My 2002 750 ace keeps backfire when I rev it up

Have the spark plugs been replaced? Have the plug wires been tested?

That would be at the same Rev range in each gear,
Exhaust/ CARBY jets/valve timing /distributor timing vacc advance/fuel pump replace [or get it checked out.]

Check the fuel pressure, could be the fuel pump

unusual, would normally say air filter but thats new. theres no restriction on fuel is there from fuel tap. so it runs out and slowly fills, time you check its got fuel again. is there a breather into tank blockedi has it once on a bike that used exhaust paste. the person had put too much in and it blocked the exhaust.

Sounds like your fuel air mixture is a little rich. Check your plugs, if they are black you are running rich which will cause some of the excess fuel to be burned off in your exhaust. Two common problems with a bikes fuel air mixture is bad plugs, if one does not fire consistently all the time it will leave excess fuel in the combustion chamber. Another would be a clogged air filter keeping the fuel air mixture unbalanced as well.

Lastly if you are able to, you may adjust your air fuel screws on your carb. However most Honda's are stock with Keihin carburetors and these air fuel screws are plugged so you cannot make adjustments without drilling out the plugs.

Starving for fuel.
Make sure carbs are getting fuel.
Make sure fuel filters are clean,
make sure fuel pump is working.

If carbs are getting fuel but wont rev, you need to clean your carbs because your jets are clogged

Your not reaching the RPM level for your rev limiter are you? The rev limiter will automatically shut fire off intermittently to keep the motor from damage at high RPM's. I have to admitt I am not up to par an all these newer bikes, but if you jetted the carbs, then I doubt you have a fuel pump on the bike. Most bikes with fuel pumps use fuel injection! Call the dealer who sold you the jets and ask them what else needs to be done, I am sure they will be glad to help.

The most common reason for backfiring is lack of back pressure from changing the exhaust. To compensate for the flat spot you get you put in a jet kit. This leaves the possibility of more unburned gas getting into the exhaust. Honda uses Kei Hin carbs with a fuel shut off diaphragm to help limit this condition (backfiring) by turning off the fuel to the carbs during deceleration. Even stock exhaust can pop or backfire. Typically it is caused by air getting into the exhaust either past the copper o-ring gaskets in the head or leaking muffler connections from the head pipes to the mufflers. Try replacing all the exhaust gaskets and you should minimize this condition.

Even though you put stabilizer in the fuel, it sounds like the jets in the carburetors gummed up. Additives wont do much good at this point. You need to remove, disassemble and clean the carburetor passages and jets.

vac leak craked boots