Another version of how not to build a model engine

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sorveltaja:
Bluechip, there is no additional capacitor, as the kit's instruction manual emphasizes not to use any. Also the resistor, that was originally attached to the coil, was removed.

Here is the current fuel tank, that is now modified to vapor carb:

To simplify the flowback valve, I removed the previous one, that was outside of fuel tank, and made it to the lower end of the inlet tube:

That is a 4mm(3mm id) brass tube, and it has 3mm steel ball, that acts as valve. Bit of tube shrinking was done, to form the actual closing surface for that ball.

sorveltaja:
So far, I haven't been able to get the vapor carb to work good enough. Engine runs with it yes, but only at high rpm.

Numerous setups and combinations were tested. None of the different size fuel hoses makes it any better :scratch:. Be it inlet or outlet hose/tube.

I have seen the vapor carb used successfully on bigger engines. Who knows, maybe it just doesn't work on engines this small.

Also the double sparking -problem still exists, even though I made proper ignition cam with lobe.

If this project had a name, it would be an ML. Murphy's Law.

sorveltaja:
Addition to that vapor carb thing, I suspect that the engine ran too lean mixture with it, no matter what :scratch:.

In the meantime, I've changed back to ordinary needle carb, that gives more richer mixture, that the engine seems to prefer.

madjackghengis:
Sorveltaja
A couple of things regarding your fuel mixtures: I suspect Jan stopped the fuel inlet just before going beneath the surface because of the substantial increase in vacuum necessary to get air flow through the liquid, when there is a layer of fuel vapor hovering above the fuel. Secondly, there is a need for space between the check valve and the intake, as the intake charge is of low mass and rather low velocity, and it is its inertia which opens the check valve, and it needs room to build up velocity, given the low vacuum presented by the intake stroke. Check valves on torches are right at the valves because a "backfire" provides plenty of power to the flame that is trying to climb backwards up your hoses and blow you up, by the energy of the ignition and its "pop". In low velocity systems, inertia is the single most important energy factor, which is why long intake manifolds work well with small engines. As size goes down by the square, mass and volume go down by the cube, and it is mass which is the smaller of the factors in inertia, and velocity which is the major factor in the energy of the intake charge. For what its worth, that's my input. I hope you get it to a happy state soon.
jack

sorveltaja:
Jack, thanks for the reply :wave:.

I tested the vapor carb also with vertically adjustable inlet tube, so it was possible to take its lower end as close to fuel surface, as possible. Still no low rpm running though.

--- Quote from: madjackghengis on January 22, 2010, 09:38:41 AM ---
As size goes down by the square, mass and volume go down by the cube.
--- End quote ---

Hmm.. that's interesting. I'm thinking of some simple formulas to calculate needed values, but there seems to be so many factors (and variables?) on the engine and fuel tank, and between them, that I can't figure out how to interpret all that mathematically :smart:.

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