A friend is visting and drives a 300TE and it suddenly quit starting (been starting fine until tonight, but has idled a little rough at times he says). If we take the breather cover off and manually push down the "choke" (not sure what the piston is called) it will start. It will run with some extra RPMs on it, but sputters at idle and will stop when that piston closes. We can push the piston down and it will start again.

So, if it isn't premature to ask, what sensor is controlling the air flow into the engine that we override by manually allowing air in?

Or is it some other source?

I searched through, but didn't see any posts with such a specific problem with the starting that could be manually overridden.

Need help quick - he has to get on the road tomorrow. Thanks in advance.