Funny how they left that out in the SRB, which one is #1, like O.M. says, one closer to the radiator as you sit in the seat.
This applies to 3T's with the 2 cyl vertical starting engine, older type prior to 17A series and beyond.
Line up that flywheel mark as shown, you can put a cork in #1 cyl plug hole or feel the air push out as you rotate the motor, so #1 is coming up top dead center on compression.
I had to get one of these running, first time, had never done it before, it's one procedure I won't soon forget
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If have not removed the mag, you might have to for repairs if it does not have a hot spark.
Once removed, you can turn the mag over, check its firing order first spark is #1 cyl., 2nd is #2 cyl., I had some trouble lining up the coupling when mounting it back on and having the mag set to fire #1 meaning tension was built up in the mag BEFORE it snaps over and fires a spark, install the mag with it coming up to fire #1, having #1 coming up on TDC, compression stroke, with that flywheel mark moving toward alignment like the SRB shows, it will be correct. Probably easier to show someone than explain. I used to fire the mag in my hand and actually touch the contacts to see what one fired 1st, it's a little jolt so I don't recommend this practice, but it did not bother me too much to snap it over and check the spark that way, still good to see the spark, nice bright blue spark is good if you can ground it out to the top of the plug, should be able to see it.
These starting engines do like a hot spark, use solid conductor wires not carbon filament or whatever they are, I've actually used copper wire in a pinch. Champion plugs don't seem to perform like they used to, NGK, autolite equivalent might better. They also like good fuel flow, tank can be rusted up with coffee grain size rust particles, clean the tank out if need be, sediment bowl and the filter near the carburetor inlet, remove and clean carb if necessary.
Once you have fuel and spark sorted out, it will run, but if you are not familiar with these and at risk of sounding like a broken record here, some things to know about them that may help you;
1.) Once running, you need to engage the diesel and get it spinning to circulate coolant through the starting engine, or it will overheat and probably seize, old school flathead motor, spinning upwards of 2500 RPM, don't take much.
2.) CLEAN OIL - check the crankcase and make sure to change that oil, clean the sump pits if need be, take off the inspection cover use suction, drain plugs are hard to get to, but can be done, and don't get anything in there, wash the track down and keep all foreign materials out, even lint from a rag if you open that cover, I used to do this on mine seems easier. Always check for gasoline in the oil, change it immediately if you find gas has leaked into the crankcase. Oil thinned by gas will damage this motor in short order. I actually used to drain the bowl on the carb once the diesel fired up after seeing the crankcase oil over flow with gasoline once, I forgot to shut the fuel valve off, you can let it run out, but installing a petcock under the bowl with a hose is quicker and eliminates any gas from leaking down.
3.) When running, and you are spinning the diesel, once the diesel fires, if the pinion does not release from the diesel in time, it can overspeed the starting engine, once I open the fuel and place the compression lever on run, I hold the carb linkage and am ready to turn the throttle down on it if it does not release, even when the pinion latches are adjusted properly to release at the correct rpm's, it's just a precaution and will slow the pony down and it will usually idle or shut down. I forget how the governor works on these, just a precaution, over speeding one of these, say with gas thinned oil, it's going to fail, toss a rod or seize.
You may already know about these things, but if you are unfamiliar with the starting procedure use care and be safe, make sure it's out of gear etc., the operators instructions would be a good thing to read. Good luck with it, hopefully you'll keep that scrapper away