Try the 4.2 pro as stated above. There are two 42000rpm motors called 4.2 and 4.2pro. The 4.2 has no torque and is worthless.

Try the 4.2pro and/or the motorworks motor. It is much faster than any 3.8 motor. I have tested many motors on zipzaps, zipzaps se, bits and many clones. If you need more assurance, use the search engine.