250-500cc Flat slide TMX38 vs newer "D" slide TMX

If I wanted to use a wr300 top end on my 2000 wr250, would I need to use a newer "D" slide TMX carb or could I use my older flat slide TMX? I think the older carb might work but I'd have no idea how to jet it since all the info on jetting the wr300 is with the "D" slide TMX carb. Does the newer TMX work better than the older version?

I think the newer TMX is at least less temperamental. I had a 2002 Honda CR250 with the older style flat-slide TMX. It was temperamental during different engine and ambient air temps throughout the day while at constant elevation. My new TMX (some say TMXX??) with JD Jet Kit is not temperamental. If yours isn't temperamental then it should work fine.

Yes, it should work fine but will need the newer intake manifold. I think 09 + or carb will rub and wear hole in bottom of it.

carb basics a flat slide has a snappier feeling so its preferred for motocross where the low rpm is not as critical
the round slide carb is the best trail carb due to its signal across the needle
the flat slides usually are motocross but found their way into the trail bikes because of the price, as in they are cheaper
now there are also newer flat slides than the TM, and the "D" you speak of is a trade off for trail/enduro

Im glad you posted this question as I don't know what carb I have.. My carb says TM in big letters and the slide is flat on one side and round on the other.. Any help would be great as when I read others jetting I'd like to compare apples to apples..

It's a Mikuni TM instead of the TMX which was developed after 1995. You could see what early 90s Suzuki RMs were being jetted at on the Pro Circuit website. They have a jetting guide there and those RMs used the TM.

Thanks for the replies from everyone. I purchased a brand-new wr300 cylinder sometime ago for some crazy reason and when she needs a new top end this time around I may do the swap. I also picked up a used 2011 wr300 pipe for it. I'm still not sure if I will use my old flat slide TMX or get a newer used TMXx. With my current setup and the org. TMX carb, the engine is definitely not clean on the bottom end and then it comes on the pipe very strong. Last thing I need to complete the 300 swap is a cylinder head...

There's probably a dozen guys here on the cafe that would sell you a newer Mikuni since they have gone to a Lectron... Me included. I have one off my 09 WR250 with 2 needles and a pile of jets that been in a plastic bag for two years. PM me if you're interested but my recommendation is buy a 38 mm Lectron.

I like the old flat slide carbs . When swapping back and forth between new and old one on a 09 CR125 I liked the old one better .
With a fresh top end my old 08 wr125 ripped with that carb and no jetting issues .
The old ones are also 10mm ? or so longer too so you d need to look at the intake boot length possibly as well.