rough running, low power Herc IXB3 in OC-4

[HGETZ]

After sitting for about a year, I fired up my OC-4 to plow some snow, and she started right up. However, after about five minutes, she lost a lot of power and started missing. I thought I had the magneto timed correctly, but I can't see why it would only happen once it gets warm.
I've got great compression all four cylinders, new plugs and wires, mag was rebuilt a few years back.
Maybe the timing mark on the flywheel is off a few degrees?
It sure seems to be a timing problem to me.
Any ideas?

[Robert Barbour]

Have you checked the fuel supply??  The other thing that will do that is a bad condenser in the mag.   It does the same job as the condenser in a distributor, when they go it will often start and idle ok but have no power.  Do you have one of the old type of timing lights that are flash tubes that go in series from mag to spark plug.  It will confirm the miss is ignition and not lack of fuel.

[oliverchris]

Junk in the tank getting sucked into the tank outlet [drain & clean tank]; rubber fuel line breaking down inside, causing rubber flakes to restrict fuel flow (same possible at carb. jets) [new fuel line hose]; ground wire flapping about or other short in electrical system [check connections and/or disconnect kill switch & lights]; water in oil cup of air cleaner [check]; mouse nest in air cleaner assembly [add cat to air cleaner  ].
I have seen all of these in OC3s and early OC4s. Some or all could give your symptoms.

[Blake Malkamaki]

Does the fuel that has been in it for a year contain ethanol? If so it could have absorbed moisture and phase separated, or just gone bad. Therefore you could have a big layer of water in the bottom of the tank and bottom of the carb bowl, causing all kinds of problems. It could even deteriorate the inside of your fuel line or cause rusting in the bottom of the tank. I would drain the tank and put in new fuel. I've had good luck with SeaFoam to clean up the fuel system after such contamination without having to pull the carb and clean it.

Blake