I'm not sure how late timing affects an engine with the ignition
turned off, but it does. First, find whoever told you to set the
timing 10° ATDC and beat the tar out of them.

Then set the timing to about 20° BTDC, and adjust the idle so it idles
right. This may not cure the problem but it will surely make
your engine run better.

If you have vacuum advance, DO NOT disconnect it when you set the
timing to 20° BTDC. That compensates for the vacuum advance which
also will make you have to adjust the idle at least twice. If you
leave it connected you'll only have to adjust it once.

You didn't say what year or whether you have carbs or FI.
That will make some difference.
Let us know how you do.

--
George Downs Bartlesville, Oklahoma