Motorcycle Repair: yamaha banshee back fire!, fmf pipes, float bowls

Question
I just bought a 1999 Yamaha banshee ,FMF pipes, ported & polished, tores eliminators, when I first got it a month ago it ran fine, now it idles great but back fires before the bike is fully wormed up it will bog out when I give it throttle(when its in gear),when its in neutral it wont back fire and it idles fine I live in Michigan and are weather has been around 50 degrees when I bought it was 65 70, could the cold weather do this, I checked all areas wear exhaust would leak or air would leak, around carbs boost bottle...EXT. nothing the left carb does open a little faster than the right but by less than a 1/16 of and inch, could this be it, compression is around 120. but one good rip down the street and than it works fine.  Thanks

Answer
Matt... two stroke Yamahas are REALLY not my specialty!

I would drain both carbs first, to see if you have contaminated fuel from sitting. You really cannot leave old gas in motorcycles or ATVs or Watercraft anymore. It separates, grows fungus inside and does all kinds of bad things. Always drain the float bowls and fuel tank between rides, if you are not going to ride it for a week or more.

With a high-strung engine like yours, the carb synch needs to be dead on, not off by 1/16th. These cylinders are firing every stroke, so the slightest inaccuracy is going to be noticed by the engine. Pull the carbs or bowls and check all the jets for cleanliness and for any debris in the bottom of the float bowls. If everything is clean and tight, synch the carbs and try it again with fresh fuel.

Part throttle issues are governed by the needle position in the slide. Drop the clip down to richen it up and raise the clip position to lean it out. Set idle screws for best idle, when hot, then turn them in slightly and test throttle response.

Temperature and humidity will affect jetting. Read the spark plugs to see how they are burning and what adjustments might be needed to the jetting. Even compression is important, however 2 strokes can suffer from bad crankseals, sucking oil from the transmission and air from the ignition/rotor end of the crank. You have to pressure test two-strokes to really see if they are okay or not.

Remember that it is now 10 years old and stuff decays over time, so you should go over the whole machine carefully, especially checking rubber parts and moving parts for security and good sealing.

I'm sure that there are Banshee forum sites that can be more specific for your setup.

Bill MrHonda Silver