What make engine shoot on load?

An engine may "shoot" or backfire under load due to a variety of factors, including improper air-fuel mixture, ignition timing issues, or exhaust system leaks. When the mixture is too rich or too lean, it can lead to incomplete combustion, resulting in unburned fuel igniting in the exhaust system. Additionally, a malfunctioning ignition system can cause misfires, contributing to backfiring. Maintaining proper engine tuning and addressing any mechanical issues can help mitigate this problem.