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

I bought myself a bit of a conundrum followed by some work that’ll be needed to sort it.
The ongoing talk of making the M series run slow got me thinking.
I decided to play a bit.

My M91 hasn’t been started in a whilst, and I thought I’d play with ignition timing a bit. It’s never been easy to cold start, regardless of priming ritual or mixer setting. It’s easier once warm, but even then it’s never hand started, requiring a brisk rope start.

Thinking back to late in the last century, the one battery ignition gasoline two stroke giant scale RC engine I owned (a big six horse brute that pulled a 28 pound 1/3 scale plane straight up) I remember it was so easy to start because the ignition timing plate was slaved to the throttle linkage. At a tick over idle timing was severely retarded so you could hand start by flipping the prop very reliably. Under throttle load it advanced timing a bunch.

Ergo, I theorized, might be my M91 could stand the timing pushed a little later in the party so it would start easier and Tick over better at the bottom end of the speed curve.

So far, so good. Then I made a tactical error. First I thought I’d start and run it before adjustment, but it didn’t want to cooperate. Fuel was fine, mixer fine, primed and no bubbles, verified spark, and all I could get with a brisk rope pull was a single pop late in the spin. Then as I tried a few more times, it started popping loudly, like a car backfiring, and still wouldn’t start.
So I thought maybe the timing had slipped?  I’m thinking ignition sensor slipped maybe, so I observed when the exhaust valve JUST cracked open, right as the slack came out of the rocker, and sure enough, the trigger plate was nowhere near BDC, Ignition nowhere near tdc. So I mindlessly undid the set screws on the trigger wheel, and paid not enough mind that they didn’t seem loose. I hauled the ignition around to agree with the valves, secured the set screws, pulled the rope and it popped indignantly and stopped dead. After an astonished moment the dim light lit. I’d forgotten this isn’t a keyed gear or chain timed cam.  The timing gear set screw is what had apparently slipped, not the ignition trigger. Now of course I’ve got it all out of whack. I’m done for the night. Tomorrow I’ll pull the spark plug to verify tdc, get the valve timing straight, then and only then get the trigger plate timing back where it belongs, this time a skinny hair after tdc.

I felt like a complete moron assuming it had to be the trigger plate, forgetting the timing gear, and not physically verifying piston placement, relying on the exhaust valve instead.

So you have those brilliant brain waves that turn into a total brain fart Moment to ?
I have done the same thing before had stupid brilliant ideas .
Not checking all possibilities before rushing in .
Never mind have a good sleep and try again .
You will win eventually.
Cheers
Dennis

Thanks. I did it the old fashioned way, pulled the spark plug and stood a 1/8” dowel on top the piston.

Thanks. When visualizing spark to time an engine, my default is to pull the lead off the plug, and arrange it so when the box fires, the lead arcs to the engine head. I set it up with maybe 1/4” or less gap, and that’s always given an arc.

As Tony said, you may not want to risk your ignition box. It is small and the components are quite delicate.
If you should leave a gap too large for it.... it will fry itself near instantly.


I simply remove the plug, make sure the wire is on all the way and hold it's threads against a head bolt and turn the wheel slowly with my other hand and rock it back and forth to establish where it actually fires, while looking for the firing point on the clockwise turning of the wheel.
(hold plug by the hex nut part of it and as long as it is touching the head bolt, you will not be shocked)


Last step.... Have fun...!