I've seen a lot of dissappointment with the kits, both over quality control and performance issues.
If they kit isn't delivering, you check the obvious stuff first, and leakage is common problem - if you're not gettin any serious performance boost that's what I would check, doublecheck, and triple check.
I used to check for leaks by purging the fuel line and blowing smoke into it - if any smoke came out of the carb, manifold, etc, you got a leak somewhere...
Lets also make sure of the parts in question...
70cc head, jug, piston and rings.
15mm Amal Carb with large jet.
Now, if you happen to be missing any of those parts here, no, you will not get serious performance, cause they all take advantage of each other.
In fact, the 70cc cylinder is the least helpful of the above, if you are mounting it solo, the BiTurbo is the most.
Also, bear in mind that a really bad idea is a 70cc Cyl with Biturbo and the original carb and jet....that's like handing a lit bomb to an anarchist...sooner or later, boom.
Also, there's been good results (at least so far) with removing the airbox entire, but the jury is still out on long-term effectiveness of this.
Confirm your parts layout, and let me know.
Oh yeah, and one more thing to check...did any of those pathetically wispy piston rings crack when you put it together ?
That'll happen sometimes too.