2012 yz250f stumble

I have a 2012 yz250f that I recently purchased. I noticed after purchasing it my had a 450 muffler on it. It was a Yamaha Canada demo bike for pros to try apparently and it has Lots of gytr goodies on it so I imagine the carb isn't stock either. Anyhow I race this bike in enduro and harescrambles and need to be below 94db so swaped muffler to the 250 complete with db dawg. Tried it out and it had a real noticeable stumble on +- 1/8-1/4 throttle. It feels like it is starving for fuel which I have seemed to have confirmed as it's better with the choke on. Here is the problem though, shouldn't the db dawg and longer pipe make it richer? How can this combo have leaned out the bike? It rocks when your hammering on it but down low putting around it sucks. It's better when the engine is hot hot hot like after racing or riding for an hour or two. I'm probably about 400ft above sea level and current temps are 10-20celcuis. I think that's about 55-70F.

Thoughts? I'm assuming it fuel or air screw but I'm totally baffled with what appears to be a lean condition when conventional wisdom says it should be fat.

Assuming from your post that prior to the muffler swap it ran well. If it was me I'd put the old exhaust back on to confirm it still ran well. If it does then I agree with you, that more restrictive 250 muffler should have made it richer, not leaner.


A rich condition can also cause problems. You may have crossed a line and now it's rich enough that it is having problems when the accel pump squirts. Multiple ways to address it based on what the root cause is. Fuel screw is a good place to start. Turn it in and if it runs a little better that confirms a rich condition. Then it's on to lowering the needle, smaller main jet, and/or fiddle with the AP.


BTW, I've got a 2013 yz250f and love it. It's stock except for some carb mods and an FMF 4.1 slip on. It runs awesome. The carb work I had done was an "Athena dual spray accel pump nozzle" and an after market "aspiration tube". Jetting is bone stock. No bog or stumble anywhere. Pulls seamlessly from bottom to top. That's in contrast to my 2009 ktm 250 xcfw that never ran well until I did a big bore and similar carb mods.

Edited by moleson10

the dbdawg will probably make you go 1 clip leaner on the needle

I know it should be rich but why if I pull the choke it runs fine? Choking makes it richer as we are giving more fuel right? This leads me to a lean condition on very little throttle. I can't see it being the main as it's one sweet ride when your "using" the bike. It has to be in the needle pilot or the fuel screw.

I'm a carb dummy, scared shitless to touch it for fear I making it worse. the fuel screw sounds simple enough. Where do I find it? I assume it tells me in the manual?

if using the choke makes it run fine then yes it is lean, but that indicates a clogged or too small pilot jet (pj).

if using the choke makes it run fine then yes it is lean, but that indicates a clogged or too small pilot jet (pj).


Ditto, classic clogged pilot jet scenario. You may have both a lean condition off idle and possibly rich after that. Swapping out the pilot jet is easy and a good first step with any carb issues. A number 42 pilot jet is pretty common if you're not sure what size. You can try to clean the one you have but its better to just buy a new one. They don't cost much and are hard to clean.  Once you get it out the size will be marked in the jet. You can remove the pilot jet without removing the carb or even removing the float bowl. There's a wealth of information on this site. Here's a good place to start.



Edited by moleson10

Ok. Will start there. Thanks guys!

What's the next size richer from 42?

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