Hi Oh Knowledgeable ones, please help.

Bought refreshed /Restored DT400B 1975 from UK importer. Was missing air filter to carb rubber, exhaust baffle, air filter, but supposedly ran well?

It starts, but runs super oily and I think rich and will not pull, felt like ignition so bought Rex speedshop brand new fit and forget ignition. All fitted great. Still does not run through the rev range.
I have bought airbox to carb rubber from the US (Very $$$) does not fit the carb as carb is 34mm! not the VM32mm 50061 it should be. I have bought air filter, made the rubber fit and fitted baffle in exhaust, but even after cleaning carb which seems fine, just cant get it run properly at all. It just about idles, but seems super oily.

Not really sure where to start, but must assume the ignition is now perfect. Guidance please. How finicky is the float height?, should I be trying it each time with everything connected r is it ok to try with open carb? Considering disconnecting oil pump and testing on pre mix to eliminate that issue?  Bit stuck really.

 from near Salisbury (UK), Brandon.

If not done already, check that all of the carb parts are genuine Mikuni - they will have a logo of a square in a corner of a larger square stamped on them. Aftermarket carb kits have been known to cause problems.

It could also be that there's a shed load of two stroke oil sitting in the crankcases - is it smokey when it does run?

It has been very smoky each time I have had it going. I’ll check all that cheers.

foes the carb need to be connected to the air box each time I make a change?

also does the 34mm cause concern regarding air flow speed?

Burn the exhaust out so its totally clean & dry. Oil gets into the packing along the walls. & even more important is the tail muffler which gets blocked in the 'fixed' spiral [non removable] spark arrestor deep down in the tail muffler. It's down in deep about where it starts to taper down to the pipe size. But wait there';s more haha--where that tail pipe fits in the main exhaust it can grow a ring of carbon growing smaller id than the pipe itself. Ok i might be wrong on all of it but i had that on an IT465 that everyone else had given up on. & a PE400 come to think, many years ago. Do that, then give it a good rev out. I'm guessing a DT400 'Doesn't' have the jet setter Oring like older 360's or i'd go there first--on older bikes it seals the bowl to the needle jet so the side bowl mounted main jet does its job or gets bypassed & effectively sucking about a 600 size jet & drowning it on open throttle. Ignore that but i'll check partzilla to view the carb type.

Here's a "74 DT360a Carb with Oring #4. I Don't think yours will run that carb, but "75 400 is missing off Partzilla & going to "76 doesn't show that earlier carb.
www.partzilla.com/catalog/yamaha/motorcy...74/dt360a/carburetor

Sorry, i'm woffling on, but here's an extreme case on my 100 twin years ago.
yamaha-enduros.com/index.php/forums/1972...aust-on-ls2-100-twin 

Could get away from the public & remove the end muffler [complete end muffler not just the end cap & bits] then give it a good full song rev out standing still, not riding initially, then if you have the room run it up through the gears on full song.

looks like Cheech and Chongs bong

Lots to think about. I don't think the pipe is blocked at all from the sound of it and the way to caught a few times and tried to wheely!!

I will revert back when I have tried a few more things. Thanks folks.

For those who may be interested.

My bike is running a 400mx carb and I am looking for a standard 50061 Mikuni 32mm. It is not a blocked exhaust. The carb to cylinder rubber is very perished so I can't do much more until a new one arrive from the states at lots of £££. Thank you all for your help I will revert once I have a air leak free set up and start again.