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Is my bike running rich or lean?

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A few questions, first I have a yz250 bored to a 300 and it idles at around 1300 rpm and this seems low but I'm not sure if it is normal or if it is a lean or rich bike. 

My rpm hangs up for a few seconds after raving the bike in neutral or with the clutch in. 

There is a bit of black stuff coming out of th exhaust.

I don't want to check the spark plug because my gas tank is in the way. 

 

Anothe question, could the air fuel screw fix it or do I have to re jet it?  Could cleaning the carb fix it?  

If I leave it like this for too long will the top end blow?

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7 hours ago, IanRogers said:

I don't want to check the spark plug because my gas tank is in the way. 

If it's way too lean, you'll have to remove the gas tank anyway to rebuild the top end...

How long will it last if jetted too lean, depends on your riding style.

 

IF the engine is in a decent state of tune: compression, good reeds, no air leaks (crank seals), proper float/fuel level settings,

pilot jet and needle settings up to 3/4 throttle are tuned by 'feel': off-idle transition, acceleration etc.

main jet (wide open throttle) is done by plug coloration and ideally also observing the piston wash pattern.

 

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It's  a very recent problem and I haven't been riding it hard at all. The top end is good but the bike is just acting a little weird. Would an air screw adjustment fix it or do I have to put a need jet in?

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Jetting doesn't go bad by itself, (of course it has to be adjusted per temperature variations)

if the engine ran fine before with the same settings, the problem is caused by something else.

How would you describe the engine 'acting weird'.

 

Has the engine been re-jetted to run as a bog-bore (often requiring leaner settings than stock) ?

Have a look a what your current jets / needle are versus the stock specs, also confirm your fuel/float level is properly adjusted.

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It was fine but the temperature has made a difference along with the seller living higher. He put a jet in for winter just before selling it and he hadn't tested it. 

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6 hours ago, IanRogers said:

It was fine but the temperature has made a difference along with the seller living higher. He put a jet in for winter just before selling it and he hadn't tested it. 

Reading this statement You are  lean

 

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Not sure how the yz's are but usually you can take the plug out with the tank on.

Taking the tank off is a 5min job and is better than blowing your bike up.

Loosen up your carb boots and twist your carb to the side and take the bowl drain out and your main and pilot are right there. Take them out and read the numbers. Then research what stock jetting is for your bike.

To get jetting in the ball park is very easy.

All of this I told you is a 30 min job max. Somebody who has done it before it would take them 5 min.

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