I have a mower with a briggs and Stratton 18 hp single cylinder motor on it and I can't get the thing to turn over on the compression stroke. Here's what I've done,,,,,,adjusted the valves several different times. checked the compression release and it's working (intake valve opens fully then again only part way) charged the battery, checked the ground, cleaned both battery terminals and the starter is fine. Once it's warmed up it works great. Sometimes when cold it locks up on the compression stroke. If I turn it by hand then try at times it will go but other times it'll stop again. Any ideas?