You should check battery voltage versus ignition requirements. The Vlach EZM ignition goes to low rpm setting on low voltage so the pilot can still bring his plane in. I believe it is 4.2V (check battery, leads, switches) It is the ignition voltage that counts, not the voltage at the battery.
If voltage is OK and steady, look the seller of the engine deep in the eyes and ask the truth about this engine. He may have had this problem before he sold it to you.
The Vlach ignitions are no longer available.

PS
If the engine has the Vlach ignition, it is older than three years