took the bike out today and as above I think the bucking problem now solved. It took me 8 kicks to start some harf harted, initial run was rough at low speeds prob due to now a rich lower end, tickover to 1/4 throttle rough after that ok, i stopped for 2 hrs then tried to start in the past i found i needed choke (before bottom adjustement) i employed the same regime and 10 kicks later 1 with a kick back that I think has twisted my ancle it started all blubbery. I still need to get the top dead knack sorted out, but was thinking now I have richened the bottom end do I re need choke to start?

Garydean...glad you are happier with the lower end richer. Mine also is quite blubbery down low until warmed up then runs good with minimal bucking. I'd still try lowering your idle...1500 is still higher than needed and lower will help reduce bucking on closed throttle coming to a stop or slowing down.  Only thing is mine is not hard to start cold or warm. Three modest kicks at worst. Maybe try as manual says...start cold with choke on and throttle closed. Start warm with no choke and crack the throttle. Of course it is subjective on just how warm or cold it is. If pretty warm just crack the throttle when kicking. If doesn't start in two of three then choke on.  Good luck!

Is it possible the two issues are unrelated? Seems the hard starting could be due to something impeding the flow of fuel in the starting jet. Regardless of how these bikes run under throttle, I've begun running thin wire through all the passages possible repeatedly and I almost always find some small impediment when I have hard starting issues. Just a thought. A first kick start is certainly preferable.

Hi Guys, thanks for the replies, my ankle is quite sore today so its going to be a few days before I have another go with it, but I will adjust the tick over and see if I can get it down without cutting out, as at tick over the engine is popping and rough, taco needle is not smooth at tick over
what figure do you use for your tick over?

Good point on the carb, I have bought a carb cleaning wire and brush kit for poking through jets etc, so while healing I will pop the carb again.
When I fitted the Rex speedshop ignition 6 weeks ago it would start on the first or second kick cold so maybe there is a restriction with dirt happening now preventing the an easy  start.

subsiquent starts I just feel each kick is kicking back or trying to kick back till eventually its caught right and starts. I still dont think I have the tecnique yet for starting.
Ive checked the de compressor and that seems to be operating correctly but still a massive amount of compression there when starting. other than that  the bike pulls like a train on steroids

After reading all of these posts I decided to purchase a larger Pilot (55) and a 2.5 Throttle slide.The slide arrived today and although it is a genuine Mikuni the overall length is some 10mm   shorter than my stock.Have I ordered the wrong one?Please see attached pictures  

Yes, and you probably won't be able to get one from Mikuni. 

Yamaha specified a tall cap carb which needs the tall slide.  Short slide can create a dangerous stuck throttle condition with tall cap. 

You can either swap to short cap (might need to mess with cable) or get a tall slide. 

Hi, this is all good information. I’ve got similar issues with my DT400B, so am keeping a close eye on developments. Thanks for posting…

Ignore me if i'm way behind.!! If it's kicking back with a Rex's non-keyed flywheel, could it have moved foward firing sooner [advanced].

Hatrick ,
Later model DT 400 's use a 2.5 slide  pn   1M2-14112-25
Check it out , i'd expect it to be the same length as your early 400 slide .

Hi,
Is the UFO slide based on a 2.5 or 3.0? Or is that irrelevant for a UFO slide?