So I have been going through my engine (I believe it is a 25mph model) trying to get it up to a solid 30mph.
I started with the carb. I opened up my SHA 14:12 to a 14:14, removed the intake shim and used a puch 14mm which fits perfect without the shim, BTW.
Then on to the cylinder. I opened up the intake to 14mm, with the exception of the window on the actual inside of the cylinder because it widens there anyways and I cant reach with my tools to do a proper job. I also widen the exhaust port in general. At the exhaust window I raised the port 1mm and widened it 1mm.
After all that I was topping out at 27 which is a 0 mph speed increase. So I double check the timing and jetting, nothing. I come to the conclusion that the pipe is holding me back here. So this is what I did.
How I De-Restricted the Pipe:
First I removed the stinger. It was hard to get out. As you can see from the picture, I had to get handy with the vice grips.
Then I drilled a hole for the middle tube on the stinger, with the belief that the 90 degree turn that the hot exhaust gas has to pass through is the restriction.
Replace the stinger thing, otherwise it will be super loud.
I don't know if I did this right. It is raining and I don't want to dirty my nice white wall tires. I will report back with my findings. Also examining the pic I just posted, I think I have to move the hole I drilled over to reap the full benefits. (edited)