Flatside carb in an XR400?

Has anyone had any success running a flatside carb in a 400. I have very restricted room due to a 400 motor in a 250 chassis.

The '96 that I am currently riding has a 34mm flatslide Mikuni (non pumper) on it since the bike was new, big improvement across the board over the stock carb.

You can do many mods to the 34 flat slide,Also bore to 35mm.

34mm seems pretty small for a 400. I read some are running as big as 42mm. I have a Mikuni TMX38-18 Boswell modified flatside but I am finding it almost impossible to get it to run lean enough. Its so heavily modified its hard to know which way to go even after to speaking to the man who modified it. I am happy to buy a standard version of the same TMX38-18 if anyone thinks it would be possible to get it working ok. After spending a lot of time looking it looks like this carb and another TMX38-1 which I thing was used on 1998 YZ250's may also fit. On my TMX you cant remove the needle jet which means the leanest needle I can get is a 59. Not sure if the TMX18-1 has a removable needle jet?

The 34mm is small for the Xr4 but for a trail or woods bike works good. Wide open running ,hill climbing,it may hold it back.

Anyone tried a 36mm or 38mm flatside or are there any posts regarding this?

None of these carbs are really optimal, you really want a pumper or CV to get rid of the off idle bog.


38MM PWK should be a good carb, the TMX doesn't have as good a selection of needles.


I have to ask, why are you stuffing a 400 in a 250 frame? Why not buy a 400.

The pwk looks to have a round carb top which may make it too tight to get in. The tmx38-18 is tight even with its semi circular top. Do you know of anyone fitting the pwk with success and what sort of base settings it ran? Are these easier to set up than the mikunis and do they have a removable needle jet?


I bought the bike semi finished with the motor fitted. Also has electric start which makes it even tighter. The tmx touches the starter cable so is at max depth downwards. 

The Boswell modified mikuni flatside tmx I have now keeps sooting the plug. I have very large bore headers. Could this be making the problem worse at low revs?

Ok. I've junked the Boswell. I've tried 10 completely different settings and nothing works. I found a TMX 18 -1 that's standard. I've had to hack the frame about to get it in. Will update next week.

Whilst mine is only a XR250R, not a 400, I am superhappy with my Mikuni Flatslide 34mm from XRsOnly. The Edelbrock pumper carby that I had before was a smoother unit, but kept clogging inside. The Mikuni definitely performs, but the power can come on quiet abruptly. If the bike is regularly riden I'd consider the Edelbrock, if not the Mikuni seems to not be as sensitive to being left alone for a while.

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