This lawnmower engine is 4 months old and surges slightly at full throttle, but cuts off when in the idle position. Any suggestions?

has it been like this since you bought it? if it was, or has just started, is it still under warranty, because if it is, it needs to be taken back, as for it surging it sounds like a governor problem to me.

It has run fine until yesterday. It is under warranty, but I was hoping to determine if it is warranty issue or something that needs cleaning.
Any way to test a governor?

hold the arm on the airfilter side of the engine still, if it still surges and doesnt run at idle, then your carb is plugged a little somewhere
my gcv160 did this about 4-5 months after i got it, it wasnt the carb of governer though, the cylinder was scored pretty badly
that, ended up with 35psi compression, and a snapped timing belt 4 years later

if when you hold the governor arm and it stops surging, then it may be under warranty (don't know for hondas or what brand mower its on) if the carbs clogged, well you may have to dig into it, and those engines would have a auto choke epa regulated carb on em i beleive, but hey if its covered, take it back in to them and ask about it. what brand mower is it on?

I think I found the problem. I disassembled and cleaned the carb. I started it again and had the same problem. I pulled the plug and found the plug ceramic was cracked and missing a piece, and the top was bent. I will install a new plug soon. Another question? I unscrewed the main jet and another 1 inch piece with holes fell out, and I am not sure which way it goes back? Do you know where I can find a exploded view of the carb?

that sounds like the nozzles.

oooh, the ceramic on the plug was broke
mine did that when i snape3d the timing belt, but if yours runs somewhat then your belt is still in place or in one peice
it may jump on the pulleys also