99' yz 250

Hello all,


I just picked up a yz 250 a couple weeks ago for 1100 bucks and im still not sure if it was a good deal or not. The bike is in overall good condition other than its going to need the back shock replaced which i was figured was 200 bucks most. The previous owner is an old friend and he traded it at an auction from a guy that was pretty wealthy and old, so we both figured it was in great shape engine wise. Of course there is a decent amount of rust on the underside of frame, but not much else where. 


Sorry for the short story.. but either way i rode it before i bought it and boy did it have some balls. It kicked over first kick and everything but my only concern was that after it warmed up, when you would stop or pull in the clutch some times to stop it would idle erratic and kind of high and not normal sounding. now we both figured it was just an old gas mix or just a wrong mix so i ended up buying it. I replaced gas with 93 octane pump and mixed 32:1 with yamalube, checked coolant good, and checked trans oil and looked pretty clean... i then proceeded to take it down to woods at my house and it was running good and great but after a while it started doing that high idle shit again so i was messing with air screw figuring maybe it was too lean so i turned it in which seemed to help but not really? i then took it home and took apart carb hoping it was just dirty or had plugged jet but it looked spot clean? then the day after it i started it and it stalled on me then would not start again? i'm thinking i possible got some water in the carb somehow so i drained carb took out plug and let her dry out while kicking it a few times with plug out. it then fired up the next day but still high idle!? it also oveflows coolant alot and drips oil from pv overflow which seems to be kind of milky brown looking which is the main worry i am having..


Sorry for saying so much but i just wanted to be as descriptive as possible so if someone can help me out all the details are there. i also am just wondering if i should try and just get my money back if it is a blown head gasket, ring, or water pump.. basically something i do not have time for right now.


Thanks ahead of time if anyone can help me.

Edited by Banger92

Erratic idle or high idle could be ignition side crank seal air or maybe air leak at the intake. If you fill the radiators to the top when it warms up a small amount of coolant will be lost as it heats up and expands. It should only happen once. If your always pushing out coolant it maybe overheating or it could just be the cap is not holding pressure. Pressure test the cap. A little oil out of the PV vent is normal. Milky oil ? Condensation?

alright so i guess i could take off the ignition side cover while its running and spray some starting fluid or something similar? and i'm not sure i never filled the radiator. he said he filled them up and i'm guessing it was when it was cold all i know is i checked it with everything else and it was pretty much up to the top. and would it not be leaking from cap then? if not how do i go about testing that? and i'm completely new to 2 strokes so i didn't know if i was pulling coolant into cylinder some how or was it just from getting water in carb? i have no clue. i read another post on here and allot of guys were saying that is normal.. but that doesn't seem normal or good to me unless there is some explanation. but back to high idle is replacing the crank seal involve removing the crank? sorry if that's a dumb question. some seals don't  need part removal..


thank you for the help and suggestions.

Edited by Banger92

i checked for air leaks around carb with starting fluid. and i dont think there is any.. idle didnt seem to change. also i dont understand how it could be a carb problem becuase the jets are already bigger than stock? so im guessing its leaking somewhere else, like at the crank seal what you said?

Change the jets back to stock.

And adjust the fuel mixture screw (underside of carb) while it's warm

ok i will try that and see if that helps. but isnt there only a air screw on 2 strokes?

Hello Banger

Well done on finding a YZ250.

First thing is the check that you've actually got a '99 YZ250 and not something else. Either post a pic or run a vin check:



Next, head over to the make and model specific forum for YZ 2-strokes. They have a whole host of YZ setup info including jetting specs, links to service manuals, a few gurus and a poodle. Your YZ carburetor most certainly has an air screw.

Here's the link to the YZ FAQs found at the YZ 2-stroke home: http://www.thumpertalk.com/topic/1032448-faq-yamaha-2-stroke-read-before-you-post/

hello swappa


thank you your very inviting and helpful.. i realized a day after i posted this that there is yz 2 stroke specific forms, duhh. but i will head over there and post some pics and ask on there what people think. im pretty sure it is a 1999 because i checked vin and i think its the 10th digit which shows the year, i could be wrong..


but thanks again

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