Question about Yamaha YZ 250 Motorcycles

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Hole in bottom end power

I live at 1000 ft and ride between sea level and 3500 ft. The bike has an FMF Burly pipe and a turbine core 2 silencer. For some reason it has a nasty hole in the bottom end and in the beginning of third just wondering if there is someone who has had this problem before or if they know of a fix

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Possibly your powervalve is not operating properly or you have a rich condition happening just off idle caused by your slide needle position or a malfunctioning float valve..

Usually, the only thing wrong with a powervalve is that it's stuck and needs a disassembly and thorough cleaning.
To lean the slide needle, remove the needle and move the circlip up one notch.

If your float valve is malfunctioning, you'll probably notice some trickle of gas out of the overflow tubes when sitting. This will often clear up from the vibration of a running engine, but may have already loaded up the crank chamber. Remove float and valve and clean the needle and seat with a q-tip and carb cleaner. Look for scarification in the seat or a pronounced
ring around the rubber part of the needle. Replace defective parts.

Posted on Jun 12, 2010


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I just put a new fmf fatty pipe and a power core 2 silencer on my bike (1994 yz 250) and now when I start my bike it will run for like 10 sec and then It will bog down and die on me. Then it wont start,...

if the bike was running fine before you replaced pipe,you may need larger jets,or richen fuel mix,also check the reed valve it may be on the weak side.try a new spark plug also if you have not already replaced it.

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Hi, when you change your pipe to the FMF you reduced the back pressure and gave it more flow. That causes it to run a little "rich". Look at your spark plug and check the color of it, it should be a nice tan color. If its a dark brown then you need to "lean" out your bike. The easiest way to do that is by taking the slide out of the carb and taking out the main jet, the lock washer is set to the middle position for a stock set-up. Move it down 2 notches and then run it and see what your plug looks like. The black "goo" is the unburned oil from not running lean enough. Another trick to try is run a NGK-EV plug. it burns hotter so it will help with the oil problem (and make it a little more snappy also). And as an added bonus it will be easier to start.

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This is not uncommon.
Things to check first would be your fuel mixture. You might be running a bit rich. You should be using a mix of 3.5 percent synthetic oil.
Being a 2 stroke, the tail pipe cartridge needs replacing from time to time, as it does collect oil with time. If the bike is a bit noisey change the packing.
Always remember that there is oil in fuel used on 2 strokes.
Lastly check the type of oil you are using. Mineral oil does tend to be a bit messy as the mixture requires around 5 percent oil.
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Stick to good synthetic oils !

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gas/oil ratio should be 40:1 synetic.empty tank drain carb bowl thats it l have 92 250 exc. It has fmf pipe &silencer boyesson rad valve, been ported ,crank has been balanced,PVL ignition. I use spectro synetic 36;1 to keep it well lubed at wide open. ,,,

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I have an 04 125 with a FMF fatty Factroy expansion chamber and a Titanium 2 FMF silencer (shorty) the carb settings are Pilot a size leaner than stock main IS stock jet needle is a setting toward lean and air screw is two turns out. My gas:oil mix is around 32:1 - 40:1.. My spark plug buget is killing me, and I havemessed with it til I was blue in the face can anyone help me?,don't worry I didn't take it wrong it is a good question, and let me put it this way.. I run it like a two stroke lol I have had more than my share of two strokes so I don't lolly gag it and usually for the trails I ride you really can't get out of 1st most of the timeI spend most my time around 3/4 throttle. When I 1st got the bike I had a friend messing with the idle screw on it cause it wouldn't idle at all and he messed with the mix screw by accident so I think it was screwed out more than two turns but I set it according to FMF recommendations with the pipe and chamber. The guy who had it before me raced it in a AMA class I don't know which so don't ask lol, so who knows what has been done to it other than what I can see,,,

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I am not sure as to where you wwant more power. I hear the JD needle makes the bike smoother. If you need more bottom end power a switch for 13:52 gearing will help LTR power valve cover a POWER NOW PLUS POWER NOW (carb divider plates), FMF Gnarly for bottom end, and a 2K2 igntion. As for your jetting it sounds like you could go down on the pilot one size and raise the clip up one position.,,,

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