My new 2020 ZForce 800 Trail has been pretty reliable during it's first 30 hours of use but I've had one little annoying issue.  Normally it starts up with a quick turn of the key but I've had a few times where it won't start unless I crank it with the throttle pedal depressed.  This always seems to happen after I've added fuel to the tank and transported it on a trailer to a different altitude.  I've noticed it doing this both after transporting to a higher altitude and coming back down to a lower altitude.  I'm not sure if the change in altitude is the culprit or adding fuel to the tank is causing this or the movement on the trailer or a combination of all three.  I always make sure the fuel cap is on tight after refueling.  It only happens once at arrival and starts flawlessly afterward until I trailer it again.  Any thoughts on why this is occurring would be greatly appreciated.   

[2015 Z Force 800 53/Trail EPS]

My new 2020 ZForce 800 Trail has been pretty reliable during it's first 30 hours of use but I've had one little annoying issue.  Normally it starts up with a quick turn of the key but I've had a few times where it won't start unless I crank it with the throttle pedal depressed.  This always seems to happen after I've added fuel to the tank and transported it on a trailer to a different altitude.  I've noticed it doing this both after transporting to a higher altitude and coming back down to a lower altitude.  I'm not sure if the change in altitude is the culprit or adding fuel to the tank is causing this or the movement on the trailer or a combination of all three.  I always make sure the fuel cap is on tight after refueling.  It only happens once at arrival and starts flawlessly afterward until I trailer it again.  Any thoughts on why this is occurring would be greatly appreciated.   

Well, if...if what's happening is fuel entering the cylinders while off and being transported, the only thing that can do that is leaking injectors. When you shut it off there is still pressure on the system which normally will leak back to the tank but will just as easy drip out a leaking injector and while in transport the sloshing fuel will build-up a few ounces if pressure which can also leak through a leaking injector. It's however rare to have two leaking injectors at the same time unless both got some trash that got by the filter. If it were me I would pull the injectors and leave the fuel lines on and turn the key on long enough that the pump stops...then watch both for any drips or leaking. DON'T try and start it! Just pressure it up. If one or both leaks...you have your answer. If not...well the problem is elsewhere. Maybe the air temp/Mass airflow sensor is coated with oil from your Unifilter...you do have one on it...right?

Yes I am running the Uni Filter.  I understand what you are explaining about the injectors but the funny thing is that if I don't transport the ATV and just leave it parked, even for days at a time, it starts up right away with just a quick turn of the key.  If the injectors were leaking by flooding the engine I would expect this would happen all the time unless you think the jostling of the machine on the trailer causes the injectors to bleed by.  I though perhaps it was related to the atmospheric pressure (MAP sensor).  It happened again this weekend after I went from about 6,000 ft. to 8,000 ft.  Again once it's at that altitude she starts up fine even after being shut off for awhile.  Is there fuel tank vent or return line with a check valve that might cause this?  I'm wondering if the tank which is opened for fueling at a lower altitude becomes pressurized when moving up in altitude forcing fuel through the injectors?  I'm stumped.