All NEW to me: Mikuni flatslide at 4000k (main)?

I've never dealt with the main jet sizes on a Mikuni flatslide. Just the carb on my 1993 XR638 wtih K/N filter, supertrap pipe, extra cooling fins welded on cylinder head.

The bike doesn't seem to be very strong up top or through the mid getting there, kinda gets weaker once past 1/3 throttle. Changed plug and it was black with soot. could be from it being flooded so many times throughout the years by previous owner. New bike to me last week.

Main jet right now is a 240. 3-4k feet eleveation is generally riding area with temps from 80-90 degrees on average. What can you recommend on the main to get me closer to the ballpark?

which mikuni is it?

230 is definately too rich.


It's the TM or TMX. I'm not sure how to tell the difference between the too.

accelerator pump?

I'm pretty sure it doesn't have an accelerator pump but how do I know, just to be sure?

Here is a pic of it. Can't see anything that would be an accelerator pump. Look s pretty basic, one manual choke and one throttle cable.


ah,2 stroke carb.

well,i dont have any jetting info for that carb really.

i would say the main jet needs to be more like 150.

lean the main jet a few sizes at a time till it revs fully.

Looks like the same carb as I have on mr XR650L. Recently got it, and it's seems I having more trouble with the bike than I should and it's related to the carb. I know the basics of how carbs work, but I'm a bit clueless as to how to how to tune it up. Is there an issue having a 2-stroke carb on a 4-stroke machine?

Trouble I'm having is setting the idle correctly, and I have to slowly rev the throttle up or else it will bogg out off the line. New to MCs in general, so let me know if there is additional info I can provide that could help someone diagnose my troubles.

Thanks in advance.

they have no accelerator order for the carb to have good response it has to be jetted extremely rich on the needle and slide cutaway.the side effect is it will run poorly at steady throttle.

I finally got this bike running pretty well, we do have to adjust the idle mixture screw now and again but no big deal.

so what were your settings?

I didn't even mess with with the jetting afterall. My bro took the bike home with him down to the lower elevation of Las Cruces, NM and Tucson area so I can't tell you exactly what the jets were that we left in it.

If I were you I'd call XR's only and see if they have a baseline.

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