If you go to East LA and walk on Whittier (more after 5PM) you'll come accross several taco trucks and some will have the tacos al postor on the spit - the one I found on the corner of Olympic had good al pastor and decent carne asada but no pinapple. On the other hand on the corner of Telegraph and Downey, by the entrance of freeway 5, there is an amazing taco stand (I think they open around 7PM and they are around 5 days a week) with tacos al carbon, they don't have al pastor but they have tacos de carne asada al carbon among other meats. The meat quality is good. They even have 3 types of salsas including the hard to find salsa de aguacate! It's run by two super friendly women and if you go there ask for Laura and say that Julia from Marina del Rey referred you. Laura explained to me that there are 3 grades of meat that are typically used for carne asada, the best one is carne ranchera followed by carne de diesmillo. Laura uses diesmillo and says that in that neighbourhood people don't really wanna pay more to get the higher end meat. She also said that a lot will be depend on how it's marinated. I highly recommend this place!