Hi Guys
I have a rich low end on my bike to prevent over run, the problem I have is I cannot ride the bike at 30 mph as im on about an 1/8" throttle at that speed and it is very chuggy due to the rich low end, at 40 it is clear.
I dont want to lean the low end up as this will create over run, so I was thinking about sprocket gearing to up the rpm (lower gearing) to get a smother run at 30 mph (dont want to get a ticket)

My 400b is running standard sprockets 14/40. what gearing (sprockets) would I need to up the rpm enough to get a clean run at 30 mph, i.e slightly more throttle at this speed?
Any suggestions appreciated

I know i'm starting off with an opposing idea but gosh if it's that rich i 'think' i'd try & get it carburating closer to good before playing with gearing. I'd pay close attention to the needle jet [emulsion tube metering part at the top] & wear on the needle that runs in it. Also be sure the pilot jet is securely screwed home in the carb body. & that airways are clear that feed the needle jet. Ok that easy for me to say but--i'm not there & might be way off track.

& i start again lol--i see ya only on about 1/8 throttle & at that you might be lucky to lean it off on the pilot screw or even reduce the pilot size but then richen the screw for idle but the leaner jet might help the next bit @ 1/8. But then there's the cutaway coming into play--or already is in partial play. All guesswork on my behalf.

Taller gearing will load the engine more, I think that's what you're looking for. 

I'm running 17/44 on my tuned up 360.  It'll chug along at 30 MPH in top gear but that's about the lower limit for 5th.  You could try 15/40 to see if it's a step in the right direction.  I would probably jump up to a 16 tooth countershaft, I like tall gears, as long as you have enough chain length for it.

I'm with Dan on this one, these big singles need to be loaded up to run nicely. I'd go straight to a 16t front. 15 if you do slow woods riding or up steep hills.

RT35
Hi, the richness is done on purpose to stop the over rev, carb has been set back to standar with all new jets etc, the only was when hot to prevent the over rev is to have it gurgly on the bottom end, apparently a common fault with the dt400
my low end mixture screw is half a turn out, if I go any higher I start to get over rev, but the motor is sweet as much leaner, its a double edged sword, hence why I thought about gearing 

I thought I would need to go the other way? as a bigger front would see more speed at that throttle setting or have I got it wrong. I thought if its a bigger rear then lowering the gearing would have a need for more rpm at 30 thus taking me passed the 1/8" throttle setting and comming off the idle jet to the main jet, so out of the gurgle zone area?

By over-rev do you mean annoying firing on a closed throttle while slowing down. I thought it was only the old RT3 type 360's that did that. Yeah its annoying for sure. 1/2 turn out is sure richening it up in that area but i'm guessing it comes on a bit blubbery when opening the throttle after slowing down. Anyway, whatever works is good for you. A fully closed throttle [stop backed off] works but of course no good if ya want it to idle. Wonder why it does it in the first place--must be a cure. I think my old almost new RT3 back in "73 or "74 i sold to a mate at work & he went the other way & leaned the pilot jet down to stop or reduce the 'bang'. Probably asking for trouble but i don't recall it seizing or doing anything bad. He probably found a slide with a bigger cutaway too like from 2.5 to a 3 then compensated up a bit by richening the needle a notch. Too long ago haha.

yes correct, I set to this from advice from other dt400 users who have the same issue, it stops it hanging when closing the throttle otherwise you have to pull the clutch and wait for it to calm down.
tick over is fine at 1300 to 1500 rpm and will sit there all day at that. I looked in the uk for a 3.0 slide but no luck so just got to go with whats working at the moment hence why I want to do the gearing to get passed the 1/8" mark. other than that bike pulls like a train with no issues

"... I looked in the uk for a 3.0 slide but no luck" 

Morning Gary.

Did you try Motocarb and Allens?