Marks Jetting 101

By Mark Cantrell - Member # 6179 on http://www.thumpertalk.com/

Visit this thread on thumpertalk here http://www.thumpertalk.com/bike/ubb/ultimatebb.php?ubb=get_topic&f=10&t=004190

I received a private message describing a rich condition and regularly fouled plugs. Although he probably has a plugged pilot, I responded to his 'I don't know much about jetting...' line at the bottom and was well into it before I realized the pilot was probably the issue. There is great info in this forum but newbies (like me last year) are often left with basics help that aren't always clear even with the excellent manual that comes with these bikes. I frequently give long responses to basic questions, just trying to help. I am posting under a subject that should keep me from writing other long responses on the same topic.
If you have coffee (or maybe it requires amphetamines) and have never jetted before, you may find this helpful. If you have, ignore it. I appologize for sloppy descriptions and if you have corrections or additions, please add to post. Motoman393, if you have pictures to help out, please post or post link to your site.
mwc
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Yes, the main problem is your jetting. I assume that you aren't sure how to correct it or you already would have. If you already know the following basics about changing the jetting, just skip this. It will be somewhat lengthy. Before I get to the meat of this topic, If you are mildly mechanical (you don't have to be a trained mechanic) and have basic metric tools, I would suggest you remove the carb, take the top off, the bowl off, and the float out. Doing this with the pictures in the manual will demystify this and you will be far more at ease.

The FCR carb has three principal circuits. A circuit is the (emulsifier?) air and gas path used to supply the air/fuel mixture at different throttle positions. There is a pilot circuit consisting of a pilot jet, a fuel screw, and a pilot air jet that controls the mixture at low throttle positions (closed to 1/4 about). The needle and needle clip position controls the mixture from 1/4 to 1/2 throttle. The main jet controls the mixture from 1/2 to full throttle. These are very rough numbers and there is some overlap. There are excellent diagrams in all three of the Inspection/Adjustment section, the Engine/Carb section, and the Tuning section of the manual. You need to look in all three sections of the manual to get the full picture.

Remember the throttle position, not the RPMs, determine which circuit is controlling the mixture. Ideally, for somewhat complicated reasons, you would select the main jet first, then the needle clip position, and then the pilot circuit (pilot jet, pilot air jet, and fuel screw).

On any circuit, if it is too lean it will pop and snap. If on the stand for the pilot or needle circuits, you can hear these best with the seat off listening through the air box. These are most often heard on deceleration but are sometimes masked by a WR only circuit called the air cut valve. If you hear popping, too lean. A bog can be rich or lean but if it is not accompanied by popping, it is probably too rich.

The main jet is a small brass threaded jet with a 6mm hex end and threads on the other. You can get to it with the carb in place on the bike. Take off the 17mm hex bolt at the very bottom of the carb with the fuel petcock OFF. 1/4 to 1/2 cup of gas will pour out. If you feel in the middle of the hole where you pulled the bolt, you will feel the small hex nut. A 6mm 1/4" socket can take it off. It should be about 1/2" to 1" long. If it is over 2" long, the needle jet came out also. No problem, just put both back in together. If you bought a US WR250F new, it came with a 170 main jet installed and a 165(or 168?) and 175 (or 172?) spare main jet in a plastic bag.

The classic way to check the main jet is the plug chop test. Find a safe place at least 1/4 mile long, preferrably straight and preferrably uphill. Warm up the bike thoroughly (at least 15 minutes riding). Put in a clean plug or at least take the current one out and notice the condition of the plug before starting. Ride the bike at over 2/3 throttle position over the safe course. Make sure the throttle is always at a steady position (so the accelerator pump isn't activated) and 1/2 to full throttle position. Full throttle is recommended if a safe enough place is found. At the end of the course, simultaneously pull in the clutch and hit the kill switch. Remove the plug and look at the circular ceramic below the center electrode of the plug (ignore everything else on the plug). If it is bone white, you are too lean. If it is dark or sooty, you are too rich. If it is a light to mid tan, you are fine. If rich, replace the main jet with a smaller one and try again. If lean,
replace main jet with a larger one and try again.

