just a piece of free advice...

i fought this same issue,but my engine was 100% sealed...it would start,idle fine,then after a couple minutes the idle would shoot up,id turn it down with the idle screw to where it would idle great,but then when i would try to fire it cold it wouldnt idle...the problem was the clutch bell bearings were going bad...so dont rule those out either...amazing how the cost of 2 bearings is cheaper than the bottle of aspirin i needed for the headache it was causing me!!!