Surprise surprise.....

Decided to replace the throttle control & did so, but not without a major oversight which almost cost it all. Yep... An Emgo which had a poorly finished twist tube. A little filing & dressing later got it working through the whole rotation, but at some point after the final check the cable had caught when I operated it...…. I swear you guys are clairvoyant to have guessed how startup went.... 0-6000+ immediately before I could cut it. I thank God it wasn't a new tight motor or I'd still be searching for bits.
Ergo more filing & dressing and the motor murderer area has been discovered and removed. Moral of story: Keep checking when you cheap out because it could become not so cheap quickly. I'll now take my flogging.

Kill switches save lives.

did it successfully pass the STATIC throttle return testing prior to starting the engine?

Sure did. About 20 times, but there was an unseen area waiting in the wings for an ambush which I walked right into.... Cable had caught on rough spot which held it slack preventing carbs from closing the last time I twisted it open.... I know.... major mistake was having put filter on before running in order for a last peek. Hindsight is wonderful. I got lucky this time.

I'm doubly pleased because I see JD cleaning house before the site turned into a battleground.

My Dad (an old time auto mechanic) always said "Never buy cheap stuff, you will end up buying it twice!"

In your case, you barely escaped buying a new engine.

Slick

I would NEVER start an engine without a functional kill switch.
A switch, not a button..

On a smaller scale I bought (and subsequently put in store) an uprated Mk3 starter that was a 'limited run'... As I was belt drive and kicking it at the time I had no immediate use for it, but the belt wasn't nirvana for me, so a return to primary chain and starter button again was always on the cards.
Months later, changed it all back and installed new starter just ahead of the yearly MOT test.
Still kicking (by now a habit) watched the MOT man fire it up on the button, starter runs fine... but won't stop!! MOT bay, no tools for miles, eventually find necessary spanner to disconnect the starter as the solenoid has stuck solid. Sod's law as replacement solenoid shows starter now U/S and one month out of warranty
Fortunately the guy who built it took it apart to find the bearing had leaked it's lube all over the brushes, hence it's inertia, but quick clean and now all's well. We live and learn, no?

It’s important to have a way of stopping your engine.

If there’s no air filter, blocking the carb air intake with your hand will stop an engine. Not so easy to reach two intakes on a twin carb engine, I know.

Agree, a kill switch, not a kill button.
For those of you not familiar with the Gardner carb, everything is upside down. Idle adjusted with a grub screw on top. Dropped down, blocked slide at about 1/3 open. Killed engine and took out the grub screw. Replaced with a hex head screw. Problem solved. Could have ruined the whole season. Molnar usually don't have Manx engines in stock for immediate delivery.