Sounds like you might need some new rings. When was the last time they were changed?

 

the bike is fairly new but i will definitely look into that. It seems to have great compression... The kick starter is very stiff and hard.h

 

ok... so i cleaned the carb and it runs a little better but not really. The bike has an upgraded fmf gold series exhaust... when i opened up the carb the bike didnt seem to have an upgraded jet kit... could this be the problem?

 

well yes and no,
the term jet kit is overused. It would need to be jetted to the pipe or tuned.
fmf pipes are easy as any to tune to .
IIRC their website has a basic starting point to help you find the correct main jet and you may not need to change the pilot jet.

your problem may be a combination of things .
so let me give you an example.

you let the bike warm up for five minutes (probably about three to many) now extra gunk has built up the plug is starting to foul (not even close to my first choice for premix oil btw) now you run it on the track. the bike starts to burn off the carbon built up on the plug and the bike is starting to run better.
next time you start it 15 minutes later you don't let it sit at idle for any length of time so less carbon is built up and the bike seems to run great.

chances are the only thing needed is a slightly larger main jet . the stock jetting would cause your bike to be a bit lean if you were not jetted for the pipe. find out the stock jetting and then check to see if your main corresponds.
you would tell this by doing a plug check
run your bike for a while
pull in change the plug ( don't burn your hands)
with a new plug run the bike for a while at normal speeds normal riding not babying it.
without idle time.
run it , shut it off while moving without any idle time
check plug
it should be a golden bronze to black color not white
some race fuels can cause a plug check to be somewhat difficult .

 

yz250 2 stroke runs really bad...help please

My 2000 yz250 2 stroke runs really bad.. It idles great and when the engine is warm it bogs down at high rpms and doesnt want to go. I have to ride it for like 20 mins for it to ride like normal. I gave it a tune up ,new plugs...cleaned carb jets and still the same.... The bike has an upgraded fmf fatty and im pretty sure it doesnt have an upgraded jet kit... could this be the problem? What would happen if i were to put an fmf fatty without upgrading the stock jetting? Would it run like crap like it does now?

 

It could just be a tuning issue... so after it's warm it runs perfect?

 

Did it run perfectly w/o the fatty. Then its a tuning issue. Call FMF tech line they should have recommendations for your ap.

 

There seems to be a lot of guys here w/ FMF exhaust tuning problems. Try removing the air filter, better/ worse

 

I concur with Mac. the term "jet kit" is a serious pet peeve of mine. Accurate jetting changes at least by day, sometimes more than once over the course of a day. It's not something you can just order a goddamned kit and make it all better.

 

i called fmf and they said it needed a 170-173 main jet and a 50 pilot jet... is the 50 pilot jet the stock jet for a 2000 yz250? anyone know? He also told me to put the throttle needle on the 3rd clip position... is this from top to bottom or bottom to top? Is it labeled? He also said to turn the fuel screw out 1 full turn. Hope this works...

 

i called fmf and they said it needed a 170-173 main jet and a 50 pilot jet... is the 50 pilot jet the stock jet for a 2000 yz250? anyone know? He also told me to put the throttle needle on the 3rd clip position... is this from top to bottom or bottom to top? Is it labeled? He also said to turn the fuel screw out 1 full turn. Hope this works...

 

always from the top to bottom, 170 sounds right for most aftermarket systems. Do not know for sure what pilot size is in, usually high 40ish. But it is trial and error too altitude does play into it.

 

already posted

 

altitude, humidity, dust conditions, how plugged the air filter is, the ethanol content/octane/additives of the fuel, where your ignition timing is adjusted to, what the air pressure is doing that particular day and time, temperature. There's more but that's the big ones, if you're keeping track of all those and still having to change for no perceptible reason you're in DEEP shit.

 

I looked it up and it looks like the pilot jet is a 50 standard from the factory.

I would still verify this when you get a chance

here is where I looked
Yamaha online parts click here

 

thanx macattack... you have all been very helpful.... ill keep updating as soon as i get the jets put in and the carb tuned hopefully.

 

http://www.aaenperformance.com/V4_racing_engine.asp

down at the bottom of that page Olav's got a book on carb tuning. If you wanna get serious into this stuff I HIGHLY recommend that book. It's got all the Mikuni part numbers, diagrams, instructions. It's a small book like 50 pages tops, but it's really got everything you need to be a carb guru.

 

i bought my yz250 new in 2000 what a great bike , 30:1 is way too rich 40:1 is great with todays oil`s , iam at a 168 main and 50 pilot but have a boysen rad valve , that made me drop one size on the main because it puts fuel and air in faster and harder , not uncommon to drop sizes with those or the v-force 3 ,

 

the bike runs MUCH MUCH BETTER.... when im on idle and revv it seems not to revv up all the way... maybe half way. When i ride it still does the same thing... it seems like its not getting enough fuel... i opened the fuel screw alot and its still like that .

 

Try moving the needle clip to 4th position. It will liven up the mid range a bit.

 

is that 4th from sharp end of needle? or from Top?

 

Have you pulled the silencer and checked the baffle / packing ?

 

4th from top

 

Ok,
after messing with the carb and the needle for 5 hrs the bike runs much better.... when its in gear i can redline the bike thru all the gears... When the bike is on idle it doesnt seem to revv up all the way! Why is it like that?

Killswitch,: I havent checked the baffle in the silencer... does this play a big role in performance or just sound?

 

Sal

Both

It's REALLY a stretch ( especially these days with cleaner burning premix )

In the old days you had to pull your pipe / expansion chamber often and clean it out from carbon build up.

The build up would prevent good " breathing " and change ( essentially ) the chamber shape and flow characteristics ( wave propagation )

Any kind of " spark arrestor " wire screen would foul up and result in loss of performance, usually indicated by lack of revs and a much narrower " power band " ( funny since the " powerbands were pretty damn narrow anyway )

I heartily agree with all the jetting input, but with the paucity of information I've seen ( history with the bike, any recent engine work, etc etc etc ) you might want to consider the problem from backwards forward, in a manner of speaking.

General rule of thumb is if the bike " was running well " before, it's not jetting, which is usually the last thing to look at.

Just a thought from an old fart

 

could it be spark plug??? Im using iridium plug instead of the basic ngk platinums..... also... what is the reed valve???? does this have anything to do with my issue???? my main question is..... do all 2 stroke dirtbikes run like crap until they are completely warmed up... and i mean ....warm!!! The bike doesnt accelerate at all until ive taken it around the block a cool 15-20 times. Then the power unleashes.