It's a heck of a mower, nice find! What behavior is it exhibiting when you try to start it? They will usually be gapped properly and run or not and don't, not too much middle ground most the time. I find that a business card is the best tool to employ when galling an ignition system. Just put it on the flywheel and let the magnet pull the magneto in. Tighten the securing screws then pull the card out.

I've had two Honda OHC engines but neither mower had a 'clutch' (I guess this is the blade control). The first was a GCV 160 and now I have a GSV 190. I have not heard any knocking from either one. They do have a little 'pop' at low speed like an OHC bike engine - maybe that's what you're hearing. I'm sure if it were a serious problem like excessive bearing clearance it would get worse when hot and under a load. Have you checked the blade nuts and engine mounts? I might also be inclined to check the flywheel nut if it's really making noise. You might be able to reach up under the shroud and see if you can detect any play in the flywheel with the blade blocked.

I really appreciate the suggestions. Very kind of you. Gas cap is ok. I have a neighbor with a similar mower, and I borrowed it for a little bit. He knew I was having trouble with mine, and he pleaded with me not to take his apart. I convinced him that I just wanted to take some pictures of the linkages, which I did. The cables, springs, etc. seem all correct. Keep in mind that the mower was running fine a few weeks ago (after winter storage), until I tipped it the wrong way and drenched the engine, and soaked the carb and filter with oil. And then I did get it started and up to speed, even if smoky, before it started with the low idle situation. I'll try shooting carb cleaner into the venturi. I'm nervous about messing with the valves, but hey nothing ventured nothing gained. Thanks again.

I think the 2013 Honda Odyssey automatic transmission has an internal filter. (At the site below, I had to choose a submodel to get a selection to appear. I chose the EX level although I would think that all trim levels would have the same transmission filter.) I am not recommending for or against JC Whitney, but display it here to show that they sell a filter for that transmission. Your owner's manual should show the replacement schedule if required. It not, consult a repair manual (if you can find one). Honda Odyssey Transmission Filter JC Whitney - Honda Odyssey Transmission Filter