shadow171

Member
May 27, 2005
96
0
I'd like to know a base line jetting, just to get me started in the right direction. I know its been asked a thousand times but I couldn't find the thread I was looking for, I looked on about 17 pages of jetting info. I have a 97 CR250 with an FMF burly and a Turbine Core 2 and Mo Better porting. It has the stock jets, 175 main and 55 slow. It feels pretty good on top but I think it has a lean bog down low. I also plan on going one step hotter plug because all I ride are trails. I live in west MI, about 1000 feet, and temps are all over the place. General riding temps are from 40 to 85 deg. I've only had the bike a short time and didn't get much riding in before snowfall, so a good starting point is all I'm looking for, so any advice is good.
And for my second ?. I think I can hear my reeds fluttering at very high rpm's. I don't know if that's normal or if that's actually what I'm hearing. I plan on putting FMF Torque reeds for spring because I don't know what condition they're in anyway. Thanks for any info. :think:
 

SQUIRLEYMOFO

Member
Jun 11, 2002
310
0
Hopefully someone else will chime in.......My 03 CR250 is going thru the same thing........I think after installing the Delta reeds I need to go to a leaner pilot. I can't confirm this because I won't ride until deer season is over. LOL
 

ellandoh

dismount art student
~SPONSOR~
Mi. Trail Riders
Aug 29, 2004
2,958
0
dont know the specifics for 97 but heres something i hope 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.
 

trickin30

Member
Dec 14, 2005
8
0
Jetting Help

I understand about plug readings and jetting leaner or richer but I can't tell which sounds the engine makes are rich or lean. I have an 05' CR125 with the V-Force reeds, FMF fatty, Shorty silencer, Power Now and Power Now Plus. I know the jetting is rich as evidenced by plug fouling and oily plugs but I could use some help. Stock is a 55 pilot and 430 main. I have a 45 pilot and 400 main installed. I lowered the needle by raising the clip one position. I also have a fresh Wiseco piston and ring. The bike starts easily and revs ok until I give it full throttle at which point it starts to load up and die. The plug always comes out oily. Should I go leaner on the main? I already went three sizes smaller. I am just worried about burning up my engine. Any help you guys could provide would be awesome.

Thanks
 

ellandoh

dismount art student
~SPONSOR~
Mi. Trail Riders
Aug 29, 2004
2,958
0
trickin30 said:
I understand about plug readings and jetting leaner or richer but I can't tell which sounds the engine makes are rich or lean. I have an 05' CR125 with the V-Force reeds, FMF fatty, Shorty silencer, Power Now and Power Now Plus. I know the jetting is rich as evidenced by plug fouling and oily plugs but I could use some help. Stock is a 55 pilot and 430 main. I have a 45 pilot and 400 main installed. I lowered the needle by raising the clip one position. I also have a fresh Wiseco piston and ring. The bike starts easily and revs ok until I give it full throttle at which point it starts to load up and die. The plug always comes out oily. Should I go leaner on the main? I already went three sizes smaller. I am just worried about burning up my engine. Any help you guys could provide would be awesome.

Thanks



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.


sputtering is rich , bogging is lean, put a larger main in and do the chop test
 

shadow171

Member
May 27, 2005
96
0
No I thought he might see this, but I just wanted to know with my mods if it's typical to go up or down on the pilot, main and need. Just to get started, I know all bikes and conditions are different, so that's all I really want. Good read ellanduh Thanks all :cool:
 
Top Bottom