Petrol is getting into my engine oil on my crf 450 2007 . checked valve clearences and they are fine .

Not all modern engines need you to set the valve clearances. Most engines use oil filled 'tappets' that self adjust the clearance as the oil temperature changes. Depending on which engine it is yours has a lot of the time you can only check the valve clearance but not adjust it.

clean the carb and check settings , also timing and valve clearences !

This could be a few things.

1.) too much oil in engine.
Too much is just as bad as not enough. check and correct level.
2.) sticking or worn piston rings.
Most likely problem as these little bikes tend to get revved real hard.
replace worn parts or upgrade to the 47mm kit 72cc barrel kit, £40 uk pounds
3.) Valve stem seals worn.
cheap easy replacement if this is all it is, but you can get a complete cylinder head with cam, lifters, valves etc ready to fit for about £45 uk pounds

hi this engine when dry takes 2.8 litres of oil,20w40. if you mean valve clearences you will have to remove the covers on top of the valves to gain access to the cam folowers, with the engine set at tdc using a feeler gauge check the clearence, the inlet should measure 0.05-0.10mm when cold in inches 2-4 tho
the exhaust should measure0.12-0.17mm when cold " 4-6 tho

hope this helps paul

need to check valve clearence, valves to tight, i have a yz450 you need a shim kit to adjust valves

your carb float valve is not adjusted correctly you have to take the bottom off carb and where the needle valve is the tab that holds it in needs to be bent toward the needle a touch careful not to go to far or it will not allow gas in just a toch

Back firing usually indicates a timing problem.

Yes you can same as most bikes.

doulbe check the grounds, & you might need to add and extra ground wire to the bike, closer to the starter. This should help. There is a valve that release the compression when you kick it off.You might see if you can pull this release the compression valve while pushing the start button????