That air filter is horrible. Unfortunately it was inherited. It's now in the bin. Stock carb-to-airbox tube and K&N RC-1060 filter will be going on it. Original DT2 airboxes simply not available in the UK.

Have you removed the needle jet from the carb? It's stamped on the outside. Three numbers are the series and a letter -dash- number is the size. Should also have the Mikuni "square in a corner of a square" trademark stamp.

Stock needle jet for a DT2 is Mikuni 164- N8

Swoop: brilliant, many thanks.

Mark: NUUUURRRRRGGGGGGHHHHHHH.... Embarrassment upon embarrassment. The diagram I've been looking at - Haynes manual - names what I now know to be the jet setter as the needle jet. My carb man is referring to the needle jet nozzle.

See the image from the manual....

I suspected that, which is why I asked in my first post which manual you were looking at! I should have rephrased it as something like, if it is a Haynes manual, bin it! Genuine manuals are free to download from the Tech Library here.

Glad you're getting there.

Thank-yeeeewwww... That could be a life-changer.

Right, so, detail. Official Yamaha manual is extremely useful. However, there's a detail I need to understand: does the needle jet setter rest recessed (as per picture attached) or should the base be flush with the recess?

Is there a washer missing, or does it look like the washer is gunged to the base of the needle jet?

Meanwhile the needle jet itself: it looks like the semi-circular part at the top is damaged, perhaps by somebody trying to unscrew it using a tool not designed for the job? Is there a tool or knack for removing the needle jet? It seems pretty solidly in there. Being soaked with WD40 now.

Also, judging by the general condition of the carb main jet and banjo, it looks like it may have had water in there for a while at some point. In the pic they've both been cleaned up, but you can see how it has been chewed up.

Finally, where the main jet feeds in to the bottom of the float bowl, should the hole in the main jet centre with the hole in the float bowl? It doesn't on mine.

O-ring, main jet, needle jet and needle all on their way to me.

I've never seen a jet setter (#4 in the diagram) recessed like that but have less experience than others here. I'd think it'll be sitting too high up and the o-ring wont seal to the main jet.

Main jet, the hole doesn't need to align, that opening is larger than the jet opening by far - but you are missing a thick fiber washer on the main jet holder (#10). It'll probably line up better with this on.

The K&N RC-1060 has a high chance of being a nightmare. The oiled uni filters are cheap, filter dust better, and don't create any jetting issues. Measure the carb inlet and order away, get some filter oil while you're at it.
unifilter.com/online-catalog/clamp-on-air-filters/.

To add to Darins post , there sould be a brass washer between the needle jet and setter .
Without the washer , the O ring won't be sitting correctly in the well at the bottom of the float bowl .
Without that O ring sealing correctly , your carb will perform badly , running rich .

Brass thick washer No6 is missing. Jet setter should sit almost flush but not quite. I see they call the jet setter No5 the needle jet & No6 the main jet washer. All very confusing but so long as you know where everything goes & ignore their wording. If you need to remove the needle jet column just screw a 5mm screw in a couple of turns until tight as thread will be not exact, then give it a tap & it'll come out the top. Has an alignment pin when refitting it, just up from the base. Side mounted main jet won't line up with the jet setter/needle jet as it's cross drilled. Well i think it is--it is on smaller models. The Oring on the jet setter is super important for it to seal into the bowl.