1-4-2-5-3-6
Here is a diagram of the firing order:
+-------------------+
| 1 | 2 | 3 | 4 | 5 | 6 |
+-------------------+
Note: The cylinder numbers are arranged in the following order:
+------+------+------+
| 1 | 4 | 2 |
+------+------+------+
| 5 | 3 | 6 |
+------+------+------+