Motorcycle Repair: honda cb400F, honda cb400f, adjustment screws

Question
QUESTION: Hi,
I got a cb400F (1975 european) and since rebuilding it from scratch (engine overhauled etc.) I keep getting a knocking noise when low in revs.  Couldn't get it out, even when carbs were synched and ok. After checking blogs, I thought it might be the primary chain (slack).  So I opende her up again and put in a new one (the old one was stretched a bit). But still the same problem. Valves are ok, ignition timing and gaps ok, compression good. However, I always had trouble getting the carbs synched (adjustment screws on two almost entirely screwed in).  Could it be the clutch? There seems to be no slack but the sound comes from around there. I would be very happy to get some advice. Thanks, Jef.

ANSWER: Jef, what you describe sounds a lot like what we used to call clutch rattle. Does it reduce when you pull in the clutch?  If it gets a little quieter, it could be the famous clutch rattle the 4 cylinders were known for. I've never found a fix for it.

Regards
Rich

---------- FOLLOW-UP ----------

QUESTION: Hi Rich,
it seems to get a little quieter. Frustrating, since I also rebuild a cb350F which runs perfect. Could it have to do with the clutch of the 400 being different in design?  Also, the bolt for fixing the clutch is slightly damaged because it had been removed without the special tool, but since it doesn't seem to touch anything I don't think this can cause it. Do you have any suggestions regarding the synchronization? I cleaned them, checked them but it stays very difficult to get them good. I'm using a Carbtune.
Regards,
Jef

Answer
Jef, those carbs are a bit sensitive to over tightening of the adjusters. If it were me, I'd start over and loosening up the adjusters so all of the adjusters showed equal number of threads above the nut.  Make sure you have a fan blowing on the engine to keep it cool while you adjust the carbs.  You need to get the carbs to within 1 inch of mercury of each other.  You also need to be sure the ignition timing on each bank are as close as you can get them.  If cylinders 1&3 are more advanced they will hit harder than 2&4 so it will aggravate the clutch knock.  I'm assuming you have already adjusted the valves but if not do tha first.

Regards
Rich