CR250 need jetting advise

[jmx271]

2001 CR250 seems too rich...just set up with FMF Gnarly, VForce3 reeds, new top end. FMF suggest:
main jet at 400,
pilot at 30, and
needle in 2nd position
Still fouling frequently and spewing oil so went to 390 main jet, 1st position on needle and running 36:1 high grade pump gas...should I go back to the needle in the second position and run 40:1 or 50:1 or keep needle in first position and still lean out gas mix?

 

[ellandoh]

Spanky's jetting guide:

A correctly jetted carb makes a tremendous difference in the torque, midrange pull, top-end pull, and over-rev of your engine. If you have never jetted your bike correctly, you will almost certainly gain some performance at some point in the bike's powerband. A cleanly jetted pilot circuit can be the difference between having to clutch the bike out of a turn or not. The needle can make all the difference in the world for the power of the machine in most situations, as it controls the throttle range that most riders spend most of their time using. A correctly sized main jet could mean the difference between being able to rev out high enough to not have to shift one more time at the end of the straight, or the power falling flat on top and requiring you to make that extra shift.
Are you fouling plugs? Many people will tell you all sorts of band-aid fixes, from running less oil, to running a hotter plug. Both are incorrect fixes for plug fouling. It's all in the jetting.
The only way to know what jetting changes you will need is by trial-and-error. No one can give you jetting specs, because every bike is different, every rider has a different style, and jetting is totally weather dependent. Unless the person telling you what jets to use is riding an identical bike, on the exact same track, at the same time, his recommendations are meaningless.
Jetting is fairly simple, and is a useful skill to learn if you ride a two-stroke and want it to perform at it's best.
It's very important that you start with the pilot circuit. The reason is simple. The pilot circuit affects the entire throttle range. When you are at full throttle, the main jet is the primary fuel metering device, but the pilot is still delivering fuel as well, adding to the total amount of fuel that your engine is receiving.
Before you start to rejet your bike, you need a clean air filter, a fresh plug (actually you need several plugs to do plug-chop tests for the main jet), and fresh fuel. One important detail: Make sure the engine is in good mechanical condition. If your engine has a worn top-end, fix it first. Trying to jet a worn out engine is a waste of time. The same goes for reeds that don't seal properly, and a silencer that needs re-packing. Worn reeds will mimic rich jetting, and worn rings will mimic lean jetting.
Before you start the jet testing, install a fresh plug. Set the float level to the proper specs, an incorrect float height will affect your jetting all across the throttle range.
Warm the bike completely, and shut it off.
As already stated, start with the pilot circuit. Turn the airscrew all the way in, then turn it out 1.5 turns to start. Start the engine, and turn the idle screw in until you get a slightly fast idle, or hold the throttle just barely cracked, to keep the engine idleing. Turn the airscrew slowly in, and then out, until you find the point where the idle is fastest. Stop there. Do not open the screw any farther, or your throttle response will be flat and mushy, and the bike may even bog. This is only the starting point, we will still have to tune the airscrew for the best response.
Now is the time to determine if you have the correct pilot installed in your carb. The airscrew position determines this for you, making it very simple. If your airscrew is less than 1 turn from closed, you need a larger pilot jet. If it is more than 2.5 turns from closed, you need a smaller pilot jet.
Once you have determined (and installed it if it's necessary to change it) the correct pilot jet size, and tuned the airscrew for the fastest idle, it's time to tune the airscrew for the best throttle response. Again, make sure the bike is at full operating temperature. Set the idle back down (the bike should still idle, despite what you read in the Moto Tabloids), and ride the bike, using closed-to-1/4 throttle transitions. Turn the airscrew slightly in either direction until you find the point that gives you the best response when cracking the throttle open. Most bikes are sensitive to changes as small as 1/8 of a turn.
The airscrew is not a set-it-and-leave-it adjustment. You have to constantly re-adjust the airscrew to compensate for changing outdoor temps and humidity. An airscrew setting that is perfect in the cool morning air will likely be too rich in the heat of the mid-day.
Now, it's time to work on the needle. Mark the throttle grip at 1/4 and 3/4 openings. Ride the bike between these two marks. If the bike bogs for a second before responding to throttle, lower the clip (raising the needle) a notch at a time until the engine picks up smoothly. If the bike sputters or sounds rough when giving it throttle, raise the clip (lowering the needle) until it runs cleanly. There isn't really any way to test the needle other than by feel, but it's usually quite obvious when it's right or wrong.
Last is the main jet. The main jet affects from 1/2 to full throttle. The easiest way to test it is to do a throttle-chop test. With the bike fully warmed up, find a long straight, and install a fresh plug. Start the engine, and do a full-throttle run down the straight, through all gears. As soon as the bike tops out, pull the clutch in, and kill the engine, coasting to a stop. Remove the plug, and look deep down inside the threads, at the base of the insulator. If it is white or gray, the main is too lean. If it is dark brown or black, the main is too rich. The correct color is a medium-dark mocha brown or tan.
Once you have a little bit of experience with jetting changes, and you start to learn the difference in feel between "rich" and "lean", you'll begin to learn, just from the sound of the exhaust and the feel of the power, not only if the bike is running rich or lean, but even which one of the carb circuits is the culprit.
The slide is also a tuning variable for jetting, but slides are very expensive, and few bikes need different slides, so we won't go into that here.
Keep in mind, even though this article is intended primarily for two-strokes, four-strokes also need proper jetting to perform right, although they are not quite as fussy as their oil-burning cousins. The only real difference in the two is with the pilot circuit. Two-strokes have an air screw that you screw in to make the jetting richer, and screw out to make the jetting leaner. Four-strokes, on the other hand, have a fuel adjustment screw that you screw in to make the jetting leaner, and out to make it richer.

