Plug carboning up - needle position

[EddieDay]

A few times after short runs the bike (1958 M18S) won't fire. I have taken the plug out and it is very sooty. Once I clean it and put the plug back in she fires first kick. I am thinking the carb needle is in the wrong (too high) position as, according to what I have read, from 1/4 to 3/4 throttle the needle controls the mixture. The strange thing is that various threads I have read and the manuals say the the needle should be in the middle position. The needle in my carb has 5 grooves (I thought normally 3) and it was already positioned in the second from top groove. So I have moved it down to the last groove. No test ride yet given the weather. I assume the carb is a 389/1 but the only stamping I can see is "N31" on the underside of the flange. The bore is a little over 1.1" so I guess the correct 1 5/32". My questions:
- Is the stamping N31 for a 389?
- Is the needle with 5 grooves wrong?
- Should I be doing something else to stop the carboning up?

Thanks for your thoughts.

[dave16mct]

You may have the needle from a 376. For a 389 it should be 106mm long and stamped with a 'D' near the top. 5 grooves is correct.
Dave

[Joker_Bones]

At a little over 1.1" bore... Could be 1 1/8" bore?

Many more 1 1/8" bore 389s out there than 1 5/32

[EddieDay]

You may have the needle from a 376. For a 389 it should be 106mm long and stamped with a 'D' near the top. 5 grooves is correct.
Dave

Dave,
Would a 376 needle be shorter? That would explain a richer mixture.

[dave16mct]

Yes a 376 needle is 95mm long and is stamped 'C'. It will make a 389 carb bike run very rich.
Dave

[EddieDay]

Great Dave. I will check next time I remove the tank.