intruder 1500 poor gas mileage?

It needs rejetting to match the aftermarket pipes.

Check the engine oil, if it smells a small like fuel you may have a leak in the main jet seating.

Did you rejet the carbs when the aftermarket (guessing freer flowing/louder) pipes were installed? Bigger jets lets fuel flow so more fuel.

Needs intake adjustment to match pipes. Either jet kit for carb or Power Commander (or the like) for FI.

Try cleaning fuel tap and screen, inline filter; if any and not EFI. Rust from tank, gum/varnish can clog idle circuits and pilot jets and make you have to give her throttle a lot to compensate.

Optional reading: Could clutch be slipping? Low tires?

Opened-up exhausts often causes harder starting, lower performance, harder starting, bad idle, lower mileage. But, not by 37 %.

Also check for vacc leaks. IDK if this is carbureted or not (no year given and "carb cleaner" often used to clean Injectors, too.) Spray some WD 40 around carbies/boots/whatever intake fittings while running. If RPM changes at all, a vacc leak. tighten boots or patch w/ black Shoe Goo or silicone.

Some carbs cant be fixed even with cleaner in tank., Like a VStar 650. Carbs must be rebuilt, vac diapragms replaced. We went round and round with a dead cylinder on one; no, not ignition/timing/vacuum/compression. After two carb cleanings; I replaced carbs, only thing left. 55 MPG, now.