How do I make the bike run richer or leaner?
Carburation is affected by the elevation you ride at, a change of exhaust system or air filter, etc but, although tuning is a science, it's not that hard and with a little patience you can get your bike running great.
There is a fuel/air screw on the
front of the carb down near the bowl. More of a star type wheel. On the TTR250
this is a fuel type device which means, more turns out equals more fuel. This
adjustment will only make a difference at idle and just off idle. It is meant to
supplement the pilot circuit. So here is the rest in basic terms:
Idle and low rpm is the job of the pilot jet. Larger pilot jet will make things richer at low rpm, smaller--->Lean
From about 1/4 throttle to 3/4 throttle is the needle jet. The basic adjustment here is the height position. Higher--->Rich, Lower--->Lean.
Full throttle is the main jet, large--->Rich, Small--->Lean. This is where most people need a change when a free flowing pipe is added. More air means you need more fuel to keep the ratio right.
This is a brief description and much simplified. It's not like one circuit turns on then the other turns on, all of these circuits work together to give a specific horsepower/torque profile. When you make changes to pipes, air box volume, valve profile, intake/exhaust tracks you change the shape of the horsepower/torque curve. You really need to determine what the bike needs. The best way to do this, short of installing a wide-band O2 sensor and analyzing things electronically, is to learn how to read a spark plug. The colour and condition of the plug will tell you everything you need to know. Start with a new plug, go for a rip, then analyze the plug.
A glowing red header pipe is a thing should not happen under normal riding conditions but, if you are letting the bike run with no airflow over the engine, the pipe will start to glow.
When a bike has been is storage, the pilot jet can become plugged with old gas that has turned to varnish or the float becomes stuck or both. A plugged pilot jet usually causes a lean condition and the bike might not run unless the choke is on. Or, if it does run, it will stall coming off idle especially if the throttle is opened quickly. A stuck float will cause over rich condition and is usually accompanied with black smoke at the pipe and a fouled spark plug. Either is no big deal to fix. With that said, your next step is to remove the carb to get at the jets.
Basically there are three circuits in this type of carb; the pilot, the needle and the main. In the most basic sense they control low, medium, and high throttle fuel/air mixture. I will describe how to service the pilot, the main and float because I think this will cure your problems. The float does exactly like it sounds, it floats in the fuel bowl and allows fuel into the bowl or closes a valve when the fuel rises to a certain height in the bowl. If the float is stuck or has a hole in it won't stop fuel from pouring into the engine, causing an over-rich situation. If you don't have a manual, you can download a copy from here – 25Mb worth though! Don't worry if the year doesn't match as TTRs have not changed in many years.
With the manual and the carb in your hand to look at you will have no problem figuring things out. What you want to do is make yourself a nice clean work area, take your time and lay things out in order on a clean rag. You will need some compressed air. If you don't have a compressor you can buy a can of compressed air from a computer store or the like. Also, get a can of carb cleaner, the stuff in the spray can. Oh yeah, wear rubber gloves and don't get carb cleaner in your eyes. READ THE LABEL.
Remove the bowl (bottom of the carb) then the float and float needle. Make sure the float does not have a hole in it, If it floats it's good, if it fills up with fluid and sinks, you need a new one. You should check the float level after you have reassembled the carb, look to the manual for details. Next remove the pilot jet and the main jet. They are right next to each other. The pilot is the one with the real small hole. Hold the jets up to the light and look through them. If you can't see through them soak them in some of the cleaner and spray them clear with the air.
For the rest of the carb all you need to do is spray some of the cleaner into the small orifices of the carb then follow that by spraying out the cleaner with the compressed air. You don't want to soak the carb in that stuff because it will eat anything not metal. After you have cleaned and sprayed out the carb and jets reassemble and reinstall the carb back on the bike. Make sure you get the carb back in properly and the manifolds on the engine side and air box side make a positive airtight seal. Now check the float level. You have new gas in the tank right? If the level is not right the carb will need to come off again, but deal with that if and when.
This basic cleaning should get things running better then you can start to tune for your mods and riding conditions if needed.
Just to put things in perspective, after you have done this a few times you should be able to do the job in less than 30 minutes.
I hope this helps and good luck.