Do you know what the stock jets are in a 04 RM-Z 250. Live and ride in Arizona. It had a 180 main and a 40 for the piolot. The pipe is a Leo Vince and the bike back fires bad and doesn't want to idle. The carb was cleaned and new gas was put in plug was changed. Could it be time for a valve adjustment?

That's a wide open answer. Is it totally stock with the crossover pipe in the exhaust ( 95)? Do you have two into one racing pipes ( 110)? Do you have drag pipes ( do you want it to idle or run good wide open throttle?)(125) ? Do you have stock air filter? Stock ignition? Dynojet kit?
The correct answer is to tune it. That means: put it together and ride it for 30 miles ( unless its obviously way off) and check the plugs and adjust and ride it for 30 miles again and check the plugs. Once you get it close you can adjust with idle air screw but my limit is 2 and 1/2 turns out. You get past that you need a bigger jet.

Start with a 48 slow jet and a 180 main jet. You may have to go up to a 190 main, but try the 180 first.

For a 250, you can get away with a leaner mix like 40:1 but I still race a 250 2-stroke from time to time for fun and I run 32:1. More oil = less friction, less heat, better sealing, etc...

Any idea what it was jetted to run? the difference between 40:1 and 32:1 is fairly minimal and if the bike is jetted fairly well I doubt you will tell a difference. If the plug is black it is too rich.

your factory jets should be ok with stock carb and exhaust,unless it came from way down south or someone changed them.factory jets are main jet #380,pilot jet #30

Any reason you had to change the jetting??New pipe,Intakes.?But,even though you may have it a bit rich on idle the pipes WILL get very hot,very quickly without some sort of air flow over the front of the motor and radiator.At the very least,have a strong fan blowing directly on the front of the motor if you are going to let the bike run and you are not riding it.I have seen the white plastics in front of the radiator distort from the radiant heat.Don't idle them for in more then 20 seconds.It can do heaps of damage.Ride the bike and see if its running well.When you stop,turn it off.All the yz and wrf will get very hot at idle.Hope this helps

The stock main jet is #350. As to fuel, it is best to run premium gas.

Stock Jetting for a 1994YZ250 is:

350 main
45 pilot
J8-6EJ33-61 needle

Adjust your jetting down one size for each 1000' of elevation above sea level. Needle clip position on the needle fine tunes the pilot circuit.

Scott

Several things are possible. The baffles inside the muffler may have partially collasped causing a restriction in the exhaust. The fuel / air mixture may be too lean. A lean mix burns hot. The stock main jet is a #178 (stamped on the jet). Try a richer jet, #180 or #182. Also, use only premium gas without ethanol. Be sure your gearbox oil is clean 10 w 40 motor oil. NO SYNTHETIC LUBRICANTS. Check the radiator coolant flow and temperature to be certain the engine is getting cooled as it runs. Be sure mud /trash is not blocking the radiator fins. Also check the spark plug. The stock plug is an NGK CR8EB. Replace it with a colder plug, NGK CR9EB

The stock plug is an NGK R6918B8. At only $22 each.

Scatter gun approach > thoughts to check out > > >
Clean and dry the air filter? Go back to a stock main jet, at least to check things out? Are "O" ring seals on the main jet going bad?Partial internal collapse in exhaust (remove exhaust pipe & do a quick run down the alley), Install a new stock NGK spark plug, Be sure battery connections are clean and tight, Be sure intake manifold and carb are good and tight.

Hope this helps!