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...you'll get lots of ideas on this one, some of them contradictory. Don't worry too much about it. It's HARD to diagnose sight unseen! Some of us will be right and some of us will be wrong, and this is no reflection on any one individual's ability. It's just the nature of the 'net. Here's what you might consider doing: (1) Check the governor for operation. I think it VERY HIGHLY UNLIKELY you have a governor problem, but this test takes only a few seconds, and can be done without disconnecting anything, so go ahead and do it to satisfy yourself the governor is not the problem. With the engine OFF, grab the throttle rod (the one that goes to the butterfly valve in the carb) and move it gently through its range. You should feel VERY little resistance; if you encounter resistance, check the linkages. In addition, some carbs have bushings for the throttle shaft, etc., that can wear and cause binding. Assuming the throttle rod is nice and free with little play: start the engine and move the throttle rod again (showing the proper respect for the belts!). You should feel a greatly increased resistance. If not, your governor may need a rebuild. If you DO feel the increased resistance, you may assume the governor is OK for now; although it may need adjustment, this usually shows up as an incomplete RPM range (won't idle slow or rev fast enough), NOT as rough running with the choke rod in. (2) Engines with weak spark will sometimes run better with a higher ratio of fuel to air in the mix, which is what you get when you pull out the choke. Check your spark by running the following test: get a plug and gap it at 3/16 inch or so (I keep a spare around pre-gapped for this purpose) and pull a spark wire. Connect the test plug to the wire and hold it up to the engine block (don't worry about paint: at the several thousand volts the secondary of the coil develops, the paint is just a dialectric that will break down and allow the current to ground with little trouble). The spark should jump the gap with a nice, wide, blue arc. If the arc is thin and/or yellowish in color, the spark may be too weak to do the job. If the spark is weak on the test plug, check the points for condition and gap. If the points check out, try substituting a coil. You can take resistance readings of the coil with an ohm meter. It should show a couple of ohms or so, but these are static readings and tell us little about how the coil is actually operating under load. It is impractical to check the coil while it is in operation with anything much simpler than an oscilloscope, and anyway, the shelf life of a coil is many years, so I just substitute. If substituting the coil makes a difference, the old one goes in the trash. If not, I haven't found the problem, but at least I've now got a spare coil! If the test plug DOES show a nice bright, blue spark, pull the plugs and check them for condition and gap. Is one or more fouling frequently? Time for a compression check! (3) Check the fuel system for correct flow. IIRC, M has a gravity feed fuel system; unhook the fuel line at the carb. Let it flow long enough to run a gallon or so through. You should see a nice, even flow with a stream about the diameter of the fuel line. I know you've cleaned the carb; but if the governor, ignition system, and fuel flow check out, the next step would be, IF POSSIBLE, to borrow a carb off a similar M that is known to run well, and substitute it out. If you still have to pull the choke to get it to run with the substitute carb on, your carb is probably ok. If everything else checks out, you can consider running a quick compression test. Pull the center wire from the distributor and crank the engine using the starter motor. The starter motor should run at a constant speed and you should hear a nice EVEN "unh!" as each cylinder goes past TDC on its compression stroke. If the starter motor speeds up and/or you hear a "whiff!" as one cylinder passes TDC on its compression stroke, time for the more scientific compression test with a calibrated compression tester. My GUT instinct is that this is either a spark problem, or a carburetor problem, though. Good luck and keep us posted.
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