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.

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now

  • Similar Content

    • By Sammy_1101
      Hi Guys,
      So I recently bought a 1997 XR400. I've done some research into getting more power through some basic mods and rejetting the carbs. Though all of the ones I've found have been for post 98 XR400s so the stock carb set up is a lot different. Usually people are opening up the airbox, exhaust ect. ect. and Jetting their carbs up to 160/162 for mains around 60 ish (I think) for the pilot. However for the 97 model this is close to stock for the jets.
      So essentially what Im asking (Just to cut to the chase ) Has anyone got a 96/97 (maybe 98, not sure) XR400 and has done these similar mods, and what sort of jetting have to done as a result.
      Thanks in advanced, also its my first post so apologies if I havent done everything correctly.
    • By Jturn
      38MM Cast SmartCarb. Purchased a couple years ago for a yz250. Don't have the bike anymore, and it has been sitting around for about a year. Used, but in fine working condition. You can also purchase a different needle for different bikes, as well as rebuild kits from their website.
      Asking $250 shipped to continental United States. I can arrange other shipping options at the buyers expense.
    • By faizakhtar125
      Hey guys I just bought a bike last night. So the seller said the bike will turn on but then die. We didn't try it because it was really late at night and it was raining (seat was wet). But he said if you bump start it, it will turn on and ride. So what do you think the problem is? I just want to make sure it runs... I probably won't be able to bump start it till Saturday (rain)... It's a four stroke, and I know absolutely nothing about them. Thanks.
    • By knotduke08
      I know this topic has been discussed in the past, but I have not found any options available for the new 2018 bikes. I live and work for a new Beta Dealer (Auburn Extreme Powersports) in California, and we are having a difficult time mapping the RR-S bikes for full FMF exhausts. We have tried contacting Beta Support several times for this issue to no avail, and we typically are given the runaround when transferring to techs and secretaries. From what we can gather, the FMF flash is designed for the RR bikes, but not the street-legal counterparts. Every time we try to install the FMF map onto an RR-S errors occur, and the programmer screen goes virtually blank. We have heard through the grapevine that some are using the RR maps on the RR-S with success, but we have yet to try it, even on our own bikes for fear that it will permanently erase or damage the CDI. Any and all help or suggestions regarding this topic would surely be appreciated! Thanks, Ben.