I have a 1972 914, 1.7L always started on the turn of the key. Last week I turned the key, engine turned over but didn’t start up. I pressed the throttle and it started but only stayed running with the throttle about 2000 rpm. As soon as I eased off the throttle or took my foot off, the engine would stall. Fuel filter is new. Checked all vacuum hoses and connections, appear to be fine. Any suggestions as to what the problem could be and how to fix it would really be appreciated.