My Sears woodchipper with a 7.5 HP Briggs & Stratton engine cranks up great even after sitting for months (I use gas stabilizer). Then it works fine for about an hour...then starts losing power/rpm (sound of engine changes). If I move the choke towards "Choke" it regains some power. But eventually I am at full choke and it keeps losing power.
If I let it cool down completely, it will repeat the cycle.
What could be causing this run-time related problem?
This could be caused by a clogged gas cap vent. As fuel is used, air comes in through the vent to replace it. If the vent is clogged, then air can't replace the dropping fuel, and a partial vacuum can be created in the tank, causing fuel flow to decrease.
It's simple to test for, just loosen the fuel cap and see if the problem goes away. If it does, then cleaning the vent with some compressed air will probably fix it.
Overheating is another possibility; you mentioned that it runs fine for up to an hour hour before it starts to lose power. Is that an hour under heavy load, or mostly idle? The engine should be at operating temperature well before an hour, so I wouldn't think it's overheating unless it happens only under heavy load.
Another quick diagnostic is to remove and inspect the spark plug. Black fluffy looking deposits are carbon, indicating too rich a mixture. Dark greasy deposits may mean too much oil, but I doubt either of those is your case. If the plug has light deposits or appears "too clean" then you may be running too lean, which is what it sounds like.
A possible reason for the mix running more lean over time is a bad gasket surface or bad mating of parts. As parts heat they expand, possibly causing gaps to appear.
I just had a wild thought. I wonder if the return spring on the throttle vane loses some of its holding power when it gets hot, then regains it when it gets cold? < scratching head > The throttle vane is what controls the throttle while the engine is running. There is a fan on the engine within the upper shroud. It blows against this vane, which is tied directly to the throttle of the engine. To keep the vane in check, there's a little spring which pulls back on it as the vane is pushed by the air coming from the small fan. As the engine speeds up (for whatever reason), greater air is pressed against the vane which closes the throttle plate inside the carb and slows the engine down. The spring counterbalances this by pulling back on the other end of the vane to open the throttle plate and speeding the engine up. Between the air and the spring, there becomes a delicate balancing act which allows the engine to maintain a certain predetermined speed (and power level). If this balance is upset in any way, it can cause the engine not to run at the speed it's supposed to. If the spring were allowing for just a little bit more give after the engine is fully heated, it might be allowing the throttle to close a little further than it should, thus slowing the engine down.
What I'm suggesting here is a long shot. I'm not sure what it would cost to replace the spring, but it would have to be cheaper than replacing the whole carburetor, which is the next thing I'd look at. There are rebuild kits for the carbs, but it's been my experience rebuilding them is an exercise in futility, especially considering the cost of replacing the carburetor. To me, just reducing the frustration factor down to zero is well worth shelling out a few more bucks to replace it with new.
Replacements can easily be found online from many different sources. You can find them directly through Sears online, but usually at a premium. If you dig around a little more using the information off the engine plate (not the woodchipper), you can usually find the replacement for them very easily.
