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Some 1-cylinder small engines have instructions that before long-term storage you should take off the spark plug, pour few milliliters of oil into the cylinder, and very slowly pull the starter cord (with fuel supply and ignition off) to lubricate the cylinder with oil, and re-install the spark plug.

My inverter generator is one of those where the instructions say that. Unfortunately, the manufacturer has made the spark plug access port so small that I can't with my large hands reasonably change the spark plug without dismantling lots of the covers of the generator, which would take so much time that I have decided instead to not do the "oil in spark plug hole" trick.

I have an alternate plan which might provide some of the benefits (namely, improved lubrication, not improved rust protection), without requiring so small hands that it's possible to reach the spark plug through the small access port.

The plan is to run the engine out of gasoline before long-term storage (which would be advisable in any case, since old gasoline can gum up the carburetor, and also fuel lines can deteriorate if they are constantly full of gas). Also before storing the engine, the starter cord should be pulled until you feel you are in the position where there starts to be a little compression (so the engine is stored valves closed, to give at least little rust protection). Then when re-starting the engine after a long-term storage, the plan is to fill it with gas, move it to the operation area, and do few rapid pull starts with choke valve off, fuel supply valve off, and ignition off:

  • The fuel supply valve would be off to avoid recently filled gasoline getting in to the engine.
  • The choke valve would be off, in case there still was gasoline in the carburetor, which could flood the cylinder if choke valve was on
  • The ignition would be off, in case there still was gasoline in the carburetor, which could start the engine so fast that the few pulls of the start cord wouldn't distribute oil before engine is started

The idea here is that since this is a splash lubricated engine, few rapid pull starts should rotate the engine enough that oil is splashed around, and hopefully oil would coat the cylinder wall due to those splashes.

I have read that typical gasoline engines have about 7-10.5 MPa peak cylinder pressure. On the other hand, a 9:1 compression ratio engine would have 9^1.4 = 21.674 times air pressure without ignition (where 1.4 is the ratio of specific heats), which is about 2.2 MPa, far less than peak cylinder pressure. So theoretically the forces on the piston should be much smaller without ignition but with compression, which should mean the cylinder wall should be coated with oil with less wear than there would be if there was fuel and ignition. Then when the engine is finally started, lubrication would be already assured during the first fractions of a second.

I understand that hybrid vehicles can do something similar. They don't have a conventional starter motor and a small battery but rather a more powerful motor/generator and a big battery that could rotate the engine for tens of minutes at least at any speed. So they can rotate the engine at idling speed, wait for oil pressure to stabilize and for oil to coat all parts of the engine, and only then inject fuel and start ignition.

Does this idea make sense? Or is the rotation of crankshaft during pull starts so slow that oil doesn't splash efficiently enough?

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  • Can you squirt motor oil into the carburetor directly while someone pulls the rope to have oil ingested into the cylinder? A little is better than a lot to avoid too much oil creating a compression lock, described as hydrolocking (where too much) fluid isn't compressible. That's the main reason to pull spark plugs for squirting oil then rotating the engine without compression for easy cylinder oiling prior to storage. Excess oil is blown out the plug hole.
    – F Dryer
    Commented Sep 19, 2022 at 0:56
  • Why not fog the motor with fogging oil when shutting the generator down? I would argue against running dry and instead suggest using stabilized gas, but to each their own Commented Sep 19, 2022 at 14:36

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I agree with Brydon, spray oil fog into the carb as it is running out of fuel. The reason to put oil in the spark plug hole is so oil will lubricate the upper cylinder with more than normal amount of oil so it will last for the length of time it will be stored without drying out. Pulling the cord will distribute oil inside the crankcase, but will do little to add enough oil to the upper cylinder which is where it needs it most, so as not to allow rust to form in a dry environment with raw cylinder metal. Oil fog has been used for a long time, especially in 2 cycle engines for this very reason. This is probably the easiest and simplest way to deal with your situation.

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