Runaway Engine?

First off, grab a foot or two of fuel line hose from your local NAPA or whatever. When you replace yours with new high-quality line, do a "free" upgrade on his. It'll be too stiff to kink the way the stock line does. If you match the curve of the tubing (from it being coiled before you bought it) to the span between the petcock and the carb it not only works really well it looks 'right'.

I think the fuel line kink is definitely part of the problem. Carbon buildup can be a big part too. I had an engine with similar troubles. It didn't want to die even with the kill switch held down. At first I thought it was the wiring of the switch, but shorting out the wires leading to the switch had the same effect as pressing the button. The bike would die, but it liked to spit and sputter and bog down a while, then it would finally shut down. I figured it was running either super lean, or super hot, or both.

I pulled the plug and it was white or at least a very light grey. Checked out the carb and intake and found that there was no O-Ring, and the carb wasn't seated right. Pretty big air leak there. Then I pulled the jug to check out what the internals looked like, since it was a "well used" engine. The combustion chamber was surprisingly thick with carbon build up, the top of the piston was mostly black, with some brown areas around the edges. I grabbed the old Dremel set and got to cleaning and polishing. I used a stiff wire bit to get the majority of the gunk off, then switched to a softer metal bristle bit to polish it smooth so that the carbon would have nowhere to stick. the combustion chamber looked like chrome when I got done with it. I put everything back together, and there was lots more oomph at lower RPMs. The bike would shut down with just a press of the kill switch. It felt like I lost a little top-end, but that may be because I was running richer than before. I love how good these little engines run when they're a little lean, but after sticking a piston on one rather lean running bike, I've leaned more toward the rich side since then. Too much heat when lean.

If you're wondering about the air-fuel ration, do a plug chop. If it's real light in color, you can show your brother that yes, it IS running lean, and the pinched fuel line can be the leading cause. Also, see if there's an O-Ring between the carb and the intake. Without one, there can be big air leaks, even when there doesn't appear to be any gaps.

Yes, I was pondering this combustion process just last night. Why do RC model airplanes use glow plugs for a spark plug? Can't we invest in a glow plug for the 2-stroke engine, which is basically a large RC scale model plane engine on acid, and do away with the whole timing thing and the magneto?

first thought is that getting it started would be trickier

crassius said:

first thought is that getting it started would be trickier

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Use a magneto as a generator to charge a small battery. The battery would also work to preheat the glow plug for starting. Once she gets fired and popping, that is all she wrote!

You need alcohol fuel for a glow plug to work.
The glow plug has a platinum coil that has a catalytic reaction with the alcohol fuel. The only reason you "light it off" with a battery is to get it started, and warmed up.

Bikeguy Joe said:

You need alcohol fuel for a glow plug to work.
The glow plug has a platinum coil that has a catalytic reaction with the alcohol fuel. The only reason you "light it off" with a battery is to get it started, and warmed up.

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Well it is dependent on the characteristics of the fuel, you are right. However, old deisels used glow plugs to generate enough heat to cold start an engine, which is high compression and fuel oil basically.

I think it is possible to manufacture a Glow plug for a large 4 stroke. I may look into it and query some manufacturers.

Diesels are a completely different animal.
There are two stroke diesels as well.

There are of course diesel conversion heads for two stroke glow models, but I'm not sure our cranks would not hold up to a diesels' compression.

Bikeguy Joe said:

Diesels are a completely different animal.
There are two stroke diesels as well.

There are of course diesel conversion heads for two stroke glow models, but I'm not sure our cranks would not hold up to a diesels' compression.

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Well the point is; can a glow plug be improvised into a China Girl motor? Assuming the right fuel conditions and heat range of course (hypothetically)

At any rate, if you converted our engines to run on a glow plug, you'd still need alcohol fuel.

Then they'd use 3 times the fuel, and glow fuel is really expensive.

The trend in R/C planes (all the way down to .40) is to run an ignition.

But yes, if you wanted to make a glow plug, and run alcohol, you could certainly do it. I just don't think there's any advantage to be had.

Bikeguy Joe said:

At any rate, if you converted our engines to run on a glow plug, you'd still need alcohol fuel.

Then they'd use 3 times the fuel, and glow fuel is really expensive.

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Why do you assume that you need glow fuel? I know it is out of the scope of my experience and maybe yours as well, but I think it might be plausible considering the applications that glow plugs are being used with different fuels. I know it seems a mute point, but considering the only major failure on the China Girl engine is the electricals it seems it should be re-engineered. Just a thought off the top of my head though. I like to think outside the box sometimes.

Diesels use a completely different type of "glow plug" to heat the combustion chamber and help the diesel start when cold. They don't run on a glow plug like a glow engine.

As I said, glow engines need alcohol fuel to sustain combustion by a catalytic reaction with the platinum element of the glow plug.

I am not assuming anything, that's just how they work.

Bikeguy Joe said:

Diesels use a completely different type of "glow plug" to heat the combustion chamber and help the diesel start when cold. They don't run on a glow plug like a glow engine.

As I said, glow engines need alcohol fuel to sustain combustion by a catalytic reaction with the platinum element of the glow plug.

I am not assuming anything, that's just how they work.

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True. Good post.