Another version of how not to build a model engine

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sorveltaja:
I made ptfe washer for the spark plug, and it seems to 'live', so that it's required to tighten the plug occasionally.

That's what I did. Until the exhaust valve hit the sparking plug, and got stuck :bang:.

Anyway, I got the exhaust valve to work, after some persuasion. Not sure how well it seals, but before starting to make a new valve, I'll test if it's possible to get the engine running with current valve.

DICKEYBIRD:
I just finished catching up on this thread. Great job and a wonderful example to all of us to persevere with our projects!

One thing to try is an exhaust throttle. I have had great success with my small (Cox .010 to .049 c.i.) model airplane engines using exhaust throttles and fixed venturis. Granted, these are 2-stroke engines and yours is a 4-stroke but it's certainly worth a try. You might be surprised.

Very small throttleable carburetors are difficult to make work properly due to scaling issues. Reynolds numbers and all that rot.:)

sorveltaja:
Luckily I opened the cylinder head, as whole combustion chamber and valves were covered with black goo. No wonder, that the engine refused to run.

Maybe I used too much oil in the fuel. It's a run-in season for the engine anyway, so I thought, that more is better, than too little . Apparently this isn't glow engine after all, but fire breathing plain gasoline one.

That led me to discover another problem with recently made spark plugs.

Distance between center electrode and plug body was too short, making it prone to conduct between them, or worst, cause a shortcut.

So I made a new spark plug, after seeing some commercial plug cutaways:

sorveltaja:
Dickeybird, good point. I have heard about that exhaust throttle thing... but as far as I can remember, it's only good for two-strokers, as their performance might even advance, when suitable, counter-pressure creating tuned exhaust pipe is used.

Back to the project. I've made some running tests, after the recent exhaust valve hit spark plug -thing. The engine barely runs. Exhaust valve doesn't seat well enough. I'm surprised, that it isn't bent at all :thumbup:.

The engine is now disassembled, and I'm going to re-lap both valves (may be the 30th time, but who counts, as it's test engine :wave: ) .

sorveltaja:
The current crankcase cover is made of 1mm brass, and it's too thin, as it holds the breaker points assy also. When in use, it bends just a bit, letting the crankcase oil to leak out.

To fix that, I'll make a new one using 1.5mm mild steel:

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