Yz 250 the rock has finally let me down

I can't get my 06 yz250 to start it's always been the rock. 

About two months ago I took it apart to swap the top end it was running fine no issues. Upon inspection the corner of the one intake port flaked a little  bit in the corner was the original plating. So sent it off to power seal fresh plating looks good.

New wiseco piston even replaced the reed petals with oem. I'm getting spark I'm getting fuel but it won't even pop a little bit. Took carb off and cleaned it out good again. Tried a couple different spark plugs compression feels excellent.

I didn't mess with the jets they are stock I rebuilt it completely only two years ago crank and bottom end are good. 

I took out the plug and kicked it over and I was getting some smoke out of the silencer. I feel like it's acting like fuel starving but there's plenty of fuel in the carb and nothing was blocked. 

Gonna check a few more simple things to test. Kind of bummed first time the rock has let me down since I bought it new in 05. 

3 hours ago, SmokinJoe said:


Timing ?

Edited by kxrob

Maybe there is a wasp nest blocking the inside of the pipe. Or you left your intake cover inside the airbox when you took your air filter off. :) 

Perhaps a carb leak around the intake/reed block. Or something wrong with the choke circuit so it's not pulling fuel.

When you say "tired a couple of different spark plugs", were they new plugs, or something you had laying around.  Don't do ANYTHING until you put in a new plug (a BR8ES is $2.49 at any auto parts store).  

The second thing I would do is lay drain the float bowl to make sure you have fresh gas in there.  

After these steps, then you can try other things.

Edited by rpt50

No they are just the plug I had in it last when it ran and like a previous one. It's getting fuel into the bowl. I am getting some new plugs to try I hope it's that simple. I didn't touch the stator or fly wheel area basically all I did was have the cylinder replated and put in a new top end and some new reed petals. 

Pretty simple nothing's in the intake tract by accident it should be starting. 

I don't use pump gas only race gas choke on or choke off doesn't make any difference right now.  It ran awesome before I took it apart the plugs I have do show good spark I smell gas on them and see gas on the reeds. 


Am I reading it right that it hasn't started at all since replacing the top end and getting the cylinder replated?

Hopefully it's as simple as a new plug.  I've had some that visibly sparked but would not start the machine.

With proper compression, fuel, and spark, it's difficult to get it to not start (running well is a different thing, heh).  It's a fuse begging to be lit.

We had a mouse nest plug an expansion chamber once; bike wouldn't start until we installed a different pipe. :shocked:

Also, "race" gas (or fuels with higher octane) don't always have the many additives regular gas has -- like detergents, anti-gumming agents, etc. We have had pilot jets plug-up when bikes (running race gas) have sat for too long. Just a thought.

Good luck with The Rock, and let us know what you learn. :thumbsup:



Double check the pilot jet and its passage into the carb throat.

Check float hieght, sounds like a possibility, just had this happened a few weeks ago when putting top end in daughters bike, I don't know how many motors or carbs I rebuilt, but must have bent the tang by accident, first time for everything 

Is there a chance the new piston was installed facing the wrong way? The arrow should point towards the exhaust port.


Nope piston is in correctly that's not the problem. 

Spark is a good arc?

Try and screw the plug cap on a little tighter for a better connection.

What you did and replaced should have no effect on why she shouldn't fire.

New plug nada still so off the carb again I better check and see if anything is worn out too. I haven't had to touch it much since I bought it new in 12 years. The joys of the process of elimination just a bit of a wtf moment. 

Blow on expansion chamber to make sure it's free flowing same with silencer. Had a mouse nest in my dads bike. Make sure all electrical cables are plugged in tight from removing. I saw that you put oem reed petals on, is it the one reed cage? Hand crank the bike slowly and see if you can feel any thing grab? Did you change fuel mixture for break in? Definitely check each jet for proper flow of fuel. Check the intake box.

You try unplugging the tps?

Put a cap full of gas down the sparkplug hole and quickly install the plug and kick, see what happens. If it fires it's a gas issue

Put a cap full of gas down the sparkplug hole and quickly install the plug and kick, see what happens. If it fires it's a gas issue

Or a SMALL squirt of starting fluid in the intake
3 minutes ago, Doc_speeder said:

Or a SMALL squirt of starting fluid in the intake

Do not use starting fluid, too hott

Do not use starting fluid, too hott

A small squirt of starting fluid will not hurt it. I've done it lots of times. Don't keep spraying and running it. Just a little squirt to see if it fires.

That's like staight 2000 octane, I didn't actually measure but damn hot. Do not do it! There's a time and place and this isn't it.

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