Loss of power

Ok, now I'm sure this is a question that has been asked MANY times, and I've been doing my research, but I wanted some more opinions.

I have a 58cc 2 stroke, and it's around 120 miles now. I'm running it at a 24:1 fuel mix. It has a lot of power and gets up to 26-28 without hesitation after start up.

But, after around 5 minutes of running, it begins to lose power, and it seems like I have to push it to get it above 20.

What could be the culprit?

Thanks a lot for the replies everyone, I'll give all of it a try, keep you posted. Much appreciated!

Just an update, I replaced the spark plug with a b6hs and the plug wire/boot with one from SBP. It helped some with the power loss.

The main problem was that I had it running way too lean, which I believe was overheating the motor. I dropped the c-clip down a notch, switched to a 32:1 mix, and the power has not dropped since.

I also realized that I am using a 44 tooth sprocket, and I weigh 195 lbs. So my cruising speed is around 23-26 mph. It seems like the motor still needs to rev a little high to get to that speed, but maybe that's just normal for the engine size and sprocket I am using?

I did want to put another question out there. I have noticed this since the day I brought the bike above 20 mph, but I am hearing a thumping/knocking noise in time with the engine. I don't believe it is hitting the plug, the gap has not been shortened any. It seems to happen only when I have a load on the motor, not when I rev while idling.

I have not taken off the head to inspect it yet, but I routinely ensure that it is properly torqued down.

Any thoughts?