My 94 chevy truck with a 305 idles at 500 rpm in drive with brake on. Idles at about 800 in neutral. When I hit the gas it spits andspuuters. If I ease into the gas it takes off fine. Once it is running down the road, everything is fine. I was gonna just turn up the idle screw, but there is no access to the head of the screw 9welch plug covering it). So what is causing the low idle? Throttle position sensor or something?