Exhaust comes realy orange

I have a 2002 yamaha yz 250f 4 stroke and when i start the bike the exhaust comes realy orange in like 2 or 3 minutes. so can you help me with it? and when i stop the bike a blue flame comes on the exhaust.

There is a excess build up of oil in the exhaust, remove the exhaust and pour some petrol inside the exhaust and light it, leave it for around 30 mins to let it burn away the oil. Be VERY careful when lighting as sometimes you get a little flame puff out of bith ends.
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