250-500cc 2004 WR 250 with 300 big bore kit - power issue

Hey guys,

Just brought a 2004 WR 250 with a 300 big bore kit, heaps of extras like black excel rims, v force reeds, aftermarket pipe, magura hydro clutch ect.

Just took it for its first ride and it goes great, heaps of power down low, pulls great however doesn't seem to hit powerband like it should.

Before this bike I had a 2005 Wr 250, stock as a rock, and that seemed to have more of a kick in it as you reached the higher revs. Now its my first with the big bore kit, but the power curve isn't like any 2 stroke I've every owned.

Its got me thinking that possibly something isn't set up right like jetting for example.

At higher revs would incorrect jetting restrict the bikes power?

Very happy with bike however I definitely think it should rev out a little more and have more of a kick, thinking that possibly the jetting hasn't been changed much since the addition of the big bore kit.

Jetting will affect the powerband. So will a few other factors. The 2 things I would check are jetting, like you said, and the powervalve actuating rod. Both are really pretty straightforward. I replaced a top end a while back, bike ran great down low and in the mid, but was a dud up top. Like it wasn't getting into the powerband and really getting on the pipe like it used to. I took off the left side powervalve cover, and voila! The little black connector rod was cracked and had disconnected from one of the balls it press fits onto. It was about $2 for a new one. And has been running like a beast ever since.
General jetting specs for your carburetor, altitude temps you can find on a stick on this forum.
Good luck and let us know what you find out. Oh, and that bike looks like it is super clean! Nice find.

I suspect power valve. If it is not opening up all the way bike will not get on the pipe like a mad woman.

Thanks guys I'll check the power valve out, if that checks out I'll have to play around with the jetting. The only other thing I've noticed is it has a 2C Racing pipe, which I think is a Wr250 one, I wonder how that's affecting the power since the addition of the big bore kit.

Have a small issue with coolant coming out the overflow, gonna try a new cap tomorrow and take it from there. It's got a 1.1 Bar I was going to buy a new 1.4 or 1.6 Bar would that be ok?

Any of those would be fine. BUT, these huskys hardly ever spit coolant. So I would just systematically go thought your system checks. You will find the bug. Check our powervalve. It may be connected, but not opening all the way. Easy adujust. if that is good to go. Check the jetting, if you haven't already. It never hurts to put a higher pressure rad cap on. But like I said, these bikes hardly ever spit cooland.
Just take your time, have a beer and run through the bike. You are only at about 3 beers of time. Should be an straightforward fix.

Bike has blown the inner oring on the head, previous owner suppousedly changed them however im starting to doubt him a little. the outer looked new and had fresh silicon sealant, however the inner looked fairly old. Anyway ive ordered new inner and outer O-rings, apparently there the same from 250 to 300? anyway gonna get the head checked to see if its warped and if not put it all back together.

If the head is warped can I have it machined? and when refitting the head should you use silicon sealant on the mating surfaces? or should the orings do the job? Ive taken some photos so ill chuck them up now.

Havent taken the powervalve cover off, that's tomorrows job, all I really need to do is pull the cover off, give it a clean, make sure everything looks in good order, then open the throttle up and make sure its opening fully? also I noticed on another thread about checking the adjustment is at 50.5mm is this worth checking?

No silicone sealant is needed or should be used. As far as getting the head machined, it's not simple grinding job on a surface grinder, since the 300 head has the lip that protrudes. Are you in the US? If so, RB Designs is a good place to go for head work. He can also get your squish dialed in which will give you a little more oomph. http://www.rb-designs.com/head.html

I'm in Australia, so if it's out of whack I'll have to find somebody locally. Interesting on the silicon sealant as previous owner put some on the outer oring.

I read that an air leak can cause overheating/blown orings? I've got v-force 3 reeds in, I've noticed some cracking on the rubber carb intake boot doesn't seem to bad yet.

Lastly after installing the new orings how should I treat the bike As in should I do some heat cycles just idling in the yard?

Ride it like you stole it, orings don'e need any break in period.

pics

Would those cracks in the intake boot be causing me to blow out orings?

Pulled the power valve covers, right side fairly oily, marks on the cog and arm were lined up everything seems connected? Heres some high res photos, does it all look alright? how do you check if there opening? I grabbed the cog and you could turn it a little but not much.

The left side wasn't as oily. The small nut on the arm/rod was really loose like about to fall off (see photo) so I tightened this up (just as tight as I could get with my fingers from fear of breaking it. Apart from that all seems connected.

Never played with powervalves so please let me know if im missing anything.

At this stage im thinking of just putting it all back together tomorrow with the new orings and see how it goes, hopefully it doesn't blow them out again.

The boots develop cracks fairly quickly. It would have a lean condition if they went all the way through. I would replace it at some point to make sure. It may be possible that the previous owner didn't use OEM o-rings and bought some generic o rings that weren't the proper material.

Ok, looking at the pics of your head, is it totally flat or is there a ridge around the combustion chamber? It looks flat in the pics, but it could be an optical illusion. If it is flat and no ridge, that is the wrong head if it's a 300 jug. Here's a pic of member Flyin's head.

My head definitely doesn't have the raised lip....

The orings have turned up and there completely different to the ones that were in there, the inner one is of slightly smaller diameter, so I think that its possible that they have got generic orings.

Apart from the lip the head looks the same, im gonna try and put the new orings in and see what happens, because it runs well when together and not pressaurizing coolant.

If I need a new head whats my options? where can these be purchased and how much $$$???

Looked up the part numbers and it seems it has a 250 head on it....

Measure the bore, you might have a 250 cylinder. I don't think a 250 head is going to work on a 300 cylinder.

Ok done some research and it has a wr250 jug and head, however the bore is 70mm+ measuring with a tape measure. So my guess now is they have had the 250 cylinder bored out to accept the 300 size piston.

So im guessing I have the right head in that case, to match the 250 cylinder (bored to 300).

later today ill go borrow a friends vernier calipers and measure the bore properly, then just put it back together.