only just realised your rustyrc on youtube haha! love your videos man!!
if there is a loose clutch spring or snapped the car will not idle at all as the clutch will be loose which will engage the clutchbell hense it not being able to idle, trust my ive had my trophy truggy for a while now, the engine is perfect but hpi put a stupid cheap 2 shoe clutch in it, the only way he will get it to run at the minute is a really high idle, whip out the engine takes 2 mins and put a new spring on the clutch, i think they cost no more than £5 for a pack of two, and all your problems will go away! (well hopefully but thats nitro for ya!)
Clutch springs
https://www.modelsport.co.uk/hpi-clutch-springs/rc-car-products/31815