Trouble at High Speeds

I need some help. I've been trying to figure out what is wrong with my bike for a few weeks now. Nothing I do seems to fix the problem.

Here's the deal,
I have a 49cc flying horse 2 stroke. Everything on it is pretty much stock. The only thing I did was put soft mounts on it. For about 2 weeks the power has been dropping out when I get to around 20 mph. I thought it was a carb adjustment but no adjusting of the carb seems to help.
I changed the plug cap because I thought it may not have been getting a good spark. Didn't fix the problem.
I thought "maybe the mixture is not right, and am running it too rich." So I am running about a 24-1 mix. That didn't work.
I was running it without the muffler cap on for a while and thought that I could be floating my valve because it didn't have enough back pressure, so I put the muffler cap back on. Still nothing.

The bike runs great until I get to about 20 mph then it starts puttering as if it were flooding, the tone changes and the vibrations increase. Then it loses all power until it slows to about 15 mph and starts accelerating again.

In case you couldn't tell, I'm a noob to the motor biking world. This is my first one so I have a lot to learn. Help is much appreciated.

Wow, you guys are awesome. I'll take off the soft mounts today and see if that resolves the issue. I'll let you know. If it doesn't , I'll take a pick of the plug and post it.

I'll keep you posted.

So I am now a firm believer that soft mounting is one of the worst things that you can do to your bike! I took the soft mounts off and low and behold my frame had BROKEN IN HALF! It broke right where the bottom bracket was mounted. Apparently the guy that I bought the bike from didn't know that you're not supposed to drill through a bike frame and then attach an engine to it. I thought it looked suspect when I put the soft mounts on it but figured "well he builds these bikes all the time so he must know what he is doing". WRONG!

Anyway...luckily I have another bike and am putting the engine on it. Once everything is put together I'll see if the problem continues.

Thanks everyone for all the good advise. You can keep it coming because even if it's not the problem it's good to know.

@dumpstercrucher,
I'm not sure if the cap is "vented". It's just the stock tank that comes with the Chinese kits.