Paul, Poppin is a much different design I’ve made two of those and they ran on first flick too like yours. As you say, the only issue was getting everything tight enough not to move as it it flies around!
With my Jan ridders engine I experienced the same issues as Brian and I had cast iron cylinders. It runs, but not very fast and is very sensitive to flame position. I’ve never known a build of it where this wasn’t the case and most probably don’t run at all. Somebody came to me once at an exhibition with a beautifully made engine ( far more skill than me) and couldn’t understand how i got mine to run! The truth was I think that my poorer machining had meant that the fits weren’t as good and everything ran more freely!
I agree with Doc, the Ridders internal valve will always have performance issues due to its design.
Poppin is a great design and a lovely exercise to build, you’ll enjoy it.
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