Some questions for you. Your head pipe (header), is it stock? What condition are your reeds in? Are they stock? Do you have the original 1993 suzuki RM 250 service manual? This will tell you everything that you need based on elevation and air temp for jetting your bike. Just for general knowledge its been my experience that unless ordered other wise. All new bikes come jetted for sea level. Your only at 400 feet. For starters Id find out what your pilot (slow) jet is supposed to be and get one if you dont have one. You say that the stock main (speed) jet is a 180. Pull the carb again (I know
) and look to see where the jet needle is set at. Stock position for the clip that holds it in place is in the middle of the 5 slots at the top of the needle. Put it there. Now with a 170 main jet your gonna be close if the exhaust is 100% stock. I'd get a 180 main jet and put that in and also ensure that your pilot jet is the stock size also. By the way Jets are cheap even from the bike dealers. Should be less than $5.00 a piece. Now verify that your head pipe is clean like mentioned. Repack your silencer. If its a stock silencer I dont recall if you can take it apart with out having to drill out any rivets. There should be 2-4 screws or allen head type screws that hold the pipe into place. If not I would recommend getting an after marker silencer. FMF or Pro Circuit both make quality pipes. I run FMF exhaust on my 92 RM 250 and it performs great. To prove that I have a video in the picture and video threads. Anyway Repack and clean the silencer your engine will thank you.
Now that youve got your jetting squared away, Jet needle adjusted, air screw adjusted properly, lets talk about your pre mix oil. I tried running 2 stroke outboard engine oil and my bike didnt like it. Your bikes engine and the outboard engine are both 2 strokes. However they are 2 different animals. Run a quality dirt bike 2 stroke pre mix oil. Suzuki CCI is what is recommended. I have run it and it works good. But I run Klotz Super Techniplate that has 20% castor mixed with the synthetic oil. Ive never seen a cleaner top end when it comes time to do a top end. But thats just me. Red Line also makes a quality synthetic 2 stroke oil. Everybody in here has there own personal favorites for there own reason. But if you want to follow your manual the Suzuki CCI is what you should use. Your spark plugs. Yes I know what your thinking $20 a pop is kinda spendy and I feel the same way. I run these kind of NGK plugs write these down you can buy 2 of each to try for less than the OEM type. All of these are NGK plugs B8EG, BP7ES, and Iridium BR8EIX. The first number in the part number is the heat of the plug. The lower the number the hotter the plug. The higher the number the cooler the plug. This part number plug r4118s-8 is a resistor type plug. I hate them. I am currently running the BP7ES and have been for the last 6 month and it works great. But dont do it because I said so.
Now with all this being said. Get your bike carb tuned for sea level 0 ft to about 1200 ft is considered sea level. Run a quality pre mix at 32:1. This should help you greatly and you manual will tell you what you need for the elevations and temp you ride at. Granted most of it is guide lines for a completely stock bike. If you have a complete after market exhaust and reed set up your jetting will be a little leaner. Typically one size down on the pilot, and one sometimes two downon the main, and up or down on the jet needle. Plus you need to ensure that your floats are adjusted correctly. Also you may need to replace your needle jet (not to be confused with the jet needle. Make sure its not worn. It should have a smooth taper with no grooves from where it sits. A clean air filter I cannot stress this on enough.
Now if none of this info works I clearly dont know what Im talking about................... Or you need a new top end or.............. You may need to take your cylinder off and get it replated. I had mine done last year. Best thing I ever did to my engine. But lets hope you dont have to do that. Its kinda spendy. Anywhere from $250-$500 depending on where you send it and what you have done. If this stuff works out for you listen to your engine it will tell you what it wants. Read your plug it will help you determine if your too rich or lean. Too lean the bike will make lots of power and then fall on its face. To rich and it will blubber. Trust me you will know what Im talking about when it happens.
