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Looks like we are getting somewhere now. checked the Fuel (Gas) cap. No vent there - the tank is vented via a carbon filter to the air intake. Air flow is fine. Disconnected the fuel pump from the carby and stuck the end in a jar and ran the engine up (the carby holds a fair bit pf fuel and the engine will run for a while with no fuel supply. At idle and low RPM the flow seemed ok but as the RPM increased the flow decreased and eventually stopped. I wasnt able to get another pump here at a reasonable price so pinched the pump from my lawn mower which has a 450 - 500 cc motor so should be close enough to work and tried the test again. The first thing I noticed was the fuel flow was much higher at low RPM and at high RPM while the flow did decrease it still kept pumping. The second test only ran for about 20 secs and I collected substantially more fuel than I did in the first test which was about 40secs. So I have left my lawnmowers pump in the Mule and took it back out to the farm and took it for a good run up some hills loaded up a trailer and dragged that up through a boggy paddock and she didn't skip a beat. So it looks like the fuel pump was the culprit (after the gummed up choke was sorted). Interestingly though I put the Mules Fuel pump in my mower and cut the front yard at home and it seemed to work fine. Though the mower doesnt tend to run at as high an RPM as the mule and any heavy loads are generally short lived so the issue would not be so evident - Ive got a new pump on order at any rate. Thanks for all the input and suggestions - I hope the discussion helps someone else in the future.

Great news! Glad you got it figured out. Although we didn't do much more than throw guesses at it. Just remember that we were with you all the way lol.

I recently acquired a new 2020 Coleman Outfitter 400 and overall am quite pleased with it. I had the "too be expected" loose bolts (first thing I checked), all related to the final assembly conducted by the pimple-faced kid "mechanic" at Tractor Supply.--the bolts in the upper driver's side roll bar joint were just hand tight, as were those of the upper seat belt anchors--but other than that it had been pretty well assembled. The included owner's manual is not current; it speaks of using the choke to cold-start the motor (it's EFI, no choke), and shows the ignition and lighting switches as dash-mounted when in fact they are on the steering column. It also direct you to remove the seat to change the spark plug--except that on this revision the cylinder tilts rearward and the cylinder head in found beneath the dump bed. Other maintenance tasks are similarly ill-described and it is generally useless. I did find the exhaust note to be a bit strident and devised the following to tone it down a bit. I used one of those inexpensive 1" NPT female inlet B&S "muffin" mufflers that have been around for decades, and a steel 1" EMT to steel box adapters that have been available for a like time: 1" EMT has an outside diameter of 29.5 mm, the tailpipe extension on the beast is 28 mm O.D.--so I use a partially overlapped cylindrical shim spacer of 28 ga. (0.47 mm) galvanized sheet metal to fit the EMT adapter to the tailpipe . Bedded well in muffler putty the single set screw on the adapter mounted it up quite firmly.On my first ride I found the motor to feel and sound a bit constipated--so i drilled out the 106 1/8" holes in the faced of the muffin to 9/64". This is a 26.5% increase in area [(9/64)^2 / (1/8)^2 = 1.2656] That made the difference needed to restore proper flow without making it too loud again:It is arguably not a pretty as some of the $100+ aftermarket alternative, but at $15 for the muffler and adapter it better fits my budget.Make sure you get the genuine B&S part (# 392989) as the 3rd party clones lack an inner baffle that makes them inherently louder than the B&S piece. FWIW--I also removed the front anti-roll bar--and have found little to no adverse effect on handling (and arguably some positive effect)--it also got rid of an annoying rattle from the low-quality driver's side roll bar tie-rod end. Put in an iridium spark plug too... [update] For anyone who cares. I have re-drilled the 106 outlet holes to 5/32" which seems to be a sweet point. I had the beast running 45 mph (indicated, 40 by GPS) t'other day and it felt a bit "held back" so when I got home I hogged out the holes to 5/32"--another 23.4% area on top of the 9/32' holes (56% more than the original 1/8" holes). Went out today and got it going 53 mph (indicated, 47 per the GPS). Coleman and Hisun claim 38 mph top-end for the HS400 but I knew it had more than that. So, I Think the 5/32" holes are about optimal. I need to take the B&S thingy off and see what it can do, however it (the B&S thing) is so nicely mounted and sealed now that I don't want to mess with it... [/update]