Onan RS1200

My WAG is you have bad fuel that will easily ignite in summer but not in winter. If this generator ran on a liquid fuel (gasoline or diesel) then my WAG would have more validity.

Have you checked your propane source? Maybe water or other debris has corrupted the fuel. Maybe cooler temperatures have affected the pressure regulator.

Sounds like it doesn't like the cold.

A heating pad against the oil pan and then some insulation could keep it warm enough to start anytime. Maybe might be worth checking out.

That's as good an answer as anyone could make. GA there.

One other interesting symptom of a weak spark is poor cold starting. One reference said to try reducing the spark gap to 0.025” to help cold starting complaints...

also some tips about not priming, but your ’choke’ fix indicates lean mixture, unless the primer flow is reduced when the velocity through the carb is reduced...

Propane and natural gas require from 50% to 100% higher temperature to ignite a charge as compared to gasoline...

https://www.propanecarbs.com/questions.html

Sounds like the gas pressure is dropping with the temperature, as the air density is increasing, this resulting in a lean mixture...So there are probably a few ways to go, heat the propane tank, add a starting fluid injector.....

"... summer it is 75 to 85 degrees in the building and during the winter time it stays 50 to 55 degrees. when this is going on in the winter time after it runs for 15 to 20 minutes i can fire it up 1/2 hour later ..."

When you run it for 15 minutes during the winter....

1. does the air in the room heat up appreciably, and does it stay warmer for around half an hour after the 15 minute run?

Or...

2. Does the fuel supply (tank, regulator and supply lines) heat up appreciably and stay warm for around half an hour after a 15 minute run?

Reduced air temperature and reduction in fuel delivery could be part of the problem, and your manual choke cold start proceedure would lend credence to that theory. However, the engine might also started better with your manual choke proceedure when warm.

A cold engine with thick viscous cold oil is much harder to turn and therefore start. Engine oil is probably up to a decent temp after running for 15 minutes and likely stays warm for a period similar to the time you describe for which easy starts are available.

Implementing an inexpensive system to heat and insulate the oil pan to keep oil from getting below 75F could have additional benefits of reduced wear, reduced start up emissions and improved start up efficiency.

Is this engine liquid cooled or air cooled? I ran propane on 300 cid Ford van for 6 years in Ontario Canada. We had to run 50/50 antifreeze/water to prevent the propane freezing and not flowing in winter. As propane evaporates it drops in temperature much more than ambient. To allow it to get to the carburetor and not freeze en route you need a stronger blend of antifreeze. Now if it is air cooled I have no experience but try applying some heat to the evaporator with a heat gun or similar source to see if the propane is indeed freezing at the evaporator.

This sounds very plausible for Ontario but that's because it can get much colder than 50°F in winter there.

I would be doing some basic maintenance checks - compression, how hot the spark is, and replace the spark plugs for good measure.

If the manufacturer allows - use a multi-grade winter oil weight.

I don't know your carburetor - but can you set it slightly richer? ie: verify the mixture. You are probably running on the lean side. Is there an automatic choke that is not working?

If all other efforts fail,use a radiator hose heater block that mounts in the lower radiator hose.It will keep the engine warm,as well as the oil.This will probably solve the problem.

I do not know your radiator hose size,so I have provided a link to an assortment of them below:

https://www.amazon.com/s/ref=nb_sb_ss_i_3_30?url=search-alias%3Daps&field-keywords=block+heater+lower+radiator+hose&sprefix=in+block+radiator+hose+heater+%2Caps%2C181&crid=8F5WSIL1GRUY&rh=i%3Aaps%2Ck%3Ablock+heater+lower+radiator+hose

Another note concerning LP fueled engines:The oil changes should be performed on time.The oil will not look dirty,because it is a clean burning fuel and there is no wash down during start up,and less blow-by than a gasoline engine.

Most of the time the oil will look just as clean after 500 hours as it did new,but it must be changed anyway.

If all else fails, then arrange a service call-out from the original equipment manufacturer's service team.