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250f Throttle response

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Feb 28, 2000
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#1
My 03 250f does not respond well right off idle when i whack it. It makes an odd bogg or even a backfire. Sounds like a lean condition. I fiddled with the fuel screw with no real improvement. I would think the needle needs to be raised a position or two. Any thoughts????

Making adjustments on this carb sucks. Not easy access like my 125's.
 
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May 20, 2001
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#2
Accelerator pump. For a quick fix, fiddle with the adjustment (probably a screw inside the cover the throttle cables go into) and make damn sure you know which position that screw was in when you started.

Also, adjust the idle as high as you can stand it, it will alleviate the symptoms, if not solve the problem.
 
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Apr 4, 2002
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#3
Yes, the carb is a PITA to work on, but it gets easier with a little practice. If you pull the hot start plunger, you can rotate the carb CW to pull the needle and pilot + main jet. Also, with the carb tilted, you can remove the float bowl to get at the leak jet.

The jetting you will need will depend on your exhaust system as well as temp, elevation, & humidity.

I have the FMF PC4 system and here's my jetting at 60F & 1200 ft elev- 172M, 38P, #3 clip position, and #70 leak jet. The bog is pretty much gone with this combination and the bike pulls VERY strongly. If this combo does not quite work for you, then there are richer L jets are available, but not necessarily from Yamaha. Instead, you can hunt down a power jet from Keihin carbs, which would be found on recent year KX & RM models. (lower number = richer acc pump action)

I have also heard of one guy who soldered his leak jet completely shut with good results. Probably not a lot of risk doing this, since the stock #100 is much too lean.

Good luck with it!
 
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#4
I soldered my Leak Jet closed. It made all the difference in the world. I went from the stock #100 to a #70, that helped but then I soldered it shut. Instantly better. Raised the clip up 1 position to #3. I will mention that before doing any of this, you need to check your AP squirt strength and duration. This is the thing that makes everything else work. Check out this webite.
www.thumperfaq.com
 

KDXfile

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#5
Bandit,
When riding that tight SERA single track and if you stall the bike, does having the leak jet closed tend to flood the bike enough to make it harder to re-start?
 
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#6
Gotta love Breezy Hill. Stalling? What is that? LOL. I haven't really noticed that. I stalled the crap out of it last week. I rode like crap. Had one of those days. Even got clotheslined by some vines right off my bike. I now have the official "Old School Enduro Look" with a nice scar on my neck. I think it is cool, my wife thinks I am nutz. I rode like 30 miles and said to hell with it. Didn't seem any harder to start than usual. I see your point though. I really think not twisting the throttle and using the hot start drill is the key though. Just clear it out a few kicks and then it starts. Even if it did make it harder to start, I wouldn't change it though. The response is just too good, especially after having to deal with that bog for so long. I tell ya, once you get the jetting down and a good soft seat on the 250fs, they are some good woods race bikes. I am going to start experimenting with flywheel weights next. Oh and I have an 03YZ250F just in case that mattered.
 
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#7
Thanks for the responces. I think ill have to learn to live with it. The thing that sucks about it is i had another 03 250f before this one and it didnt do it nearly as bad as this one. Oh well..
 
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#9
Originally posted by Bandit9
Dude, you don't have to learn to live with it. Can't you read?
VERY well stated, Bandit.

Malden,

My bike was pretty much as you describe before rejetting. The diff you may be seeing could be as simple as if the fuel screws were set differently. My advice- Get down to the Yammie dealer and come home with some jets.

Good luck with it.
 
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#10
Yes i can read, Thank you. What i was insinuating was i dont think ill be soildering anything in my carb shut any time soon. I dont know enough about this carb or tuning the bike to take that step. I dont even know what the heck a leak jet is!
I changed the needle height and it seems better but i can still get it to bog. Turning the idle does work though.

thanks for the help.
 
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#11
I know the new FCR type carbs can seem intimidating. Really, I know. I actually have never even touched a carb until about a month ago. I just starting reading and researching this bog issue on this site, www.thumperfaq.com, and thumpertalk.com. Seems like just about every single 250F has this off idle bog. I read posts from 2 years back. I read my manual and got familiar with the terminology and stuff. Once you understand what the leak jet is and what it does, you will see that it is no big deal. Totally reversable too. Basically what it does is control the amount of "extra fuel" that the AP squirts into the carb when you twist the throttle. Larger # leak jets leak less fuel hence the smaller diameter hole in the jet. Smaller # leak jets leak more fuel, hence the larger diameter hole in the jet. That is all it does. What is wrong with the 250F is that most of the AP squirts aren't producing enought fuel when we wack the throttle. The #70 isn't big enough and Yamaha doesn't go any lower the that. Just do yourself a favor, go down to your local scooter shop and purchase an $8 #70 leak jet and screw it in the same hole as the stock one you took out. Do not test on the stand. Go ride it for about 10 minutes. Continued below in new post
 
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#12
Notice how much more responsive it is. Now it will still bog, but not as bad. Then take your stock leak jet and have it soldered shut, filing the solder smooth, completely sealing it off. Now screw that puppy in there and go ride it. Your off idle bog should be gone. Again, you must set your AP squirt timing to where it just misses the throttle slide before it will be completey gone. You should do this after installing the soldered leak jet. Then go test it and it should be gone. All the leak jet stuff is mainly just for the idle to 1/2 throttle low speed circuit. Now you will need to experiment with the Main Jet. Easy stuff there. Don't be scared of the carb, just save all your stock jets and if you get nervous you can always go back to stock. I don't see why you would though. Riding that bike with stock jetting is like riding a YZ175F. You are not even riding the 250F.
 
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#13
" Just do yourself a favor, go down to your local scooter shop and purchase an $8 #70 leak jet and screw it in the same hole as the stock one you took out. Do not test on the stand."

Isnt it going to run the same when i replace a #70 for another #70??????? Anyway it sounds dooable so im going to get a leak down jet and try it. If the bog is produced by a lean condition whydoes blocking the whole leak down system fix it???? Wouldnt it get worse?
 
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#14
The leak jet controls how much fuel the AP squirt puts out in a round about way. Smaller the # more fuel. Blocking it off, gives it the most fuel. Therefore making the squirt richer to solve the lean bog. I don't understand your question about the #70 Leak Jet. Do you already have one in there? If so you can use any leak jet that fits your bike to solder up. It doesn't matter b/c it will be closed off. Good luck.