Originally Posted by GearNut
If your spark plug is a milky brown then you are good to go for an engine that is broken in.
Since yours is still in the break in mileage, I would like to see it more like a dark chocolate brown. This slightly rich fuel/ air ratio insures that there is a little bit extra oil getting to the engine's internals and it burns a bit cooler than a leaner ratio.
The too big of a jet syndrome leans towards a very rich fuel/ air ratio, which would make the spark plug black in color. This also causes "4 stroking". This may still be possible in your case and will only show up during 3/4 to full throttle operation.
I now suspect that you may have a bad spark plug or spark plug wire. Either of these problems will cause poor operation at higher engine speeds. With all the previous questions and checks, we have pretty much ruled out carburetor jetting and oil ratio problems.
Oh yeah I almost forgot, are you using the stock plug in wire connectors between the engine wires and CDI wires? Those are notorious for being a problem. If so, suspect them before a bad plug or plug wire. Solder them and use electrical tape, or for the best job use heat shrink tubing, to seal and insulate them.
Today I picked up some solder, gonna jump on that tonight after work. Just to see how much of a difference it would make I cut & twisted them back together w/electrical tape. It got up to speed pretty quick without the 4 stroke sound. But with me cuttting and gutting out my exhaust it sounded like a chainsaw going down the road...lol. So in short my motor never got the chance to warm up to run right. In that short little run it seemed to not do it as bad on my 8 block run. But not sure if I would be able to notice since the motor wasn't warmed up. Tomorrow I will be able to braze the end back on my exhaust just to tone it down a bit. I might end up having to buy a new exhaust, which stinks. I had to cut and bend this one so it would fit because I changed the angle of the mount.