'03 CR250 with 265cc kit jetting?

[DVS ME]

Hi just wondering if any one has had experience with jetting big bore two strokes.

Helping a mate with this bike 03 cr250 mods are 2mm bigger wossoner piston ported full fmf pipe boysen reeds carb machined out 1mm to 39mm.

Fresh rebuild when we got it most of the work had been done it was 1mm smaller and worn out its 265cc now.

The plug is very oily the muffler has spooge smokes a bit.

seems ok to ride down low but doesnt have much top end doesnt pick the front wheel up much.

Have gone back to stock jets but seems like it wants to be leaner had bigger main and pilot jets but ran better leaner goes against what I would have thought bigger ccs more fuel?

Any help would be great

Thanks

[scott411]

big bores do tend to lose there top end, and not want to rev out,

i would suspect with boring the carb, and a big bore you would have to change jetting,

what oil mix are you running, and what sort of oil, as the spooge is likely from that, ?

[Tony.OK]

Very diff bike but when I did my 269 kit on my wrf I went down slightly on main jet, bigger piston in turn sucks more air so should draw more fuel on its own, possibly making it run rich on stock jet. Mine ran fine on stock main but top end was soft, went down one size and now it pulls crisp to limiter.
May be diff for 2 banger but worth a thought.

[gwynfryn]

Can never quite understand why people would go for a steel liner over a replate .
You should check the operation of the power valve and if you still have the mikuni tmx well, theres a reason people swap them out for a keihen pwk, difficult to set up but ok when you get there. Sounds like you need a selection of jets and some new plugs and start chopping.

[DVS ME]

Running 30:1 castrol tts
Just picked up a smaller pilot jet (27.5) and a 400 and 390 main so will give them a try as well as checkng the power valve operation.
I just want a bit more power less smoke and to clean the plug up a bit as its so oily hasnt fouled yet but i think it would in some situations.
let you know how I get on

[scott411]

i normally ran a bit less oil than that with my 250 2 strokes, normally ran 40:1, otherwise they would spooge, and that was running top 15 in the national class,

so that may help what you are looking for, and by the sounds, leaner may help as well, do you know how to do a plug chop?

google "reading a 2 stroke spark plug" there is some good info on their,

[DVS ME]

Bit more of an update changed to 27.5 pilot and 390 main rode well far less smoke sounds nice and crisp did a chop looked lean so fitted a 400 main.
Decided to take it for a ride with a mate and half way we swapped bikes for a bit and it made me realise how slow it is.
It comes on fine down low but has no pull you just have to change to a higher gear theres no point in revving it out the engine note doesn't even seem to change.
pulled the plug part way into the ride and it looked pretty good so dont think its jetting.
I am running out of things to check took the side cover off the power valve and double checked it.
It was missing the dowel stopper for it to rest on when closed so was closing a bit more than it should but started it and revved it and seem to open all the way havent test riden it since.
one more thing its hard to start when cold as in kicking for 5mins pretty good when hot also seems to have to much compression hard to kick over.
next things i will check are float level and do some checks on the spark side of things.
Any more help or ideas would be appreciated
Thanks

[noobi]

Checked the reeds lately?

Sent from my HTC One using Tapatalk

[DVS ME]

We put new boysen reeds in there a few hrs ago when the bike was built.
might be worth pulling out again to check may be installed the wrong way?

[DVS ME]

Ok so thought i would update as we have had some progress.
Fitted a dowel back into the power valve stopper.
Adjusted the float level which was low.
changed to what i thought was the std 420 main but on closer inspection its a 380 so looks like its been jetted for a pre 02 the bike is a 03 and 420 main is std for the 03.
went for a ride sounds and goes so much better I think the float level made all the difference.
Did a compression test and got 125psi but found that i wasnt kicking a full stroke when you flip the kick start lever out and put you boot on it if you lift your boot up higher than normal you heard it click one more and that allowed me to kick it up to 152psi.
fitted a 390 main and 30 pilot and it picked up a bit more so one the right track.
went for a quick test ride today started first kick from cold which it hasnt done before was nice and snappy.
Almost done with it now another good ride and a bit more fine tuning should have it sorted.
Thanks

[cheese]

I think that they compression is a bit low, my 04 CR is near 200 psi from memory.

[barty5]

i would have thought that 152 should be ok if its a new piston etc then once it beded in a little more it should come up some more. wouldnt want it to be much less though