I'd guess the OWB might be slipping. I'm not familiar with HPI's setup. If the owb is inside the engine then pour a little fuel straight into the carb or glow plug hole and tilt the truck so the owb is lower than the engine and work the crank back and forth a bit. That works for me. If the owb is outside the engine then you should be able to loosen a screw or two and squirt some fuel or wd-40 to it. You may need to remove/replace if these ideas don't help.