90 yz250 running horrible.

[Willum]

i bought this bike for next to nothing knowing it didn't run. I got it running good for maybe about 2 hours and ever since Ive been chasing my tail in circles.
When I bought it, the guy who was selling it couldnt get it started. He swore to me that it ran and that he couldn't get the front brake to bleed.
I bought the bike threw a new plug in it and it ran. Took it out and ran it the next weekend and it wouldn't go into the power ranged. It seemed to be killing itself with to much fuel so I threw a 390 main in it, it had a 410 in it before. Bike ran great!
I was coming back from a trail to the truck and it started acting like the choke was stuck wanting to die and not run at all. I limped it back and ever since I cannot to get it to run passed 1/4 throttle at all.
Parts replaced
New Reeds, Boyesen Rad Valve, old ones were alll tore at the tips
New Air filter
Tired new plugs
I bought other main jets to lean it out like Boyesen told me.
Whatever I do it still doesnt run past 1/4 throttle just boggs down and wants to die. It smoke a lot more then it did when running right.
I think I had some clutch/trans oil dripping out of my exhaust pipe at the head. Could it be my crank seal?

[AJ_YFZ]

What color is the smoke? If it's white then it sounds like the crank seals

[Willum]

Yes it's white.

[xrrider1230]

could it be the carberator floats sticking not letting enough gas in to go more than 1/4

[Willum]

Originally Posted by xrrider1230

could it be the carberator floats sticking not letting enough gas in to go more than 1/4

I had the carb off 100 times and the floats seem to be working fine.

[xrrider1230]

try putting new rings in it

[HELLDOG488]

CHECK MAIN BRGS...

Do a leakdown test.

[Ford]

Yeah i recommend that you do a top end rebuild, rebuild the carb and also check to make sure the timing is right on it. Sometimes with oil coming out of the cylinder at the exhaust is a sign that someone is running it too slow and it is carboning up as the mix oil isnt being burnt up, although this is probably becasue as you said that it wont hit the powerband, etc. Did the last person who rebuilt the top end put the power valves back in the right sides?..if not then you usually get no powerband when they are installed incorrectly. Although if the top end is tired and the rings have no compression (do a compression test on the top end, check the spec for it) this too will not run very well or with no powerband, etc. Again the best bet is to spend a few dollars and rebuild the top end, rebuild the carb and check the timing on the bike too. Once you have done those things which are good things to do for a 2 stroke anyhow you will have cured lots of problems and allowed it to be diagnosed quicker too!

Goodluck!

P.S. If and when you do the rebuild, take lots of pictures, we want to see!

[Willum]

Well i got the motor out and I pretty sure there is trans/clutch oil in with the crank. I ordered a gasket kit, stator puller, rings for the piston and some odds and ends. The rod bearing feels good no up and down play and the I will dig more into it once I get the puller. The top end looks new, I am re ringing the piston just for safety sake. I wondering if I should just go ahead and order all the bearings for the bottom end. When you guys talk about timing a 2 stroke what is all involved? Just lining up all the gear marks? Is there a kit I can buy with every bearing I need? I will post pictures of my progress once my fiance comes home with the camera.

[bitshifter]

It could be the power valve. Take the power valve cover off and see if it is moving as you increase power. It is possible for the rubber gasket to pinch the arm. Here is a pic of my 1989, which is very similar to your 1990. [Only registered and activated users can see links. ].
I don't think that it is crank seals since the stator side will suck air and lean it out. The clutch side will suck oil but not so much that it will bog like you are describing unless it is really bad. I had a problem similar to what you are describing and I discovered that one of my floats was on the wrong side of the lever. It had been that way for a while because it had marked the lever.
I have been concerned that my 89 can suck oil down the splined crankshaft on the clutch side. To try and seal this I installed an oring between the spacer and the bearing. I have not had a chance to ride much because of the weather so I don't know if it is working. When you get yours apart let me know if you think it can suck oil down the splines.
[Only registered and activated users can see links. ]
Good luck.

[Willum]

Well I got the lower end all apart and both crank seals were leaking. The one behind the stator was pretty much hanging out of the hole and the other side I could see that gas/oil was leaking in. Everything looked great. The lower end is now all back together minus the clutch guts and stator cover. I hope them seals were the problem. I will try to read up on setting the timing.

[Willum]

SHE RUNS!
I haven't had a chance to break her in or ever ride her yet. But I started her up today and she sounded great! She would free rev in neutral which she didn't do before so that's a start! I will get her out tomorrow and see what she does.

[bitshifter]

Good deal! So much for my advice. Every time I saw bad crank seals It was very hard to start and it was never using to much fuel as you said initially but instead running way to lean. It almost always ends with money being spent on a piston and a cylinder.
Good for you, I think you'll like it.

[Willum]

I got a chance to break her in yesterday. I needed to put a better main jet in it started with a 350 and ended with a 370. She runs great! Pulls hard up top and has get throttle response.

[A-310]

Good to hear it runs well.