The safety bar thing reads like a brake/drive stop thing, and unrelated
to the start problem. Read how the feature works and attack that separately.
The engine is backfiring on start, not audible but "kicking back".
Wind the engine over slowly with the pull start until you feel it bind.
That should be "top dead centre" of the firing cycle.
Fully open the throttle.
Then give the pull start one solid sharp pull, the engine should start
or give out an audible backfire. Do not repeat.
Wind the engine over again to TDC and prove the same result.
If the engine starts and runs rough you definitely know then there is a
timing issue, could be valve settings or merely a corroded (rust) flywheel/coil
face.
Easy check first is the plug electrode is clean of debri, I assume you
would have done that?
When your topend is all to spec and the engine still wont start, post more.
Trust that helps some.
KK