Concentric carbs

Dccycleworks

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Jul 19, 2020
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Hey guys, I’m hoping someone can maybe offer me a nudge in the right direction. I have a 1966 Norton N15CS and I am the second owner. The original owner installed concentric carbs on it. I just got the bike running at work and decided to ride it home over the weekend and it began fouling the plugs on both cylinder. So I thought I should double check my float height. I checked both carbs and the fuel level on both carbs is level the boss for the float bowl mounting screw closest the the engine. I am checking float height with the carbs installed on the bike. That boss measures .170” from the surface of the bowl which I think is the lean side of the spec. I have adjusted my pilot air screws from a half turn out to 2 turns out. They are happiest around 1-1.5 turns out but regardless of where I put them after about 2 minutes if idling the plugs are black again. Is there something I am missing? Should I lower the float height some more? I would really appreciate any suggests anyone may have. Thanks in advance
Dennis
 
Triton Thrasher said:
At what throttle position is it rich?

If you don’t know, go and find out.
Click to expand...
At idle, I don’t have to touch the throttle. Just let it idle for a minute or two and the plugs are black. And just off of idle also, I was thinking I could lower my needles one position (the clip is in the middle position right now) but figured that won’t help it when the throttle is closed
 
Dccycleworks said:
At idle, I don’t have to touch the throttle. Just let it idle for a minute or two and the plugs are black. And just off of idle also, I was thinking I could lower my needles one position (the clip is in the middle position right now) but figured that won’t help it when the throttle is closed
Click to expand...


Don’t lower the needles, if the problem is at idle.
 
I just had a thought, these carbs have little plastic velocity stack on them, and when I took them off someone had put silicone on the threads, I assume as a sort of thread locker. I wonder if some of that silicone got into the idle air intake ports at the mouth of the carb. That would cause a rich condition I would think regardless of where the float level is at. Someone correct me if I’m wrong
 
Dccycleworks said:
I just had a thought, these carbs have little plastic velocity stack on them, and when I took them off someone had put silicone on the threads, I assume as a sort of thread locker. I wonder if some of that silicone got into the idle air intake ports at the mouth of the carb. That would cause a rich condition I would think regardless of where the float level is at. Someone correct me if I’m wrong
Click to expand...

That would be entirely possible and would cause the behavior you are experiencing. The pilot air circuits need to be checked for blockage. I would have started there.

slick
 
Dccycleworks said:
At idle, I don’t have to touch the throttle. Just let it idle for a minute or two and the plugs are black. And just off of idle also, I was thinking I could lower my needles one position (the clip is in the middle position right now) but figured that won’t help it when the throttle is closed
Click to expand...
Size of pilot jets?
I suggest you go over the other carb sizes, jet needle etc.
 
Bernhard said:
Size of pilot jets?
I suggest you go over the other carb sizes, jet needle etc.
Click to expand...
I’m not sure of the size but they aren’t removable, it’s a bushing pressed into the pilot circuit. I got home last night and blew a bunch of carb cleaner thru the idle air ports again and it seems to be better I think. I brought the bike to work with me today and put it back up on my lift and made a few more adjustment and went and rode it down the highway that the shop is on and pulled the plugs and they were cleaning up so hopefully I have it squared away now. I will put some more miles on it this weekend and see if it Gould’s the plugs again. Thanks for all the advice guys I appreciate it
 
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