YZ250 problems...Do I need a new crankshaft?

[kc1399]

Hey everyone, I'm new to the forum and I just got my first dirt bike. So I didn't have a lot of cash to spend and I really wanted a bike, so I got an old 87' yz250. When I got it, it had low compression and there was a lot of oil coming out the tailpipe. I went to the local moto shop and they suggested changing the rings and maybe the piston. So I started taking it all apart and then I saw the crank, and I'm thinking that it's not supposed to look like this. What would cause something like this to happen? I was told that there isn't an aftermarket crank, and I've called a few junk yards around the area and they don't have another crank. Is this thing done in, or do you think that the engine could run ok with the crank like this? Please help!!!!!!!

[kc1399]

So, I found this crank on ebay, and it doesn't look shiny like the one in my bike right now. Does that matter?

[MXtras]

I can't see anything wrong with the crank in your bike other than some discoloration. It looks like it sat in one spot for an extended period of time - not in water, more like in a mild acid or something.

What's more important than looks is how smoothly it rotates and if there is ant vertical slop in the rod.

To me, the crank in your bike looks better than that Ebay crankshaft.

Scott

[yamahammer01]

i agree with scott. just because it has some discoloring doesnt mean its bad. if it rotates smoothly and doesnt have slop up and down and side to side, i would leave it alone.

[kc1399]

Cool, thanks a lot for your input guys. I'll get this thing put back together this weekend and finally be able to ride it!!!!!!!!

[MXtras]

Flush out the crankcase (gasoline) while you have it opened up and drizzle several drops of premix oil on the rod bearing and into the holes on each side of the crank before you button it back up.

Throw some grease on the bottom portion of the studs and replace the base gasket. No grease on the threads, though. Torque dry.

Good luck, and report back how things go.

Scott

[bitshifter]

Since you have it all apart. A quick check for piston clearance is to put the cylinder face down on a paper towel. Take the piston without the ring and place it into the cylinder the way it should go, usually arrow to exhaust. Drop the piston and see if it hits the paper towel, clunk, or does the air in front of it slow its fall. If it is slow then it is probably ok and can go in with a new ring. If it goes clunk then think about getting a good measure. You are lucky that it is a steel sleeve and can be easily bored. If you bore it remember that the rotary valve may need to be bored also. Since you had oil coming out the pipe you should think about repacking the silencer if you have one. How much oil was in the crank? If the crank seal is bad on that side the oil could be coming from there. If I had the crank out I would change the seals.
Good luck, I love the early yz's. I have an 88 and an 89.

[kc1399]

Thanks for all the input guys, it helps out a lot. I've finally got some time to work on the bike this weekend, so hopefully I'll be up and riding in the next week. I tried that test with dropping the piston and it went clunk; I went to the local moto shop, they measured it and they suggesting a 1st over piston, with new rings and gaskets. So, I got those on order and I'm waiting for them to come in. Once they come in, they are going to bore the cylinder/rotary valve to match the piston.

bitshifter: there was premix gas/oil in the crankcase. I'm not sure how much of either.

I'm trying to get my head around how our engines work;

Is the crankshaft lubricated by the premix oil/gas?

Does the rotary valve shoot the premix into the crankcase for crankcase lubrication?

If the crank seal is bad, does it leak transmission fluid into the crankcase to get the white smoke?

Is the oil filler cap, the oil check bolt, and the oil drain plug all from the area that has transmission oil in it? And when you are supposed to "change the oil" it's the transmission oil that is being talked about?

On another note, with my carb, it looks like one of the points where a hose is supposed to be attached, the hose was broken off. It is the point on the bottom of the carb, which I believe houses the float. The other two hoses come off of each side, and were running to the back of the engine to drain on the ground. Does this sound normal? The broken piece of hose on the bottom of the carb doesn't appear to be right.

Thanks again,

Kris

[bitshifter]

Kc1399, I will do my best.
1) If you have premix in the crankcase then the float may be sticking or misadjusted. If you leave the gas on when you are not riding it may leak into the crank. It could also happen if you laid the bike over to work on it.
2) The oil in the gas lubricates the crankshaft.
3) I don't think that you have a rotary valve. You have a power valve that raises and lowers the exhaust port for a broader power band and a reed valve that acts like a one-way check valve for the gas/oil and air. The premix and air comes through the carb, through the reed valve, and as the piston goes up it is sucked through ports into the crankcase. As the piston goes down it is sucked through other ports uncovered by the piston into the area above the piston and then ignited. It is all very simple and very complicated at the same time. In a two stroke the piston is the valve.
4) If the crank seal on the tranny side is bad it will suck oil and will make gray smoke. If the seal is bad on the flywheel side it will suck air, run a little strange and destroy the piston and cylinder if not caught in time. White smoke is usually water.
5) Yes and yes.
6) The only really important hose is the one bringing the gas and if that were off you would know. The other are drains and vents, the hose keeps dirt from entering the carb. Next time you are in an auto parts store get about two feet and replace them.

Good luck, glad you are giving it a shot. I always figure that if someone can make it I can fix it. Remember that it is two hard to assemble it will cost too much to sell since most of the cost is labor. Most things are designed to be fairly easy to assemble, just pay attention and try to understand it, which you are.

[MXtras]

Great responses, Bitshifter!

Scott