Dead Cylinder Below About 1500 rpm's

I know I'm late to the party, and everything has already been talked about, and I'll throw in with everyone that mentioned vacuum leak,
somewhere on the #6 leg,
I would include lobes going bad, but not all the signs are there for that, And it would be a little strange how that could just mysteriously appear right after a service and ignition inspection.
But I just gotta include the valve lash, if you included that with the "service".. If the valve isn't closing properly when the engine heats up, it will act like a vacuum leak, as the piston squeezes the fuel charge back up into the intake. You can easily see this on a vacuum gauge.
Or if the exhaust is not closing at the right time, then the piston will pull exhaust into the cylinder on the initial part of the intake stroke. You can sometimes feel this at the tailpipe at very low rpm as your hand gets "sucked" onto the pipe every 6th shot. It's really easy to diagnose on a motorcycle with individual pipes for each cylinder.
You can easily catch this by doing a LeakDown test on a fully warmed up engine. It should have shown up on the compression test, again, on a fully warmed up engine.
I have seen this when some contaminant got stuck on the exhaust seat. What I would do was, with the LD tester hooked up, and the cylinder pressurized; I tapped the exhaust valve with the wooden end of a hammer. A few taps later, the valve sealed up and the pressure normalized. I obviously didn't see the contaminants so can't say what it was, but I suspect it was a piece of carbon from the EGR system. If you continue to run it like this, the valve will overheat and be torched, requiring the head to come off again.
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I used to see a lot of this in old lawnmower engines. I would occasionally buy up a non-running single,cheap, clean up the chambers and re-sell it for beer money. The tell-tale was always the same; it "sucked" my hand onto the pipe as I pulled the rope. Sometimes I could get it started by covering the pipe, cuz then the piston would have to "pull" from the carb.