WOT stumble
 

So I don't know if this has just started or has always been there. I must admit that I wasn't the best rider when I bought my bike but I've improved enough to actually need full throttle. The problem is when I crack the throttle wide open the engine stumbles, I have to back off and let it build rpm's. Even when going 50mph twisting the throttle wide open produces a bad stumble, if you slowly open it up, the bike will speed up. It also seems to have lost its 'hit'. Any suggestions on where to start, I've checked the power valve operation with the engine running, it does open up. I have a 178 main, lowered the needle one clip when I got it and just raised the float a little bit after I had lowered it earlier in the season due to gas leaking out all the time. I always use fresh gas but I've been using regular pump gas. Should I try some higher octane premium fuel?
Throw some stuff out that I should check, this is my first real 2 stroke bike and therefore my first foray into jetting.
Thanks

Check the reeds. Make sure they aren't chipped/frayed. Make sure they lay flat and seal against the reed block.

Check for air leaks between the cylinder and the carb and air box. Most do this by spraying carb cleaner or starter fluid around the joints while the engine is running. If the idle speed changes due to sucking in the chemical, you have a leak.
Do so at your own risk and at least wear safety glasses. I've never had a problem but...

There's 2 cheap, easy things to check that you'd want to do anyway.

Partially plugged silencer
 

Might be worth a shot to pull the silencer apart and make sure its not plugged and repack it. Had a couple of bikes sputter on the top end related to my spark arrester getting clogged with packing.
Ryno

DaveJ, does your EC have an intake restrictor?

Dave,
I've been using Shell super only. Runs great, no detonation, no bogging anywhere.
You sure you aren't riding a lean running 4T ?
It does sound like you've got the float a little too low, but many other things could be happening too.
Mike

Change your spark plug.
I've had a rash of bad NGK plugs.

Hi!

I have exactly the same problem with my 07 250. You can read the other thread about "not 100% syn oil in winter..".. Main jet 180, clip in 2nd position (it was in 3rd.. now it's better), air screw 2,5 turns out. With -10 C I tried a 185 but it moved this stumble to almost all the rev range. Right now I get much less spouge out of the silencer and the bike works great except this full trhottle thing. Should I try a 178 jet? Where is this air restrictor if there is somewhere?

Thank you!

Leon

I haven't had a chance to try anything yet, apparently nice bike riding weather is also nice 'do crap around the house' weather too.
I don't want to do too many things at once. I think I'll put some fresh gas and a new plug in it first and try that. I do have an NGK plug in it so maybe I'll try a different brand. I also tried going up to a 180 main, that made a big improvement in the midrange power but i still had the WOT stumble. I may try the 178 main and raise the needle after I figure this stumble out. Next step may be to take the carb off, clean it well and check the reeds. I'll keep this updated as I try stuff just incase someone else runs into the same problems.
Oh, and no intake restrictor and no spark arrestor so those are ruled out.

This was my initial thought or gut feeling also.

I have had that happen, but only with the standard NGK BR8EG. My bike would still run but felt like the jetting on top was way fat. Put a new plug in and it was like a new machine!

I have been using the NGK IRIDIUM IX BR8EIX for a season and these seem to last. I have never had one foul. You are however gonna pay more for them.

I think that's the plug that is in it now, the NGK BR8EG. As for paying more, it's only one plug and I'd rather pay an extra $10-15 more than chase engine problems around. First thing I'm going to do is get a better plug, some fresh premium fuel and take it for a spin, that is right after all this new snow melts. It was 15C on Tuesday and all the snow was melted, today it's -7C and woke up to 4" of fresh snow...argh.