Backfire after carb sync


XS400 Member
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Portland, OR
Hello all, I recently purchased an 82' xs400 that is a real custom job. To say it's a rat rod would be an understatement. I love it. I immediately set the valve clearance, gave it an oil change with a new filter and cleaned out the gas tank. I'm awaiting new spark plugs from Norway for whatever reason and my pod filters are definitely old. I'm going to replace those soon. I cleaned the carbs with carb cleaner and took out all the innards, cleaned them and blew them all out with compressed air. The left needle was pretty gunked up but with some 0000 steel wool and carb cleaner it is shiny and clean. I did a diy sync with glass bottles and ATF and the levels are even. It fires right up actually. It backfires though and when opening up the throttle, it pops and bogs down. Could my filters be too old and not creating enough vaccuum? Could it be the jets aren't tuned for pod filters? The person who built it drove it almost everyday with those pods on there but its sat for 4 years. The compression test on both cylinders reads at 110.
Any guesses on the backfire situation?