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Doodlefork -- Can't help with any specifics on the JD engine -- all I know about JD's can be pretty well summed up as "they're yellow and some other color" -- but the most probable causes of backfiring through the carburetor are way-over-advanced ignition timing and/or intake valves not closing properly. There are a number of possible ways for an intake valve to not close properly. Among them are bad valve timing, a sticking valve stem, a bad valve spring, trash inside the engine holding the valve open, and a broken or burned valve head or seat. There's an old trick for setting ignition timing that should get you close enough for the engine to run if you don't have any luck finding the factory's method of setting timing: Remove the spark plug from all cylinders and manually pull the engine over until the #1 piston is coming up on the compression stroke. Use a probe (a small wooden dowel will work) through the spark plug hole to feel the piston's position in the cylinder. When the piston nears top dead center (TDC), its rate of travel will slow until it stops at TDC. Stop pulling the engine through when #1 piston is at TDC, ground the base of the #1 spark plug, turn on the ignition power, and tweak the ignition timing adjustment until the #1 plug fires. Clamp the timing adjustment, turn off the ignition power, reinstall the spark plugs and ignition cables, and then start the engine. Once the engine is running, you can make further timing adjustments by ear or with a vacuum gage. John
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