Cascade Crew

nonsensekid20

Just took apart my dirt bike. (1989 Yamaha YZ250)

It needed a rebuild. I was out in the trail and it suddenly just lost all power. Kept it going for a minute, then it died and we couldnt get it going again.

So I took it apart, and the piston looks like someone took an air hammer to it. Then raked it behind their car on the pavement. (It's just the top of the piston, and the outside of the top.)

Also, the same amount of space on the bottom of the head looks the same way.

And the inside of the cylinder is all gouged up.
Kinda irritating considering I just paid 500 bucks not too long ago to get it re-sleeved.

Any idea why this happend?

BigDaddyVu · Posted: Thu Feb 10, 2011 6:25 pm Post subject:

power valve (carbon build up) not moving freely

Twilightoptics · Posted: Thu Feb 10, 2011 6:56 pm Post subject:

Yeah. It means something went through the cylinder.

Look for the broken part that's missing or missing pieces.

nonsensekid20 · Posted: Thu Feb 10, 2011 7:42 pm Post subject:

Carbon buildup in the powervalve housing?

I didn't think anything was missing. I'll have to look closer. Hands were frozen so I went in the house. Laughing

BigDaddyVu · Posted: Thu Feb 10, 2011 8:31 pm Post subject:

Scoring on cylinder walls on dirt bikes are very common. check the air box or filter if the hose clamps are tight they usually loosens up between runs and if you remove the cover for the power valve remove the servo arm and check if it moves freely by twisting it but not too much it might be seize by carbon build up. I had my 87 yz 250 resleeved and they didn't do it right my power valve didn't fit right. also check the reeds if the supports have broken off (usually cheap after market breaks)

nonsensekid20 · Posted: Thu Feb 10, 2011 9:38 pm Post subject:

Alright I'll do that.

Just a side note: Before just a little while ago the bike wasn't ridden much, but when you rode it you could go on about 2 rides and the plug would seize. But after awhile it quit doing it, and would crank over in just a couple kicks. WITHOUT starting fluid! Twisted Evil

Do you think this is related?

BigDaddyVu · Posted: Fri Feb 11, 2011 4:15 pm Post subject:

power valve opens at WOT and closes at idle they usually stick at either end if there carbon build up but sometimes. YZ need to run wide open don't treat it like a trail bike or you will get build up or foul a plug.

nonsensekid20 · Posted: Fri Feb 11, 2011 4:35 pm Post subject:

Lol. I've been ridin that thing around in like 1st and 2nd gear. The only time i get to open it up is on the fire roads. Twisted Evil

Before, when it was fouling plugs all the time it was ridden like a trail bike. until i got on it. Laughing

Twilightoptics · Posted: Fri Feb 11, 2011 4:38 pm Post subject:

Plug would sieze?

Explain...

nonsensekid20 · Posted: Fri Feb 11, 2011 4:39 pm Post subject:

or, foul. Sorry. hah.

Twilightoptics · Posted: Fri Feb 11, 2011 4:41 pm Post subject:

Oil foul or fuel foul?

One means look at carb.

The other means look at valves/piston rings

nonsensekid20 · Posted: Fri Feb 11, 2011 4:46 pm Post subject:

Fuel. I think it goes back to what he was talking about buildup. Rings had just been replaced and it was fouling plugs. Well not JUST replaced. But recently.

BigDaddyVu · Posted: Sat Feb 12, 2011 8:05 am Post subject:

You are running 32:1 mix gas right???

nonsensekid20 · Posted: Sat Feb 12, 2011 11:23 am Post subject:

Honestly I don't remember. I think i was running 50:1 and it seemed to run better.

BigDaddyVu · Posted: Sat Feb 12, 2011 11:54 am Post subject:

50:1 on synthetic for racing(racers rebuild between races) 32:1 regular (normal use extended reliablity)

nonsensekid20 · Posted: Mon Oct 01, 2012 4:16 pm Post subject:

How long can a bike like this go on 32:1 gas? I'm getting it rebuilt here shortly and just want to know how many hours I can get out of it before it goes boom again.

jchaussee · Member Joined: 14 Nov 2010 Posts: 1318 Location: renton

Two strokes can go forever on 32:1. You sometimes can try and polish the inside of the piston to get more life out of them. I dont know a lot about motors in cars, but two strokes arw about as easy as it gets. Keep carbs clean. Make sure its getting air,and ride.