You cannot do the timing adjustment by ear. If you set the timing to maximum engine-speed by ear, that is not a good timing setting (someone who knows, will indicate if it is too advanced or too retarded when/if you set timing to maximum idle-speed by-ear).
Not sure how Dad did it on my 235, but he set the timing without a timing light or vacuum gauge. Engine idled like a dream and pickup speed without a miss or a cough. I have to believe back in the day there were many shade tree and driveway mechanics that lacked a timing light or vacuum gauge. I know he always set the advance so it had just a slight ping on an uphill pull.
Not to say if you have a vacuum gauge and or a timing light use them by all means.
John
