads that shows what component in a carb does what at different slide positions.

Vic
link:
Amal tuning
 

paulthom1199

New Forum User
VOC Member
#6
Thanks Vic. I note that the original Comet carb, 229F/1DV, had a 200 main jet. I think I'll try that. Confused as to why the previous owner was running a 270 but must be worth a try.
 

oexing

Well Known and Active Forum User
VOC Member
#7
No, I did not relate to the main jet, that is not responsible to rich running in most conditions. From quarter to three quarter opening of the slide the NEEDLE jet is active. Please read ALL of the Amal tuning guide in my link.

Vic
 

paulthom1199

New Forum User
VOC Member
#8
OOPS. My bad. Didn't think about the needle jet. I believe standard size is 106 and they supply as low as 104. Will try that as well. I will check to see what size was fitted to the worn carb I replaced. Thanks.
 

Bill Thomas

Well Known and Active Forum User
VOC Member
#9
OOPS. My bad. Didn't think about the needle jet. I believe standard size is 106 and they supply as low as 104. Will try that as well. I will check to see what size was fitted to the worn carb I replaced. Thanks.
I don't think you want lower than 106.
I have found the slide needs to be weaker than 3.1/2, I have hand cut mine.
Float level could be too high, But if it's new, I would not think so.
About 200 for a main jet, But as Vic says, That should be only at high speed.
Did you tell Burlen's it was for a 4 stroke.
If you look at the burlen site it shows all the different jets, And ID of them.
Cheers Bill.
 

roy the mechanic

Well Known and Active Forum User
VOC Member
#12
A while ago I bought a new pair of mk1's, it would only run one one cylinder. It turns out the idle "bush" had been omitted. Turns out my jetting guess was not too far off. Main 220 needle jet .106 slide 3.5 idle .016. So, remove the mixture screw to see if there is an idle jet in there.
 

Bill Thomas

Well Known and Active Forum User
VOC Member
#13
There are so many needle jets, I don't mean size, Some have bleed holes, Some don't, The Mk2 has a single ring mark, Some are taller than others, The needles also have ID groves at the top, If like me you have a lot of old stuff, I have to do a lot of reading !. Cheers Bill.
 

MartynG

Well Known and Active Forum User
VOC Member
#14
I have just purchased a series C Comet and having issues tuning it to run correctly. It came with a 930/71 Mk1 concentric carb, which was worn, so I replaced it with a new premium one from Burlen services. I used the same settings as the one that I took off but it ran very rich. Wet sooty plug and white smoke from exhaust. I've reduced the main jet from the 270 that was fitted to the previous one and gone down to a 250. I've replaced the throttle valve to one with a higher cutaway and moved the clip on the needle up to the top notch. None of this seems to have made a difference. The bike runs ok for short runs but the plug and clouds of white clearly indicate it's far from right. Anyone using the same carb and can let me know their settings or suggest other changes I can try. Many thanks.
I have the same carb on my Comet. I run a 220 main with the needle in the middle position. Idle mix screw open 1 1/4 turns. Stable and reliable start and runner.
 

Alan Connor

New Forum User
VOC Member
#15
Given that you've tried so many different things without success, and are uncertain whether this problem is caused by a too rich mixture or surplus of oil in the combustion chamber, I suggest that you re-trace your steps and temporarily re-fit your original worn carb. If problem goes away, you will know that it is a fuel issue, not oil being burnt.

In relation to your new carb, I'd be tempted to check the fuel level in the float chamber - too high a level could lead to the problems you've described. I believe there has been an issue with an unnecessary fibre washer being fitted somewhere within the float bowl on these new Mk1 concentrics that can lead to this problem. A search on the Velo Owners Club forum will turn it up if you can't find it on this site.
 

erik

Well Known and Active Forum User
VOC Member
#16
i ran a 220 main Jet in my comet.but if you think your fuel Level is too high ,you can drift out the yalve seat out of the float bowl. then it is possible to fit a washer under it and this means the float Closes earlier.best is to use a fibre washer.but copper will do it ,too.Erik
 

macvette

Well Known and Active Forum User
VOC Member
#17
You say short runs, I'm just wondering if the bike is wet sumping. Try draining the sump to see if this is the case.
 

MartynG

Well Known and Active Forum User
VOC Member
#18
i ran a 220 main Jet in my comet.but if you think your fuel Level is too high ,you can drift out the yalve seat out of the float bowl. then it is possible to fit a washer under it and this means the float Closes earlier.best is to use a fibre washer.but copper will do it ,too.Erik
Not needed with the new "premium" carbs from Amal as the float has a metal (not plastic) tab that you can gently bend to adjust the fuel level in the bowl
 

Bill Thomas

Well Known and Active Forum User
VOC Member
#20
I don't get this white smoke ?, I admit I am colour blind, But the only white smoke I have seen is from a water cooled engine, When the head gasket is blown.
My Comet burns oil bad, But even I can see blue smoke.
I wonder if it's a blocked silencer with condensation in it ?.
Cheers Bill.
 
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