Burnham Alpine fails ignition, but once running is ok

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HaveWrench

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BLUF: Ignites with a whoosh/bang and goes out immediately - but I can get it to start by blocking the intake. details below:
I have a propane-fired 2010 Burnham Alpine 105W that came with the house and has always been temperamental. First, it has a tendency to fog-horn - but only sometimes. I had all of the issues on this forum - the stack-temp sensor was faulty, the high-temp sensor was faulty, the fuel/air mix was poorly tuned, the vents weren't done right, etc. I had various HVAC technicians here in the past, but more often than not, I end up fixing all the issues on this unit because they don't take time to diagnose properly. Lately, I've run into a new problem that is bugging me. For the first time ever, I got the Soft Lockout Code #11 which indicates ignition failure (5x). I bypassed the pressure switch, the condensate switch and the high-temp sensor (no improvement). When the system does run, the flame rod reads 18uA on the Alpine display, so it seems to be working. I replaced the ignitor, but no improvement. I took apart the entire vent and checked for dead rodents inside - nothing. By all appearances, the furnace seems like it should work, but when it kicks on, it will light and "whoosh", and then the flame seems to blow itself out before the gas valve closes 1 second later. I checked the 24VAC to the valve and it's coming on and off as you'd expect. It will often do this regardless of what the throttle screw setting is (assuming it's not fully closed). I can even get it to light pretty softly (no whoosh), but it still blows itself out. A far as I know there haven't been any changes to the system in years other than the last time I cleaned it (3 months ago, in the summer). I put in a new swirl-plate when I serviced it last (old one was corroded probably due to exhaust re-cycling). I have heat and hot water loops, calling for either makes no difference. BUT: If I choke the intake to the blower by 1/3-1/2 (with my hand or duct tape), I can get it to light reliably and it will stay lit. It's almost like it's got too much air? While it's running, I tune the throttle screw (while watching my combustion analyzer) - I can get it into the target O2 range at high fire (6-7% while running 66-100% seems to light with the least noise and fog-horn), and the low-fire range is within the spec in the manual, but I can definitely set off some really bad harmonics while I'm trying to tune it, especially when it's running very lean. There is about 5ft of CSST tubing from the gas inlet, so maybe that is contributing to the harmonics? Also, whoever installed it put the intake & vent too close together (9"), but it's always been that way. While it was failing to stay lit, I could smell combustion products, so I added a snorkel to the vent, which should fix that problem, but doesn't address the bad startup issue. I called in another HVAC pro and he thinks it's the gas valve. He also thinks the blower needs replacing because it squeals sometimes, but it's always done that and it still turns fine and all the vanes are good. So...any ideas? Is it really the gas valve? Why all of a sudden and why does it run fine after starting? Why does it seem to start fine if I block 1/2 the air intake?
 
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HaveWrench

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Following up on my own post with the solution (moderator is still holding my posts for review): It was the valve. The old valve appears to not have been opening all the way, even though the resistance checked out and it worked fine when it did get running. The replacement valve is audibly better (much louder click). After installing and tuning (note 1: you must use a combustion analyzer or call a pro who has one!), it purrs like a kitten and starts up better than ever (note 2: the pre-turn table in the manual seemed way off - initial throttle setting didn't allow any gas, but backing it out a 2 more turns did the trick to get started).
 

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