Office of Steam Forum for Model & Toy Steam Gas & Hot Air Engines

G'day all,

I've had this problem with the Microcosm Hit & Miss Engine since it arrived.
Anyone have any ideas on how to get it running?

Thanks Bruce, that's pretty much what I do with this mine as well but this engine defeats me.

Its the only H&M engine that I have that has the drill chuck spindle for 'easy' starting as well

I'll keep plodding away with it.

It can be seen running, begin viewing at 6:45 in your video https://tinyurl.com/2b6ufxw8 .

The way it dies out hints at a fuel feed problem, especially because you have tried another power pack to verify spark.  Remove the needle valve and blow out the tube.  A stray bit of debris may be captured there that blocks the jet orifice.

When it ran and then stopped, at that moment did it seem to have less compression?  If so, check for a tight exhaust or intake valve guide that binds and prevents the valve from closing properly when the head heated.

A low tank (and low fuel tubing) would require more intake vacuum to lift the fuel, but the M91 runs as designed.  If you had to raise the tank to make it run, that was compensating for something in the assembly not up to spec.

If an engine has spark (intensity and timing), compression (sealing ring/s and valves), and properly metered fuel flow, it will run.

With Jim’s engine, if it sucks in fuel on the intake stroke and there is a spark at the top of the compression stroke the fuel has to ignite. It has run, but briefly, so which one of the requirements is not doing its job? If it has compression, then fuel becomes the focus. The fuel/air ratio is generally the cause of starting and continuous running problems. If the mixture is too rich the plug may have gotten soaked. To fix that, remove the plug and blow it off with compressed air. Also hold down both valves and blow air through the spark plug hole to dry the compression chamber and clear the carburetor venturi. Reinstall the plug, close down the carb needle and then open it 1/8 turn. Try to start it with about five revolutions of the crank, spinning it more is pointless. If no bang, open the needle a barely perceptible amount and retry. Keep repeating and you will eventually hit the sweet spot where it runs, usually somewhere between 1/8 and 1/4 open—if the carburetor jet was drilled and position properly.  My M90 starts with 3/8 turn. The threads on Jin's carb needles are coarse which makes fine tuning difficult, so be patient. It can be helpful to glue a larger head on the needle valve which will make precise tuning a lot easier.

Mentioned before, I’ve found these engines to run best on low octane gasoline mixed with Marvel or 2-stroke oil. Naphtha fuels are less volatile and can make starting a troublesome engine more difficult. I shouldn't mention this because it can damage an engine, but aerosol engine starting fluid (ether) ignites easily and can sometimes help.  Tread lightly.

I’ve acquired four of Jin’s engines and all have functioned right out of the box, but all except the M17B needed adjustments to run better.  Some of those improvements required disassembly and examination with magnification to find the problem.  It’s a hobby.

The carburetors on these engines are pretty crude.  It’s easy to twist right past a very narrow sweet spot, especially when everything isn’t perfect.  Fuel? I’ve been there and back with zippo. It’s simply not very energetic, so it doesn’t suffer anything less than great very well.   Gasoline as sold for motor fuel is more energetic, but suffers storage issues. Old gas, bad gas, etc are real things.

So if you sent me the engine, I’d first completely flush and blow dry the carb and fuel line. I’d refill the tank with known to be good gasoline with about 50:1 marvel or two stroke oil, Then insure the engine will suck fuel when cranked. Any bubbles present in the gas line should move when you crank the engine briskly.  Set the needle at 1/4 turn open, and in steps no more than one clock position, open the needle and retry.
If you thumb choke and hand crank until the line is full of fuel and your thumb gets wet, then turn on the battery and drill spin it briefly, the engine should at least pop if it’s anywhere close.
What these engines don’t like is leaks. The carb vacuum is weak, so fuel lines can’t leak or you lose suction. Persistent air bubbles can be maddening. Get em gone. I’ve literally used a needle oiler to refuel, and sucked air bubbles out of the line with the needle, by way of the filler cap.
Insure flywheel set screws are tight. A slipping flywheel will put one of these engines out of action, and until you grab and twist, you might not know one was loose. Just stuff my m91 taught me.