 

[jmx271]

Thanks for the thorough insight. You've given me a project for the weekend (if only it weren't for work during the week). What are your thoughts (or anyone) regarding oil mix ratios? I thought I was adventerous going away from the factory 32:1 but realize many are going to 40:1 and even 50:1. Is 50:1 pushing the envelope of a shorter piston life?

 

[ellandoh]

40:1 has served me well. if yore riding all out all the time like a 125 i would guess 32;1 would be better but pick it b4 you jet and stick to it after :cool:
by the way that was a cut and paste from the net

 

[jmx271]

Is there reallya difference in needles...I've been sticking with the stock needle, but am now questioning whether I should order the FMF needle due to adding the FMF Gnarly and Q silencer. Suggestions?? (You can tell I'm just getting into it)

 

[ellandoh]

most of the time moving the clip will work other than that 1 step leaner needle will usually get you there ...........if youre a do it your selfer or would just like to learn a new skill for the challenge then go for the gusto but if you want the easy way out with a finely jetted bike get a jd jetkit ive heard good things

 

[Curt_704]

The 2001 CR 250 is a tuff bike to get jetted properly. Do a search on this forum and you'll see what I mean. I had one and change to the S-7 nozzle, with a 420 Main, and a 30 Pilot. If I had it to do all over again, I'd switch to a leaner slide,one step leaner needle, and a 30 pilot, instead of the nozzle change. I think the slide focuses more on the low to mid area where the nozzle focuses on low to 3/4 throttle. I'd had a lean condition at 3/4 throttle that cost me a cylinder replating. Again, do a search and you'll find a ton of opinions!!!!!!

 

[jmx271]

Thanks...I'll be trying things out.

 

[tyesai]

Ellandoh,

Here is something to think about, what do you do if the bike runs good but still fouls plugs. My CR250R runs great after it warms up. It runs clean and smooth down low and really tears in the middle and top end of the throttle range. Several people have rode the bike and says it feels good to them and they wouldn't change a thing, but it still fouls out plugs about every 2-4 hours of riding time. I do admit I am still nervous getting into the middle of the powerband going off some jumps and other situations were I am happy to lug a gear high and keep the throttle response a little tamer. I figure that the bike runs good and if I foul out a plug every now and again no big deal. Napa sells them for about half of what the dealership and other "bike" places sell them. Just food for thought.

 

[ellandoh]

dont go far from the truck then :(

 

[darringer]

tyesai, if you take the time to jet properly, you will be amazed at how much better the bike will pull. It's virtually free horsepower, and you will won't be buying plugs by the case. I haven't fouled a plug in years, even though on some of the trails I literally crawl at just above idle for long periods. The only time I change plugs is because the electrodes get rounded off. So about twice a year.

 

[tyesai]

dont go far from the truck then :(

I'm kind of fat and sweat alot so if I am not riding on the track I have a hydration system, 2 plugs and a wrench on me at all times. It just makes sense. :laugh:

 

[ellandoh]

I'm kind of fat and sweat alot so if I am not riding on the track I have a hydration system, 2 plugs and a wrench on me at all times. It just makes sense. :laugh:

a friend of mine couldve learned alot from you when we were an hour from the truck in the mountains of tenn. :bang:

 

[tyesai]

tyesai, if you take the time to jet properly, you will be amazed at how much better the bike will pull. It's virtually free horsepower, and you will won't be buying plugs by the case. I haven't fouled a plug in years, even though on some of the trails I literally crawl at just above idle for long periods. The only time I change plugs is because the electrodes get rounded off. So about twice a year.

I guess it is just a case of the NewB situation then, I have had people who have rode for several years ride my bike and say it feels good, not to worry and don't change a thing. I have a different pipe and silencer on it and have followed the recomended jetting changes and like I have said, it runs great, but fouls plugs. Maybe I need to get one of the really really fast guys to ride it and see what they think. "No I am not being sarcastic, but us novice to intermediater riders may be off the mark" Hope I am not stealing the thread, but in reality, I have been told 2 strokes foul out plugs, if it runs good, and you are happy, then leave it be. :bang:

 

[ellandoh]

take a look at your plug if its black and wet drop the needle 1 pos..............that is the range most people spend their time in

if you got the recommends from fmf they are full of #$@# they copied off mxa reccommends for my bike stock but left half of it out

what year is your bike and what jets are in it now???

 

[bikepilot]

Properly jetted 2-strokes do NOT foul plugs. Proper jetting for a pro my very well be different than proper jetting for a beginer. Its well worth it to keep working on the jetting till you get it right, the bike will run so much better its amazing, plus it will not smoke as much or foul plugs - ever.

There is no way MXA or FMF can give you exact jetting specs, every bike is a little different as is every rider and the conditions they ride in. Take these jetting recomendations as a basic starting point and tune the bike specifically for your situtation from there.

cheers

 

[tyesai]

a friend of mine couldve learned alot from you when we were an hour from the truck in the mountains of tenn. :bang:

Never ever ever ever ever ever everevereverever ride a 2 stroke bike in the woods without a plug but I prefer to make it plural and always have a wrench. Even if my jetting was spot on and I hadn't fouled a plug in months I wouldn't chance it, cause if you do you become that guy.