Help me to get my YZ 250 started!

Just bought a used '04 YZ 250 that was set up for woods that won't run. Appeared to run rich at purchase (lots of smoke out exhaust and wouldn't clean out while accelerating) which owner said was from brand new top end replacement and that he was running a rich ( I think it was16:1) mixture for first tank break in period. Didn't have a compression tester but it seemed similar to other YZ 250's i have had kicked over. Owner stated crank was checked and was within tolerance so nothing was changed except top end. Got bike home and barely able to ride as it would only run or clean out with near wide open throttle then just died on me! Noticed a lot of gas running out of the over flow tubes after trying several different plugs and different throttle positions while kicking over and even tried the choke once which I am sure wasnt a good choice. Decided floats must be stuck so took apart carb and cleaned and inspected with nothing clogged and no residue. Checked float height and was off minimally so adjusted per manual. Had 178 main 50 pilot. Replaced with 175 main and 48 pilot. Air screw was at 2 1/2 turns out and i tried 1 1/2 as well. Air cleaner was clean and not over oiled. Drained gas tank and got fresh 93 octane gas and mixed at 42:1. Plug was very wet tried new one no luck as it would start but then immediately stall. Interesting note is previous owner said he never had to use the choke to start bike even when cold. A lot of splooge out the exhaust from when it was running earlier. Does get good spark and bike will start for about 5-10 sec if I clean the plug but then just dies out. I have no idea where to go from here as I have never had problems with any other two stroke I have owned! Any suggestions would be greatly appreciated!

how was your needle when you looked at the carb? set in the right slot?

it sounds like you're running pig-rich

did the previous owner say anything about crank seals?

Hi there

I would not give much about the previuos owners stories. Look 1rst if you have a spark, if yes take off the carb and clean it, have a look at timing and at least pull the clutch side engine cover to see if there is movement at the crank.

Smoke which does not clear normally indicates the seal of that side is gone for good. But it should still start.

Good luck


You are really rich, and your carb is probably way out of whack. If you are dumping gas out of the carb even while the bike is running, you probably need to set your float height properly, and you may need a new fuel inlet needle valve. They are a part that is supposed to be changed out every 2-3 years. Check the jetting section in the FAQ sticky up at the top of this forum.

I would pull the cylinder head and see what your dealing with. Check for tolerances and feel the crank for yourself. No point in risking it when it takes an hour to check and just a gasket.

Did he have receipts for the top end?

+1 on what others have said.

You really sound like you are jetted super rich. That, or something is in the carb is definitely messed up.

If it were me, I would do the following:

1) Tear down the top end and check the piston and cylinder and PV linkages while in there. Also check the crank free play and see how much residual oil is down in the bottom of the crank valley. (Side story, my brother poured some oil down the spark plug hole on a CR125, never really explained why he thought that was a good idea, and it migrated to the very bottom of the crank valley. The bike would fire and run, but would not clear out or run clean.)

2) Check the carb and reset the jetting back to stock. Including the needle and position. I believe the stock jetting should be:

Pilot - 45

Needle - 55

Main - 178

Needle - N3EW

3) Check the crank seal on the clutch side.

Once you have done the above, reset the carb to stock settings and see what happens.

Also, the Jetting FAQ is a plethora of knowledge and very easy to follow. From the FAQ: "REMOVING (leaning) oil from the GAS/OIL mixture makes your AIR/FUEL mixture RICHER, effectively making your engine run RICHER (more smoking/spooge) . If you remove oil from your premix mixture, you have more gas in a specific amount of fuel. Making the mixture that really matters, the air/fuel mixture, richer. Do not fix jetting issues by changing your premix ratio."

Thanks for the reply as this bike is just frustrating and to make matters worse i unscrewed the throttle position sensor to clean all parts of carb and then noticed the manual said not to unless you are replacing it or it could cause a performance loss. Needless to say this is now out of my league so its in the shop and they suspect a crank seal. thanks for the help!

how was your needle when you looked at the carb? set in the right slot?

it sounds like you're running pig-rich

did the previous owner say anything about crank seals?

The needle was in position 3. I ended up bringing bike to the shop and they believe but haven't diagnosed it yet as a crank seal. To make matters worse while cleaning the carb i took off the throttle position sensor which the manual said i shouldn't have done unless replacing it or it might cause a performance loss. I learned a lesson - don't take things apart without reading the service manual first if your a rookie mech! thanks for your reply!

I would pull the cylinder head and see what your dealing with. Check for tolerances and feel the crank for yourself. No point in risking it when it takes an hour to check and just a gasket.

Did he have receipts for the top end?

thanks for the reply - brought bike to shop and they suspect a crank seal. Waiting to hear as i have never had problems with a two stroke like this before cause i usually buy newer machinery but wanted to save a buck!

I will share a handy trick, next time check for spark. If spark is there, turn the gas off and place an unit weed burner torch in the airbus. Kick the bike over, the propane is pre atomized and it will light as long as you have spark and compression. Once running, turn the gas on and start backing off the propane......done.

Also, no matter how hard you try to clean those pesky little pilot jets, it's just better to spend the $5 and put new ones in. I think you've got some good advice from the above posts, and be wary of the dealer....they're out to make $$.

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