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I own a XF125GY Supermoto Pioneer 4-stroke single cylinder.

I put too much oil in it so I did an oil change and put in a new spark plug then I noticed that oil had got into my air filter and that the engine misfires at around 6000 RPM and recovers around 5000 RPM.

Can anyone advise me on what I should do to fix this problem?

With thanks, Richard

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Resolution

1. Drain your oil

The reason you are having the issue is too much oil. You overfilled and now it's coming out of your PCV (positive crankcase ventilation). Your PCV vents your crankcase on the valve cover. It's the big hose attached to it. Oil gets into that hose that attaches to your airbox between the air filter and the carburetor. Drain it and refill it properly with the correct amount to prevent this from occurring again.

2. Clean Your Airbox

Remove your air filter. Look behind. Clean all of the oil out. You can use carburetor cleaner if you want. It makes it a little quicker and easier. This is the oil that was going in your engine.

3. Clean Your Air Filter

Make sure your oil filter didn't get saturated during this mess. Either clean it or replace it if the clean carburetor side has oil on it.

4. Check Your Spark Plug.

Pull the plug, clean it if it's black with carb cleaner and a toothbrush or replace it. If you can't get it not black. That's your hint to replace it. Be sure to gap it according to spec.

Once complete you should be fine. I would imagine your core issue is an oily air filter that restricts air flow and makes the bike too rich. The oily air filter cause you already know.

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Pull the plug and clean it, pull the air filter and clean or replace it, with the plug out of the cylinder cycle the motor a few times - as if you're trying to start it - but cover anything you don't want gunk to get on. If there's residual oil in the cylinder this can help flush it out.

Of course like the last answers mentioned, your spark plug's gap should be correct, and it would be wise to make sure you have the correct amount of oil in it.

If the air filter is paper, replace it. If it's a foam filter, use some foam filter cleaner, not something as abrasive and destructive as brake/carb cleaner. A mild dish washing soap and hot water in your kitchen sink will do the job but not too hot that the rim of the air filter falls off because the temp disrupted the adhesive.

Spark plugs are fine with being cleaned with the brake/carb cleaner but I'd go with the brake cleaner. I find it evaporates better with less residue.

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First of all, I hope you have drained the excess oil, you'll also need to clean the air filter and spark plugs to ensure they are all working as they should. You may have to remove the air filter housing to give it a good clean with carb cleaner or brake cleaner to remove the oil build up.

The spark plugs should be cleaned with a similarly volatile cleaner so no oily residue is left and can be cleaned with a stiff nylon brush.

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You did gap that plug correctly, right? According to the manufacturer, at least? If you're not sure, pull it back out & regap it.

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It is also advisable to ensure that there is no water or water droplets in the fuel tank and the carburreter as well

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