So after only a few miles with my new (OLD) AT1
I notice that going into neutral when reaching a traffic light or a stop is impossible !.  If I turn the engine off, then turn contact key back on, Flick gear lever up, then no problem I am in neutral.  
It seems that when engine is running I am unable to engage neutral, even when on full stop.
Obviously some adjustment to do ?... any suggestions.
Thank you.
 

First check your clutch plates/discs. Are they sticking, even one or two, such that you are still somewhat engaged when the clutch is pulled and supposed to be completely free? Have you changed/checked the fluid? Sure of what's in there? After that, then check adjustment according to manual in tech library.

If you can turn your clutch cable adjuster at the lever out a little bit (for more clutch pull) that might help. Just be sure you have a little free play in the lever or if the clutch slips then you've gone too far. I've had that too and a little more clutch pull cleared it up. Ten second fix. Let us know if that's it!

They've got a neutral detent ball & spring under a cap on the left top side on an angle. I don't for a moment think it might be the issuer unless ball or spring is missing. No30 & parts above it. Most likely just the clutch hanging on & i'd remove the plastic rings that sit under the fibers if you go in there as 'i think' they just try to drag the clutch around due to their close proximity under the fibers plus a bit of oil around the. Think they're only there to help spread the clutch in a cold climate "theory", & makes for a soft uptake when releasing the clutch but more trouble than they're worth. While the cover is off "right side cover" pull the lever & observe if it frees up good. Steels 'might' be bent but likely new fibers & she'll be good. From what you're describing i'm guessing when you stop at the lights that it's trying to move you forward slightly. It doesn't take much though to hang on & make neutral a problem. with practice you can hook neutral on the roll as coming to a stop.
www.partzilla.com/catalog/yamaha/motorcycle/1970/at1b/shifter-1
www.partzilla.com/catalog/yamaha/motorcycle/1970/at1b/clutch
No6 in the clutch parts pic.
No30 in the first link.
That's providing the clutch is adjusted ok in the first place. My old AT1 is a bit like that but never been apart but slips with a 175 top end its now grown.

what speed is it idling at? I've had some issues with hitting neutral when the idle is too high but perhaps that's been floated around already. 

My AT1 would not catch neutral with the bike running like a little clutch drag. Replaced the fiber plates and tossed the rubber cushion rings and now it is much better.

Bernard, you need to get it sorted, you're missing the action
 

I think it's something else entirely.  That nice little bike just wants to GO!  So she's fighting you when you try to put it in neutral.  

Thanks all for suggestions.....  A few more check and a few more spare from Yambits.... and little whitee will be on the road again....  :-)
I will post the result soon....

That's good. New & lubed cable plus get some lube into the moving bits of the actuator would make it even nicer. Forgot they have a tin cover on the inside to shield things, that's a good idea. Shows how long since i've looked at mine.
www.partzilla.com/catalog/yamaha/motorcycle/1970/at1b/clutch