whats most likely is that an air / vacuum leak is opening up as the engine warms and everything expands and so allowing in extra air
The best way to locate a leak is to have the engine running and warm and then spray lighter gas /propane around each joint in turn. If the engine rev's up you've found your leak.
Now you might think that spraying lighter gas around a hot engine isn’t wise, however the flash /ignition point of gas is about 400°C so you need a naked flame or spark to set it off and I’ve used this method for many years without incident.
Work your way through each possible joint one at a time and you should find it. I use a slightly flattened piece of brake pipe and some rubber hose from the can of lighter gas to provide a spraying 'wand' and allow a direct blast of gas into each area, especially those difficult to reach with large implements.
regarding the carb heater element then this typically warms as soon as the ignition is turned on and then time out via a relay unit
I'm going to opt out here and get another expert involved
don't reply to this post otherwise it'll lock back to me