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Check compression – if you have very low compression, there can be too little force in the crankcase to pull fuel through the reed from the carb and push it into the cylinder. Could also have the lower reed totally stuck or blocked by insect nest, but not likely.
Look for the easy to check stuff first! Before you go tearing the motor apart, check that lower for spark. Sometimes a motor can run a while on one cylinder without the dead spark plug getting wet.
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If compression is good, possibly a broken petal on one of the reed valves.
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So the running cylinder has the low compression? That’s odd. I’m going to take a wild guess, worth nothing. A serious water ingestion problem into the lower cylinder, from either a blown head gasket or a holed exhaust cover baffle plate. Problem with that theory is it wouldn’t be “dry” as you said..
Hint: take a look at the edges of the exhaust cover, where you will see the two gaskets with a plate between them. If that plate is thin, like about a credit card’s thickness, that’s good. But if about 1/8″ thick, that would be one of the troublesome aluminum plates that corroded through.
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Bad capacitor maybe? Sometimes that will not fire under compression. Compression release? Is the plug wet? Top crankcase seal fouling magneto? broken points cam wiper or out of adjustment? Broken reed(s)? Bad reed housing gasket? Bad coil wire? Check all three. Ive had bad new parts too.
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OK, well I am confused about what you have reported, doesn’t seem like the cylinder with low compression would be the one that is firing OK… But, anything IS possible.
Let’s start with the compression though. First, recheck it, perhaps your gage is messed up. Are you cranking the engine with the electric starter when checking compression, or with the recoil? Using the recoil will activate the compression relief system, which may explain your strange compression readings. Trying checking compression with the electric starter.
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