2 stroke questions.

[Bara]

hay guys,

well im about 3 days away from buying a 02 cr250.

condition. i would say good, but not great.

- front brake needs new fluid, better pads, and probably a braided line for extra feel ( probably just me being picky from coming of a motard )

- chain is ok, and so are sprockets, but they will need to be replaced soon.

- minor dint in the frame, i cant see any sign of cracking, but the frame has been polished.

- rear shock needs a new spring, ( for my weight )

now for the engine,

my mechanic pulled the top end of and inspected the piston and such and everything was fine, pretty much brand new. now he says its been sitting a while and the the plug fouls a bit possible because of needing a good ride and the mixture being to rich or not jetted right.

i took it for a test ride and the first 2 gears where really short, 3rd was sweet and the power came on great once it was on pipe.

but, in the first 2 gears, down low in the rev range the engine struggled, as if it was bogging down, however in third it seemed okayish- would this be because of the mixture/jetting with the gears being so short?

i took a look at the pipe and it had this black oily stuff in it.

at the end of the day he wants 3 grand firm ( the bloke selling it blew his 450 up and is selling the 250 to pay for the 450 ) he wanted 3500 orignally but a month, he went down to 3.

what do you think the bogging could be? once warmed up she started first time everytime.

just want some extra opinions before i put the cash down. i trust my mechanic and he assures me that it is definenatly jetting and or the mixture. is there anything else i should be aware of before buying this model?

thanks guys.
Bara

[albonator316]

A good mate has a cr250, yes they have short gearing but he did the finke last year and was able to gear it to do 150 in top. I road it and the engine was amazing (2005-2006) in every gear, couldn't keep the front on the ground. At the time it was gear to about 80km/h in top. It has to be the jetting, too much fuel mix going in and not all being burnt so that will explain the black glug in the pipe. if you can get the jetting right that will get rid of the engine running poorly in the first few gears. The gearing shouldn't affect the egine running so it was the jetting not being affective in first few gears but better in the later gears.

[Bara]

thanks alb, so the reason why 3rd pulled really well, and first and second were crap would be a jetting issue, so the only reason why 3rd was going sweet was because of the higher ratio it had? but i guess the main thing is that it is fixable and is nothing serious.

[albonator316]

Yeah has to be jetting, the black oil in pipe says it all really. Sounds like maybe its stock jetting with an aftermarket pipe that requires different jetting? Jetting can get confusing and can be time consuming if you're doing it for the first time. There are jetting kits out there or you can find dedicated cr250 forum and there will be guys who will know what jetting you need for what mods etc.
or do what i did what get a whole new carbi jetted to your specifications, i don't have the patience for it and have never rejetted. After read the correct process of what to do it would take me a whole weekend i reckon
what carbi does the cr250 run?

[Bara]

38mm Mikuni TMX flat-slide

i forgot to mention the bike has a full pro circuit exhaust system on it. how hard is it to jet the carby?

[albonator316]

first you have to see if running rich or lean. obviously running rich. then you have to see where it is rich. 0-1/4 throttle or 1/4 to 1/2 throttle or 1/2 to 3/4 or 3/4 to WOT.
I don't know much about the mikuni carbi, mines a keihin.
you get the bike to idle 500rpm above normal, then adjust the idle screw to where the engine idles at the highest rpm. if the revs keep building up more than 3 turns out its to rich. or revs still climbing and screw turned all way in too lean. should be around 2 turns out. To change that you have to get a different side throttle body.
1/4 to half throttle is main jet and neddle territory they kinda over lap each other. so you run your bike up the road at 1/2 throttle. then take spark plug out, if its covered in gunk its to rich so you get a different size main jet etc. to goes on like that and its hard to get right as you change the main jet it affects how the needle works. and the neddle doesn't just come in different sizes it has diffect tapers because different tapers affect the bikes air to fuel ratio at 1/2 throttle to WOT. in all this i think i confused myself :lol:
if you got the time and patience go for it as you'll learn alot, or do what i did and get a pro to do it.

i have a few 2-stoke tuning manuals from web links i'll see if i can find them again as it will give you a better explination.