[DOC on the WR426 forum has evidence that the WR's air cut valve (aka deceleration enrichening circuit) masks changes in the fuel screw and pilot jets. If frustrated, consider plugging 1/8 inch vacuum port (with washer) behind air cut valve plate.]

I go to the pilot next. Its sloppy on my part but seems easier. The pilot controls from idle to 1/4 throttle. Adjust the idle with the black nob until it is way too fast. Then adjust it back down until it is around 2000 RPM or if no tach until it sounds just a little high. The fuel screw is at the bottom of the carb in the front (engine side) recessed in a small hole cast into the carb. You can't see it and you probably don't have a short enough screwdriver narrow enough to get in that hole. You will probably have to make one by cutting down an existing screwdriver or making one out of a framing nail. Motoman393 has a picture of a couple on his web site. Before you start adjusting, count the turns required to tighten it up lightly. Mine was 1 and 3/4 turns out. Then start the bike with the slightly elevated idle and turn it out 1/4 turn, 1/2 turn, 3/4 turn and so on until you get to 2 turns. Listen for best RPM and best response to a quick 1/4 turn tweak of the throttle at each position of the fuel screw. Now turn back in 1/4 turn at a time doing the same thing. By now you should have been able to distinguish the speed of the idle and the responsiveness to tweaking the throttle. If it gets better between 3/4 and 2 turns out, set it at the best location and leave the rest of the pilot circuit alone. If it is getting better turning it in or is best less than 3/4 turns out, replace the pilot jet with a smaller one and go through this procedure again. If it is getting better as you turn it out or best at more than 2 turns out, replace the pilot jet with a larger one and go through this procedure again. The pilot jet is in the same hole under the 17mm bolt in the bottom of the carb that the main was in. It is beside the main and has a flat screwdriver head. Just use a short screwdriver to remove it. A new US WR250F came with a stock 42(?) and a spare 38(?). If you needed bigger (you almost certainly don't because you sound rich), you will have to buy one. Sudco is on the web.

When the pilot is right, you will need the choke to start cold and it won't run well off of choke for 30 seconds or so. When hot you will probably need the hot start.

[If you are working on the pilot jet or the main jet, through the 17mm bolt in the bottom of the bowl, the main jet is right there at the bottom of the bowl and very easy to find. The pilot jet is in front of it (toward the front of the bike) and recessed in a hole. You may need to twist the carb in the boot so the bottom of the carb is closer to the shifter side of the bike. The boot is the heavy rubber connection between the airbox and the carb. It is flexible and the collar to the intake manifold will turn. Just twist the carb in place. Some people loosen the collars that connect the carb to the boot and intake manifold, others loosen the throttle pulley so the cables aren't tensioned. You can move it a fair amount to make getting to these easier without doing anything.]

I can rarely predict what I need to do with the needle. Just change it up a clip, ride it midthrottle on a course with changing conditions, lower it, ride the same course, and one will feel better than the others. Oh yeah, the needle is under the very top of the carb. On a WR you will need to remove the tank and seat. There are two 3mm allen bolts on the front and back of the top of the carb. Clean off the top of the carb and the bottom of the frame above the carb with mineral spirits or WD40 and a rag. Take off the two allen bolts. Remove the top of the carb. It has a rubber gasket attached so try to do this without rubbing it around too much. Looking down into the top of the carb you will see the throttle slide. Twist the throttle and it will rise. In the middle of the throttle slide is a 4mm allen screw. Remove it and it will come out along with a spring and a collar. Dont' loose the spring and collar. Under it is the round top of the needle. Using tweezers or needle nose pliers, gently lift it. Have someone twist the throttle to raise it up if you have trouble getting to it. Pull it out. It will have 5 or 6 grooves around the top with a clip in one of them. The top groove is the top clip position or clip position number 1. The needle goes through the middle of the carb through the needle jet to the main. It blocks (emulsified?) fuel coming up from the main through the needle jet. As the throttle position is increased, it blocks the main/needle jet less, letting more gas come through. The diameter, length, and taper fine tune this process but for now focus only on the clip position. Raising the clip (to the top of the needle) lowers the needle to block more fuel and is leaner. Lower the clip (toward the bottom of the needle), raises the needle out of the main/needle jet enrichening the mixture. Yours sounds rich so try raising the needle position one clip at a time. Ride it each time you change it. If it pops or is more sluggish as you raise it, try the other way.