Good luck to you in your trouble shooting
) and look to see where the jet needle is set at. Stock position for the clip that holds it in place is in the middle of the 5 slots at the top of the needle. Put it there. Now with a 170 main jet your gonna be close if the exhaust is 100% stock. I'd get a 180 main jet and put that in and also ensure that your pilot jet is the stock size also. By the way Jets are cheap even from the bike dealers. Should be less than $5.00 a piece. Now verify that your head pipe is clean like mentioned. Repack your silencer. If its a stock silencer I dont recall if you can take it apart with out having to drill out any rivets. There should be 2-4 screws or allen head type screws that hold the pipe into place. If not I would recommend getting an after marker silencer. FMF or Pro Circuit both make quality pipes. I run FMF exhaust on my 92 RM 250 and it performs great. To prove that I have a video in the picture and video threads. Anyway Repack and clean the silencer your engine will thank you. Now that youve got your jetting squared away, Jet needle adjusted, air screw adjusted properly, lets talk about your pre mix oil. I tried running 2 stroke outboard engine oil and my bike didnt like it. Your bikes engine and the outboard engine are both 2 strokes. However they are 2 different animals. Run a quality dirt bike 2 stroke pre mix oil. Suzuki CCI is what is recommended. I have run it and it works good. But I run Klotz Super Techniplate that has 20% castor mixed with the synthetic oil. Ive never seen a cleaner top end when it comes time to do a top end. But thats just me. Red Line also makes a quality synthetic 2 stroke oil. Everybody in here has there own personal favorites for there own reason. But if you want to follow your manual the Suzuki CCI is what you should use. Your spark plugs. Yes I know what your thinking $20 a pop is kinda spendy and I feel the same way. I run these kind of NGK plugs write these down you can buy 2 of each to try for less than the OEM type. All of these are NGK plugs B8EG, BP7ES, and Iridium BR8EIX. The first number in the part number is the heat of the plug. The lower the number the hotter the plug. The higher the number the cooler the plug. This part number plug r4118s-8 is a resistor type plug. I hate them. I am currently running the BP7ES and have been for the last 6 month and it works great. But dont do it because I said so.
Now with all this being said. Get your bike carb tuned for sea level 0 ft to about 1200 ft is considered sea level. Run a quality pre mix at 32:1. This should help you greatly and you manual will tell you what you need for the elevations and temp you ride at. Granted most of it is guide lines for a completely stock bike. If you have a complete after market exhaust and reed set up your jetting will be a little leaner. Typically one size down on the pilot, and one sometimes two downon the main, and up or down on the jet needle. Plus you need to ensure that your floats are adjusted correctly. Also you may need to replace your needle jet (not to be confused with the jet needle. Make sure its not worn. It should have a smooth taper with no grooves from where it sits. A clean air filter I cannot stress this on enough.
Now if none of this info works I clearly dont know what Im talking about................... Or you need a new top end or.............. You may need to take your cylinder off and get it replated. I had mine done last year. Best thing I ever did to my engine. But lets hope you dont have to do that. Its kinda spendy. Anywhere from $250-$500 depending on where you send it and what you have done. If this stuff works out for you listen to your engine it will tell you what it wants. Read your plug it will help you determine if your too rich or lean. Too lean the bike will make lots of power and then fall on its face. To rich and it will blubber. Trust me you will know what Im talking about when it happens.
Good luck to you in your trouble shooting
Your bike is only a year younger than mine and they are great machines even for their age. But bringing your bike back to good health might be expensive. I paid $750 for mine. Not including all the little odds and ends Im almost or just over what I paid for it. The cylinder replating was the most expensive $500 but that included shipping both ways and a new topend kit with gaskets. Then I bought a FMF turbin Core sparky silencer $100. Boyesen Rad Valve $160. New FMF Fatty head pipe $200. Just with these 4 things Ive exceeded what I paid for the bike. Then you throw in expendable items. Fork seals, wipers and fork oil. New air cleaner. Front and rear fenders. Tires and tubes
Oh my god. Gear box oil, pre mix, half dozen or so of main jets, pilot jets. Carb cleaner. The list goes on and on and on. 
Dirt bikes are as bad or worse then boats. Boat stands for bust out another thousand. I have one of those too. Ive invested more in my bike in the last three years just in up keep and replacement parts ect than I have in 6 years of owning my boat.
... i dont know anything about jetting bikes...i know racing mud trucks.. you can really adjust on the fly... but these carbs have not too much adjustability..