FMF fatty chamber and silencer re-jet? cr250

[SkAgS]

will i need to re jet the bike picking up my new pipe this week. (bit of a ding in my factory one ) havnt really played round to much with the carb as it goes well and im a great believer in if something goes well dont touch , would it be worth taking it in to the shop to get done right or would it be easy enough to try myself. if i need to do any adjustments
bikes an 05 cr250 i run 40:1 haven't had any problem with fouling etc ever my rideing is mainly play rideing with trail rides now and then, anybody else have similar setup ?

hopefully ive posted in the right place sorry if i have not, new to this site !

thanks !

[cheese]

If Sam (Booom) ever gets round to it his jettign is spot on on a 06 which should be a good benchmark for you.

[CaMo]

Jetting is actually quite easy. Here is a basic guide that will help

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.

[Danger]

Welcome to Kiwi Biker Skags. Don't be fooled, you can spend a lot of time tuning the carb, it can be a moving target with changing conditions, thats why it helps to take notes as you go. Developing a feel for lean and rich helps as well. I suggest that you should tackle it yourself rather than take it too a shop. You should at least become familiar with what jets your bike is currently running and expect to change them from winter to summer and vice versa. Each bike has a temp range where it runs better with the alternative jetting. My KTM needs to be changed earlier in the summer than my Suzuki and my Suzuki needs to be changed earlier in the winter than my KTM.

One thing to be aware of is that if your main is too small ie too lean it will effect the needle zone, for this reason you should (and I often do) tune the needle area without a main jet at all. You don't want the main to influence the needles range. It will get rich at about the 5/8-3/4 zone and some carbs with the electronic controlled carbs can be a little tricky too, some people disconnect the wire and report better running, I have not tried this. I tune the pilot, air screw, jet needle and clip position all without a main fitted. Then I tune the main. Or you can tune with the biggest main you have and come back a size at a time until you find a lean condition and then go back up. Once you are in the ball park with your main (and I often run slightly rich on the main for those hard full throttle sand runs we have) you don't have to remove the main to alter the needle or clip position as long as you are sure your main is not too small. Some bikes are easier than others, like my KTM all the jets are easily accessed, my Suzuki requires sub frame removal to get at some of the jets.
Tony tells me there has been a heap of seized bikes at the bike park with the soft conditions. Your bike actually runs richer in the summer so these bikes have been running too lean for a long time and its the softer conditions that have put more load on the engines which seem to have made the difference. Even at the Dead Toad there were a few seized bikes at the end of the 2k full throttle runs on the gravel road, this is where you might like to run a main one step richer. For off road riding most of you throttle time is below the mains influence anyway. MX is a different story but MX is quite tolerant of a richer main. So what I am saying is don't be tempted to run lean on the main to try and cure faults in other areas of your jetting or bikes performance.

As for the FMF pipe it might have a slight influence. Faster flow means your bike will run leaner, but how much faster does the air flow with an FMF pipe? The specs I gave in another thread for the 05 CR250 was on a bike with an FMF pipe and silencer. Just search my posts it was only in the last week.

[cheese]

I'm going to give all this a go. Quite nervous........ If the weather stays like this for a while I might go out to woodhill and give it a go. the long straight roads should be prefect there.

[Danger]

Cheese you need to be careful of sand getting inside when you pull your carb, Woodhill might not be ideal. Its also good to have a hill that would put some load on for about 15-20 seconds without having to back off full thottle. Its hard finding some where suitable I'll admit.

[cheese]

Oh shit True!!! Didn't think about that! Maybe I'll do it down the road near my house........ I'll wait till I recon it. the funny thing is that on suday my bike felt like it was going the best it ever has, but it still has excessive oil after I ride it all around teh pipe. I might try a few adjustments on the needle and maybe a leaner jet. I will experiment anyway. I do need to rebuild it in about 10 hours.

Oh and Cheers for tha link Greg, it worked well. Now I know how to use them properly they are dam good. Now to go down to Mt eden and give them a piece of my mind. Gee they are a pack of dorks down there.

[Danger]

Glad it worked out for you.

Your pipe is probably ready for a repack as well. Once the packing is soaked with oil, it will always spooge.

And with the jetting, one change at a time and one step at a time. If it feels good it might not be far of. A rebuild and muffler repack and a slight jetting tweak might have you sorted.

[B0000M]

on the beach itself would be ok, the sand give the bike extra loading like a hill. as long as its not a windy day it should be ok (and you're careful about the sand)

or come round and pull mine to bits and have a look.... for that starting point

[Danger]

I thought that we had already established that it was a different carb or engine management system between the 04 and 05 upwards? He already has a starting point from one of my earlier posts if it is the same, which I don't think it is.

[cheese]

THe psiton is different as its only got 1 ring in the 05 and 06.

The pipe is repacked and there was a seal on the pipe that was rooted and I put a new plug in and the spooge is gone, its just oily as hell. Black build up all in the pipe joiner and out teh back of the muffler. I think that I'll check what I got and see what the factory settings are and go from there.

[SkAgS]

wow wasnt expecting so many replies, thanks for all your info , i think ill get the pipe on then go from there, i cant imagion i will need to change things to much , but in saying that i dont really kno. as long as i dont damage anything in the process ill be happy, thanks for all your help.

[skullcandy]

Ive been trying to rejet my 2005 cr250 for months now.
Currently have a FMF fatty pipe with powercore 2 silencer and vforce reeds.
Current carb setup is a James dean 400 main, red needle on 2nd clip from top and 32.5 pilot circuit. 40:1 Amsoil with 91 pump gas. 3000-4000 ft

My problem is my plug is running really really dark brown with oil coating the plug threads.
When off the throttle mid air or off the throttle while operating my bike makes a gung gung gung gung sound . Is this a symptom of a rich or lean pilot circuit?

I just replaced the top end and still the same issue. There is a bit of spooge coming from the mid pipe and it blows white/ blue if its not pined to the tin.

Any suggestions?

[Pierce]

Have u tried running 98 octane petrol? Could help. I have similar setup (*unsure of the carb settings) but mine isn't too bad, seems to be running a lil rich but not too bad.