Just got my first bike, have questions

[quino]

Just got my first real dirt bike, a 1991 CR250

Here is a pic: http://www.coserv.net/~quino/bike/CR250.jpg

I used to ride when I was a kid, but have not ridden 2 strokes in about 15 years.

It starts and idles good, but I have a question about the power band.

It seems awful peaky. It has a FMF gold series pipe, and a FMF silencer. Those are the only mods I am aware of. It sure seems like this bike only likes it full throttle. It "bogs" before you get to the power band, to the point that it sounds like it is choking... and then it takes off like a rocket, only for a split second, then you must upshift and get back on it, or it will start to bog again on the upper range of the rpms/power band.

I know this is the nature of the 2 stroke... but damn this thing seems peaky.... and really runs rough if you are not getting on it.

Is this more of a function of the pipe? Would the factory pipe give a broader power range?

I just want to kick around a bit.... and ride trails mostly... and I think this might become an annoyance.... what do you guys think?

 

[cr125_king]

Looks like you got yourself a good used bike as far as i can tell.:) . As for the on/off power you'll get used to it.Most Mx bikes have a little bog, but not as much as you've described. Maybe play around with the jetting. Whatis the plug color?. As for mellowing the power out a bit, try some flywheel weight.

 

[Anssi]

If it still feels like you've described after a few hours of getting used to it, it's time to get a more detailed look at the bike. The air-filter must obviously be clean, and it wouldn't do any harm to take the carb apart at least to the point where you can use carb cleaner and blow compressed air through the jets.

You did warm up the bike (maybe a minute of idling and 5 minutes of easy riding) before really trying out the powerband, right? Cold engines sometimes feel like you describe.

The next thing is to pull out the carb needle and check if it's set too rich, that causes smoking and bad running in middle range throttle openings. If it's strictly rpm-related(and not throttle opening, there's a difference), you may want to check out the power-valve and make sure it's not stuck open.

 

[dadshillnut]

Hey Anssi,Quino just described exactly the way my old 175 runs and then you commented about the smoke and mine does seem to smoke a lot. i know its not the crank seals cause i just replaced them. I took apart the carb to drop the clip and it was already in the top slot. does this mean i need to change the jet and if so which one the main ,pilot, or both . carbs are still a mystery to me:confused:

 

[Anssi]

First, I'm not an authority on this. I do know how carburetors and jetting work, theoretically, and have limited practical experience.

Jets and the needle affect certain throttle openings, not RPM (except power jets, and I doubt your 175 has one). See http://www.keihin-us.com/tune.html for more details.

If you have the stock needle and jets in there, the problem probably lies somewhere else , especially if you get smoke all through the throttle and RPM range. Needle clip positions usually have a big effect.

IF the problem is with the needle and you've run out of clip grooves, you will need to change the needle to a leaner (thicker or different taper depending on where in the throttle range the problem lies) one.

 

[OnAnySunday]

You may want to try a set of Boydsens' dualstage reeds.
"tourque reeds or power reeds" i think they were called.
My YZ400 was like your bike, on or off power.
You will have to rejet to compensate, but the powerband will be much more "user friendly".
IMHO the best bang for the buck mod you can do to your bike.:cool:

 

[Saratoga]

It sounds like the powervalve is not functioning properly. There is a cover on the side of the barrell the size of a quarter. Simply remove it, start the bike up, and watch the valve open and close. If it does not fully open or is stuck in one position, you need to clean the powervalve. Go buy a manual and learn to do this.

As for you guys above that recommened to buy some aftermarket parts-shame on you. A 1991 CR 250 has a very linear powerband with plenty of bottom end. Buying aftermarket parts to fix a maintenance problem is what keeps those speed shops in business.

 

[KSHAFFER]

disassemble and clean the power valve assembly. it could be sticking and not opening soon enough. that should improve the way it runs.