The exhaust is leaking, but it's still weak on one or more cylinders.
Those infra red thermometers can identify which exhausts are cooler and carbs that need attention, or need opening (sync)
syncing will even things out, but only if all the cylinders are running.
up to you to work out if it's the carb on any cylinder not opening enough (sync) or not running properly to start with, plugs, tappets, compression, flooding, starving, blocked. often some combination of all.
All at the same time.