Super rich Comet

roy the mechanic

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VOC Member
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

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VOC Member
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.
 

Martyn Goodwin

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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

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VOC Member
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

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VOC Member
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

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Non-VOC Member
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.
 

Martyn Goodwin

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Non-VOC Member
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

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VOC Member
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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