Now that you know how to do that, there are two things that are more likely than jetting. The reasons I say this is a more open pipe usually requires a richer mixture and these are common problems. The first is starting technique. It is very tempting to 'prime' it by twisting the throttle a little and letting it go to use the accelerator pump to put some gas in it. I used to do that and it really did start better but I went through a plug a ride. Its common to see people twist the throttle during the kick like a two stroke bike. Same result. Some recommend not touching the throttle by putting your left hand on the front brake master cylinder instead of on the throttle while you start. Another is starting the bike and stopping it before it warms up. I used to start my bike after washing, just to see if I had drowned it. I would rev it once or twice and then cut it off. It fouled a plug every time I did this. If I start it now, I will let it run for at least 2 minutes before I cut it off. I fouled my first plug in months last week because I was in a hurry replacing a throttle grip and got grip glue in the throttle cable pulleys. While picking the glue out, I twisted the throttle for 15 minutes, maybe 20 times. Of course when I tried to start it, it was fouled.

The second thing more likely than jetting is to have the pilot jet or the fuel screw plugged up with air filter oil. This is VERY common. For reasons I can't figure out, plugging the pilot fuel jet makes it richer. People have said various things to explain it and some or all were probably right but it seems to me if it was plugged it would be lean. It isn't. You can do the basics (pilot jet and fuel screw) of cleaning with the carb in place. Remove the pilot jet and look through it. If you can't see through it or if the small holes across the top of it are plugged, blow it out with aerosol carb cleaner. Then stick the red plastic tube of the carb cleaner way up into the hole the pilot came out of an blow it out. Then remove the fuel screw all the way out. Blow carb cleaner up the hole it came out of. If you have compressed air blow all of that out to. [Be careful to not lose fuel screw, it give off a false sense of both being started and being seated.] Replace pilot jet and fuel screw, resetting fuel screw to the turns out you have it adjust to before. The main and needle are large enough (and may not have air or emulsion air going through them) to clog. The start jet, and leak jet don't pass air. The pilot air jet and the main air jet can clog but the carb (or at least the boot) have to be removed to clean them. If the air cut vavle plugs, you will probably be better off. Removing the carb and thoroughly cleaning/blowing it out are recommended but not for everybody. If you can't jet your problem away, clean it. Clean the pilot jet before doing anything to an FCR.

Another possibility, although mine starts fine so I haven't tried it, is the sky blue wire. It is a neutral gear detection circuit that changes the rev limiter while in neutral and is reputed to weaken the spark. I can't imagine this is true but some smart people (including Motoman393) are now swearing it helps with starting and fouling. They cut it or remove it from the connector up under the tank.

Good luck, let me know what happens. I re-read your post after I got to this point and noticed you have already timed the accelerator pump squirt so you are already familiar with the carb. Sorry for the long description.

I'm going to post this under 'If you have never jetted but think you need to...' on the 250F forum. I am NOT a jetting guru. I still can't get my hands around the main air jet and pilot air jet and the combinations of needles are very mysterious. If this is basic and you need more advanced jetting, read the Jetting Q's post on the WR426F side, buy JD's jetting spreadsheet, or post for Taffy or JD to get 'real' help. Also, the classic FCR tuning guide is Patrick Burns. But they probably aren't as interested in this basic stuff.