nitro engines are not made to idle long, dont tune to idle, tune to performance. I use 20% fuel with a cold plug and mine runs fine. There is a saying that there is no race or first place for how long your nitro idles.

Second, if you cover the gears on the bottom any rock you have in the chassis will be caught there and not be able to escape and grind up your gears, thats why ALL nitro cars have a cut out on the chassis below the gears, its meant to throw the debris out.