[albonator316]

this ones for keihin carbi though
http://www.duncanracing.com/techfaq/Tech_keihin-carburetion-jetting.phtml

this ones a good read, 8mb download
http://www.edj.net/2stroke/jennings/2stroketunershandbook.pdf

[albonator316]

here is a mikuni based one
http://www.iwt.com.au/mikunicarb.htm

[Flash]

Usually the black oil is because it isn't being revved enough! Very common on MX bikes that get used on a track not quite as bad in th bush.
A 250 should be geared to do about 120 Nick not sure why your mates was so off! THe stock gearing on my 125 will pull over 100!.
The other think with the pipe being really black could mean the power valve is very gunked up this will really efect the way the bike feels and rides! If the power valve is oiled up on my bike there is no real bottom and a weak mid the top seems ok but not great. But my bike doesn't usually get oiled up unless (very very very rarely) I let someone else ride it.
You may find the honda power valve holds it closed when it's gunked up where the YZ seems to hold it open.
I'd start there and maybe also put a piston and ring in! If the piston is worn it will have a poor bottom and not so great mid but the top will feel good.
Once you've done the power valve then look at the jets followed by the piston.
The other thing is the oil ratio! Stock I think most manufacturers say 30:1 or 32:1. A lot of guys will run them at 40:1 or less! 125's need more oil because of the higher sheer strength to bore surface ratio (Piston wanting to go sideways as the rod come around) it's no where near as bad on a 250 so you can get away with less oil. but again if you can really ride (eg if you have the bike really reving the whole way around an MX track) then you will need the oil, if you can't then less may help the whole problem!

Just my .02

BTW I still think it's best to get them set up on a dyno then do the fine tuning at the altitude you would normally ride at (there abouts)
My YZ is going once it's all back together might take all the bikes at one time! It'd be nice to have them all running good at once!

[Bara]

legend mate,

heaps of info man, cant thank you enough, all my searchs came up with shops to buy them and what other people are doing to them. great find!

well the bike gets picked up on thursday so will report more when i have it in the shed and ready for pre thrashing inspection :)

[albonator316]

yeah when i road it i was surprized its gearing was so close. when he rides in the bush he'd rarely get above 80 km/h so i guess he just likes using all the gears? :shrug: i don't ask questions because he is a much better rider than me ;)

[Bara]

Flash:

where abouts is this powervalve located on the 2 stroke??? have no idea man. iv only ever ridden 2 strokes, not owned them so im a little sketcy on that side of things, on the plus side its always good to learn more so im willing to give it a go just as long as i wont stuff it up too much. and we pulled the top end of and checked the piston and ring, hardly any use at all ( i insisted that it be changed but my mechanic said there was no point )

and flash, i plan to do 50/50 mx ( appin mx track ) and bush ( stocktan, obron, lithgow ) so my altitude will change alot, how much will this effect it?


cheers for the info guys.

[Flash]

The power valve is located just about where the exhaust comes out of the barrel.
I'm not the sort of person who goes purely on what things look like I prefer to measure them and say it is right! Did the ring gap get measured in the bore say at mid stroke and top? And did the piston and bore get measured?
The piston will usually show wear but the ring won't necesarily.
Be a little weary on the fuel oil mix! On the track I can imagine you won't rev it as hard as the bush so I'd run a little more in the bush. I don't like going over 40:1 A seize will wreck your day trust me on that one!! THe other think is the oil rill allow the ring to seal better and (obviously) the oil wil reduce wear too!

To be honest I don't mess to much with the jetting if the bike is running wel I leave it. If I'm on the start line and I recon that the right jet could be make and break for a championship then I'll worry about it then! When you go to ride andit's playing up have a play and see how it goes then when you go to the other ride place soo how it goes! You may find a good mid point that will allow you to have it pretty good at both places!