Another version of how not to build a model engine

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AdeV:

--- Quote from: bogstandard on December 03, 2009, 01:16:25 PM ---I was picking up on what you had said Ade, not criticising, but trying to be of assistance

--- End quote ---

Don't worry, I took it to be an assist rather than a crit. And, after I read your post, it occurred to me that every model carb I've ever seen has quite a steep taper on the inlet, not a nice shallow one like wot I drew. IstillNAFD, though

sorveltaja:
I must admit, that I enjoy reading the discussion about that carb thing :thumbup:.

There sure isn't plain right or wrong aspects, but rather options, of how things can be achieved.

As much as I'd like to test variations of tapered venturi's, lack of required tooling keeps me on that basic straight tube -venturi schema.

In the mean time, testing goes slowly, but evenly. With 0.8mm carb throat, the engine fires, and almost runs.

In between, I tested also with the previous carb(1.9mm throat), and it has turned out to be a kind of reference. With that, the engine runs, making it possible to check once in a while, that valves are seating ok, spark timing is ok, etc.

sorveltaja:
After some more testing with 1.9mm and 0.8mm carbs, the engine wants to run only at high speed with both.

Part of the problem might well be the spring loaded intake valve, having tension, that allows only limited rpm range:

It's kind of difficult to adjust its spring tension :scratch:.

I think I'll start a new camshaft. With it, the intake timing should be more constant, than that spring loaded, regardless of rpm :dremel:.

NickG:
That is a good point S. But many slow running engines have an atmospheric intake valve. The spring must be really very weak in such a system so that the suction of the piston will open the valve. From what you are saying it's too strong and the engine can only open it at high rpm?

Nick

sorveltaja:
Nick, it's something I wonder also. When turning the engine slowly by hand, the inlet valve makes snorty sound, indicating that it lets the air in, and also, that the spring tension isn't too strong.

I may be wrong though. But for example, on hit'n'miss engines, I assume, the spring tension isn't as critical, because there is a governor, that effectively prevents the engine from running too fast.

Hmm... should I mod the current engine to hit'n'miss. Tempting option, but we'll see.

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