Mikuni 32mm flatslide awesomeness!

Decided to go with this carb after kicking the daylights out the bike after spilling it. Thought to myself this is useless if I am going to do harescrambles and made me second guess if the xr would be better than a crfx(push button). Well, since the Edlebrock pumper has gone bye bye and the other pumpers just seemed expensive and daunting to set up and tune I decided to go with the Mikuni from XRsonly. Have to say the installation was smooth, love the one throttle cable simplicity and function which came with the carb. Now, when they say it is tuned that is a yes and a no. The carb is set for "average", meaning 3200' elevation. Didn't know this until I tried to run the bike and found it was running too rich. Stumbled at low rpms, flat build up and black spark plug. Looked at the paper they sent and made me thing, wait a minute, I am at 500' so that should make me lean. Called XRsonly a couple of times and found them to be very helpful. Basically the carb was intended for bigger bore applications and the bike requires the smaller jets. Which on the paper puts me at 5000' elevation(according to the paper). So I went with the smaller jets. The needle was on the second clip from the top so I didn't want to risk going all out lean and risk the bad things that can happen when too lean. So small jets was to be tried first. I was able to check the needle and change the jets without taking the carb off! :thumbsup: Took the bike for a ride and it was great. Revs much more. Better response. Starts one kick from TDC. Now, the real treat. Flooding. Did a harescramble today and spilled it good. Top of the bike was slanted down hill. Took a few seconds get myself together and left it up. Got on the bike and thought, ok, here is the moment of truth. 1 KICK! :confused: Other times I bogged out such as on a hill(amateur, :worthy:), find TDC and away I went. Now, I can imagine what this is going to perform like with the 277mm piston I have waiting to install. :busted:

Thanks for the review. What year is your bike and what was the final jetting?

2001 XR250, stock bore, stock cam,FMF pipe, ground welds on header, opened airbox, slightly ported and polished exhaust ports.

The carb came with 30, 35, and 40 for the pilot and 190, 195, and 200 for the main. Clip was second downfrom the top. Carb came set up with the 35 and 195 but switched to 30 and 190. Still haven't checked my plug since Sunday's harescramble but checked ok the Saturday before.

Bump, for a VERY helpful thread.

Just went through the same thing, but forgot to check TT before installing :smirk:

Same symptoms: significant bog to complete cut-out when opening the throttle. Figured it was electrical (had a similar problem on my XR4), but luckily I found this thread. Swapped the 35/195 jets the carb came with, and went with the 30/190. It's been raining all day, so I can't confirm all is good, but the bog is now 95% gone in a stationary test. Zero bog on the slow speed ride down the laneway.

Hope to get out on the trails tomorrow to confirm all is now good, but I'm optimisic. Expecting to go down another jet size when I take it out of the city (3.5 - 6.5k feet). Will likely tinker with the needle as well.


87 XR250, stock muffler, header welds ground, snorkle removed

Pilot: 30

Main: 190

Needle: middle position

Air screw: 1.5 turns out

tested @ approx 600'

So its worth the expense?

Ive got my carb on my '95 well sorted, no bog, but sometimes a bit hard to start after i drop it (only when it goes upside down :smirk: ), but usually 1st or 2nd kick.

Is it worth the cost?

It seems the forums were a little lacking on info with this carb; which is why I was so uncertain about it. As far as is the carb worth it? Well, I had a '87 XR250 with a rebuilt carb and didn't have any really bad problems starting. After flooding it didn't take more than maybe 5 kicks. As for the 2001 I now have with a rebuilt stock carb it was nearly impossible to start after flooding. Terrible. Perhaps I could have messed with it until it was like my '87. Thinking that I was going to go big bore eventually and wanted to get as much performance as I could from the bike without breaking the bank it made sense. The easy starting which was boasted is what sold me. With harescrambes you could be down in some very muddy conditions and the clock is ticking. Trying to start with muddy boots takes away time but also ENERGY that I need to keep me from bouncing off of trees! I have to say I am not only pleased with the starting but also the power through the whole range. If there is any sacrifice it may be in the bottom end but that is hard to say a "sacrafice" because it is extremely marginal. I think my '87 might could "LUG LUG, PUTT PUTT" a little better. Nothing a big bore high compression piston couldn't rectify. In no hurry though, I would rather be ridding the bike right now instead of tearing apart for the bore. Hope that helps. :smirk:

I bought this carb mainly because of the reports of easy starts after stalling. I ride a lot of tough single-track in hot weather, and having to kick the bike over 30 times after a fall is really draining. My stock carb was a bear to get started after a stall. If it fixes that, it was well worth the money to me.

Just got back from the inagural ride with the new carb. Nothing too demanding, just 30k of FSRs. Elevation from 500ft to 2200ft.

Cold start: terrible. About 15-20 kicks. Would not start with the choke on, and would stall if I pulled up on the choke ever so slightly when running.

Warmed it up for a good 5 minutes, and observed the following.

