Hello all, I've run out of ideas, so I figured I'd try here for some advice. :)

I've got a 17HP Briggs and Stratton engine (31C707-0154-E1) that's not running right. It uses a Walbro LMT type carb. The engine will idle fine at half throttle and below, but anything past that and the engine pops and lacks power up to and including full throttle. It is not sporadic when it does this.... it almost sounds like it's missing on every other ignition stroke. When starting it fires up in a normal amount of time. Mower is 6 years old now and is garage kept.

In no particular order: I have checked compression, cleaned the carb a couple times (although it looked pristine, and I haven't used a rebuild kit yet), replaced the magneto (after it failed the resistance check on B&S's website), replaced the plug, cleaned out the fuel filter, dumped out all the old gas and replaced with fresh. Run with and without air filter. Checked the flywheel key. Checked valve clearances (intake valve tappet clearance is tight at 0.003" which is the minimum, I have to wiggle the rocker to make it fit though).

Should I go with a carb kit even though it looks fine inside? I've dunked it in gas and blown out all the orifices I can get to and it seems fine. Aside from that, it seems like I have little else to look at besides valves and valve seats.

A little history on this engine:

I bought it new 6 years ago. Within 2 weeks the cam broke a tooth and bent a pushrod. That was fixed under warranty and has worked fine ever since until this week. It has had regular oil changes and new plugs yearly.