if there is no packing you might as well take the can off its like running a straight piece of pipe straight out of the motor what do cars do with no mufflers they blow flames nothing to restrict the flow of unspent gases
some turbulance is good and an exhaust creates slight back pressure and a scavenging effect pulling in the exhaust out instead of the motor pushing it when you disturb the physics how they make power you actually lose power more than likely
After getting the ncvq needle and going to put the carb on i realized the boot had 2 huge cracks that you coudlnt even see unless spread apart. So what was happening when i checked for an intake leak was, when i sprayed the carb cleaner on it the cracks were pushed closed and it didn't idle up but if i blipped the throttle or really hammered it causing slight pull or torque like affect on the carb which in turn caused the cracks to open up slightly letting air in. So Im almsot 99% this is the problem and i already have a new boot on order! I will update as soon as i get it!
I bet I figured it out! So I 2004 yfz 450 and I took the carb out to clean it and maybe up the jet by a size in the main from a 175 to a 178 so i put it back in and it started back firing and popping... long story short I tried every jet setting to come and find out I was putting the piece of metal that sits on top of the slide piece upside down... i put it back in and ran great once again so try that maybe
Low Fuel Pressure or Weak Fuel Pump; Like a clogged fuel filter, this would cause your engine to not get enough fuel to the combustion chamber. A Hole in the Exhaust; If there's a hole in your exhaust system (muffler or tailpipe) near the engine, this would cause backfire.