using the UFO, the cut away is not relevant. 

Had to google a UFO slide. Looks like i've got a lot to learn haha. Whatever your bike running issue is--if they ran ok out of the crate, should you not be solving whatever's causing the 'Now' problem before going UFO. Just my thoughts without refreshing looking back.

Sorry - I’ve hijacked Fencers original query with my UFO queries. Looks like there are a few of us with DT400s with standard set up (well, in my case, the addition of a Rex Speedshop ignition) that can’t get them running correctly by ‘tuning’, suggesting they didn’t work correctly out of the crate?
So its time to swap out parts, including the slide?. I’ve read good reports on the UFO slide which MAY be a better way to go than replace it with a different size slide.
I’ve sent a mail to Rich Taylor porting regarding the UFO slide, but no response as yet, so still don’t know if I can buy a new, modified slide from him or if I have to send my slide to him to get it modified (I’m in the UK and he’s in the US)?, Cost? etc.

Hi GaryDean - I hope you don’t mind me messaging you with some queries about your DT400B..
Did you get a 2.5 slide?
Was it the same overall height as your stock 3.0 slide?
Where did you buy it from?
What size pilot jet do you have?
Are all your ‘cruise’ jerking/erratic firing issues all gone now?
I’m interested where you are on this as I have the issues you had/have and can see that you have a Rex Speedshop ignition / 12v system - the same as me.
Many thanks….Phil (Jammyyammy).

Edit….. Ooops, I thought that was going to be a private message. Now re-sent as a private message.

I am in the UK and currently have this item on fleabay.

How do the dimensions compare to your original 3.0 slide?

The orientation of the cable fitment is slightly different to yours, but I doubt that this would be an issue.

Suzuki RG125 1986 to 1988 Mikuni carb slide piston throttle valve 13551-36A00 | eBay

Hi, thanks, but the dimensions of mine are…
Overall height = 53mm.
Diameter = 35.5mm.
Any assistance in getting a 2.5 slide in the uk to the above dimensions is greatfully received.

I’ve done some more research into the UFO slide. I have received the following advice from a uk supplier of the UFOs:
”I wouldn’t advise fitting an insert in anything other than a road race or motocross bike due to the instant throttle response it will give”.

I've got my parts in the UK from Motocarb (I dealt with Steve) and there is also a company called Allens. 

Sounds fun to me!! Never heard of to much throttle might have to try on my Ct1c and rd350 if the make one that fits 

“The UFO is a simple, trouble free, aerodynamic piece that fits VM Mikuni carburetors from 30 mm through 44 mm. The UFO makes a normal round slide carburetor out-perform the more expensive flat slide carburetor by eliminating turbulence and increasing air flow and velocity, creating a highly atomized fuel charge. Normal gains are 1.5 to 2 HP per 100cc of engine displacement on 2 stroke engines. (Minimal rejetting and tuning are required).”