I am having trouble getting a new Mikuni VM 32 carb to work properly on my 2009 KLX 250 S. The only mod on the bike is a Muzzy header and silencer.

My main problems are a flat spot when I blip the throttle off idle, and an uneven idle. The engine dies from time to time when I come to a stop immediately after hard acceleration or climbing. I have a 27.5 pilot jet, the needle clip is in the mid-position, and the float level is on spec.

Has anyone tried this particular carb and can anyone help me with a list of jets that should work on this bike. I ride near sea level and up to about 5000 feet in elevation.

 

Odds are the slide cut away is too big. There are a variety of slide cutaways. The lower the number the richer the mix. It works from idle to about 1/8 throttle and still contributes up to about 1/4 or so. If you have a 2.0 cutaway you need to go to 1.5 or so. The needle doesn't really hit at that point with the round slide for sure.

To check, run the bike and block the air intake, that might work to show the lean condition. Otherwise you'd need to pull the air boot to be able to cover the mouth of the carb to cut air flow and prove the situation. My friend proved it to me on my old trials bike by sticking a couple fingers in the open carb to block air flow.

You can actually reduce the cutaway by filing/grinding the bottom of the slide to reduce the cutaway some.

Are you trying the 32 round slide, because you had it to try? Just wondering. The 90s KLX 250 ran a 36mm round slide, I wondered how it would work with the current ones when there was one of them listed on ebay for $120 or so.

 

Please print some results. I take it the carb was the standard round slide like would have been used on a two stroke.

Jetting and all?

I know it is common practice to put the round slide 34s on Triumphs, Nortons, and Yamaha XS650s. Maybe it is possible to do without the accelerator pump and still do okay. I guess we did it with the SR500s, why not the KLX250s. The round slides are way less costly and more compact.

Makes me wonder about the flat slides used on the two strokes. They're like 1/3 the cost of the pumpers. Gonna have to peruse the Planet KLX and other 300 based forums...

 

This is the standard round-slide carb that has been around for years. Even a minor adjustment makes a noticeable difference. So far I have a 27.5 pilot jet and a P-0 needle jet with the needle in the lowest position. The idle is OK now and the bike does not stall at stops any more. there is lots more snap than I had with the CV carb but I think that I am consuming more fuel than before. Makes sense, I guess.
I am still not satisfied and have more fiddling around to do. The bike starts and idles from cold without the use of the choke and in fact will not start with the choke. Also, when the bike is warm there is a flat spot if I blip the throttle from idle. The bike coughs and I release the throttle and it idles away again. Too rich? Too lean? I don't know as yet. Also, since the carb does not have an accelerator pump, maybe it would do that anyway.
This flat spot is not a problem really because when riding off-road, I'm seldom on the idle, even in slow-going. I have more work to do, but the nearest local source of advice and parts is an hour away.

 

What is your slide number? Look under the slide. Smaller the number the richer it will be. It affects the air intake, large cutaway (3 is fairly large) the more air, smaller will do less air. Works from idle up to about 1/4 throttle, needle works with it.

Few flat slides have any different cutaways, round slides definitely do.

My SR500 didn't do too bad coming off idle.

 

That's a big pilot jet. Especially on a smallish 32mm.

You not needing the choke to cold start it is the first indication the pilot jet's probably too big.

Try leaning out the pilot jet

 

I guess these carbs use an air screw to meter the amount of air going to the pilot jet? How many turns is yours out from fully seated?

 

When using a VM round slide the smaller throttle mix is not just the pilot jet/screw and jet needle, it includes the slide.



What is described in the first post is what happens when there is a lean condition right off idle. If the throttle opens too quickly it will go extremely lean and stall, if the throttle is worked enough (blipping the throttle to get the revs up) it is possible to get by the lean spot from the slide and get it up working the needle/needle jet. My one bike was so lean it was almost impossible to get the revs up enough to keep it running, totally unrideable. When a large part of the mouth of the carb was covered (cutting air flow) it would throttle normally. Too lean on the slide cut away, too big. In my case way too big. No amount of work with the pilot circuit or needle will clear it up completely. One or two steps down in slide size should help.

Here are some sources, one is a good easy read, the other is the Mikuni manual, what we used to fight with to figure out how to jet.

Been there done that with VMs on a 175 and a 250 two stroke and the 500 four stroke with VM34, 32, 24, and 38. Not done a flat slide so I have no idea if they use a variety of slide cut away sizes. The ones with accelerator pumps make up for the slide cut away with the pump.

Keep up on what you're doing. I'm still really curious about the round slide on the bike due to easy availability and low cost. As I've said before, the used them for years on the TT/XT/SR 500 Yamaha four stroke singles along with the old Honda singles from the 70s. They're also used with Yamaha, BSA, Triumph, and Norton twins. So it is clear they are a viable replacement for the CV carbs.