Idle: smooth

1/4 - 1/2 throttle while stationary: most of the bog is gone

1/4 - 1/2 throttle while taking off: very smooth, zero bog

1/4 - WOT while taking off: wow! Not sure if I was a bit shy of the throttle with the old carb, but it easily wheelies now. Zero bog.

All positions while riding: smooth, smooth, smooth. The bike ran good before, but now I think it runs a bit better. Hard to be certain without real objective data, but I was smillin :smirk:

Start after stall: wasn't riding technical stuff, so I had to simulate. Set the bike down perpendicular to the road, wheels facing downhill while running. After the engine stalled (maybe 15 seconds), picked up and tried to restart. 1 kick!!!

Tried again with the seat facing downhill. Could not stall the bike without lifting the wheels up higher :smirk: Picked it up after it finally stalled (almost 1 minute) and tried to restart. 1 kick again!!!!

Checked the plug several times, and it was perfect. Pretty impressed with this carb so far. I'll have to check the valves, hopefully that will resolve the cold start issue.

Final air screw setting was 2.5 turns out. Still may need smaller jets for higher elevations, and haven't touched the needle yet.

would anyone happen to know if the jetting would be similar for my CRF 230F with a big bore kit. I just recently rebuilt the engine and had to get a new carb for so I went with the 32MM Mikuni and i am unsure about the jet sizes. The top of my engine has been ported to fit the bike but I require different jetting so if anyone would happen to know, it would be much better than me fiddling with them! 



Each bike usually requires its own jetting solution, as cam timing and engine bore/stroke ration effect vacuum, IMO. I would start as a baseline with the 30/190 and possible have a 25/185 and a 20/180 on hand to try as well. The XR250 revs out higher than the 230F as I understand (I have a 150F but not a 230F) so your power curve is much different. 


One thing I found with the Mikuni 34mm Carbs is that the airbox, boot, and intake must be as high flow as the exhaust to get rid of the bog. I run an XR400 airbox boot and uni filter and it really cleaned it up and flat screams with stock 250 piston and XR's only long oval exhaust, stock header. running 30/190 and 35/200 is in range depending on temperature. I am selling my 34mm though as I have to get back to a push pull throttle. Cannot risk the single cable in my races anymore. Too many crashes that can cause throttle hangups. 

Thanks for the reply, we are trying out the jets today and I will let you know whats working and what isn't working. I believe we found the old jets in the old carb, so we are going to try the 25/165 i believe... My bike has a lot of mods including upgraded exhaust, airbox, intake and rev limiter. Also had some new racing shocks put on by the old owner so it rides like a race bike with a good amount of power on the trails. When we had it idling before it was fine but when we reved it up it would bog out. 


Also, do you run the Mukuni 32mm on your 150F?

Thanks for the reply, we are trying out the jets today and I will let you know whats working and what isn't working. I believe we found the old jets in the old carb, so we are going to try the 25/165 i believe... My bike has a lot of mods including upgraded exhaust, airbox, intake and rev limiter. Also had some new racing shocks put on by the old owner so it rides like a race bike with a good amount of power on the trails. When we had it idling before it was fine but when we reved it up it would bog out.

Also, do you run the Mukuni 32mm on your 150F?

My 2005 150f has 12.5-1 247cc, ported by EO, with their Stg 3 torque cam, compression release head and there 32mm flat slide carb.

The 230f crankshaft is now lighter than a stock 150f crank and the 150f flywheel is approx 8oz lighter than stock. This bike will loft the front end at will in any gear.

15-45 gearing, stock swing arm with 18" rear and 21"front wheels.

Could not be happier. But will be installing a 4mm stroker soon (262cc) and a Mike Coe "MC-1" web cam.

So I may be even more happy after that.

Edited by adnohguy

so we ended up buying this kit from a company called 6 sigma as they specialize in jets. The jet we currently have in the carb is a Mikuni jet 270 ( the sizes are different ) but the carb is dripping gas out of the overflow. Also, when we give it a lot of gas it bogs out and when we do give it gas the carb stops dripping for a little but when it is idling it drips. Anyone have any thoughts on how I can fix this? 

Yes the float is stuck if it is dripping gas. Or maybe the float valve has trash under it. Pull the bowl clean the valve inspect and set float level.

270 in TM32? If it is a genuine Mikuni it's way to large. Your bike shouldn't take anymore than a 200 or I'll be surprised.

Following this thread:  I have an '84 XR350L...the one with the dual carb set-up.  Thinking that I might have to convert it to a single carb set-up, using a Mikuni Carb and a manifold.  The folks at SUDCO claim to have an entire system all developed for this...anybody heard of this / any feedback on how well the Sudco system works?



Don't know anything about Sudco. But I'll let my 34mm flat slide go. Going to try a 33mm pumper with push pull.

finally figured out the jetting sizes and the bike is working great now! We ended up getting a 120 main and 45 pilot with the needle in the top position. The bike idles great and is going to be tested out tomorrow for the spark plug to determine richness or lean. Turns out the spark plug was fouling and causing bog on the throttle when we opened up the gas, so we switched it out and now there is zero bog! Thanks for all the help 

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