The compression of the bottle you're seeing is the air trapped inside of it compressing because of the lower temps. Try filling your fuel bottle absolutely chock full, then throwing it in the cooler again. I'll bet you don't see the bottle collapse in that case.
Double check your drivetrain too. Make sure there aren't any blown out bearings, that the brake releases fully when you grab the throttle and that your rear wheel bearings don't bind when you tighten the wheel nuts down.
Double check your low speed needle setting too. Here's the best way I've found yet. Run the truck until it's up to temp, then come to a stop and idle for 5 seconds. Grab full throttle, and see what the truck does. If it spews a ton of smoke, and stumbles over itself before clearing out and taking off, it's too rich. If it takes off immediately like it does when you're already rolling with little or no smoke, it's too lean. If it puffs smoke and just BARELY stumbles when you grab the throttle, then you're on the money.
The low speed needle being off has bitten me in the past. If you're too lean on the bottom, you end up adjusting the top too fat to compensate, and your fuel mileage goes down.