My father has had his 2001 FXSTS carb bike for 3 years and has been running great. Starting a few weeks ago (when the weather got colder) his bike started running funny. The carb started coughing and he started noticing some backfire on decel. I changed the plugs (looked good), cleaned the carb, changed his Kuryakyn hypercharger to a Arlen Ness Big Sucker (he noticed a big improvement with this change), adjusted his mixture screw, and idle. I got the carb to stop coughing and it only does it very little. But the backfiring is still there. Could it be the exhaust gaskets? He is running stock headers with Python mufflers.

It is just funny that all of the sudden the bike starting running crappy after 3 years of running smooth. I believe the backfiring through the exhaust was always present he just paid more attention to it when the carb started to cough.

Any help would be great!

 

Could be the exhaust gaskets or the flange nuts have loosened up a bit. Could also be the slip joint between the muffler and head pipe. Pull them off and apply some hi-temp RTC sealant at that joint.

 

check for intake leaks as well

 

If it's been backfiring (through the exhaust) all along, could be that someone jetted it a little too fat when doing the stage 1. Did you check the jet sizes while you were cleaning the carburetor?

When you say you cleaned the carb, does that mean complete disassembly and soaking the body in a solvent for several hours (and blowing compressed air through all the passages)? Did you check the pilot jet for partial blockage? A restricted pilot jet is a fairly common ailment, and causes stumbling/coughing when accelerating at low RPM. It would also be worse with colder weather.

 

Thanks everyone for their responses. Great feedback!

I double checked when I got home last night that when I removed the Hypercharger I cut one of the rubber hoses that T off the vaccum on top of the carb (normally it is one hose that goes into the top of the carb). This hose operated the butterflies on the Hypercharger. This hose is still sucking air into carb and I need to plug it. I will plug it tonight and see if that helps me out some.

 

That hose is sucking air into the manifold downstream of the carb, and causing the mixture to be leaner. Check the rest of the hose as well, for leaks.

Don't get me started on HYPE-er-chargers... in my opinion, one of the best scams to ever hit the market.