Unscrew the exhaust outlet from the muffler. You will find the spark arrestor screen is mostly clogged up from carbon build up. It wouldn't be long until it would not run worth a spit.

You need to secure this outlet nozzle and screen in a vice or vice grips, use a propane torch to heat up the screen to red hot and brush off with a wire brush. Do this away from any fuel or combustables as the hot carbon sparks brushed off could set something on fire.

This is caused from running at half throttle, dirty air filter or fuel mix that is old or has too much oil in it.

OR, you had one or more instances of the needle and seat hanging open a little. Raw fuel/oil mix ran into the engine and out the exhaust port and through the muffler, picking up some black carbon. Add 1 oz/gal Marvel Mystery Oil or Seafoam to your 2-cycle mix in addition to the regular amount of oil. This will clean up the whole system and keep it clean. I don't think any repair is required at this time. All 2-cycle mufflers are full of black gunk.

Is Marvel Mystery oil that good?!!!! They have been around like....forever!!! I used in my car back in the hot rod days!!! It is an oil also? Does it has any problem using with Stabil, Mobil Racing 2T or Stihl Ultra?

Hmmmm, I have a BG55 and I'm pretty sure it has a diaphram carb and not a needle and seat type.

Sounds like an overly rich condition, and I'll bet that the stain was fuel. Sounds like too much fuel entering then washing out the exhaust onto the floor.
Whats the spark plug look like? Is it clean or wet and oily?
What ratio of fuel/oil do you run? Should be 40:1.
Have you turned any screws on the carb?

You say it ran good last but has it always?

Neal

Diaphram carbs have a needle and seat.

After reading Neil's response,the problem got me thinking. At one point it ran like crap. No power and no high speed. I then tried adjusting the idle screw and both the High speed and Low speed screws with no results. Come to find out, the spark arrestor screen was plugged so I cleaned it and it ran just fine after more adjusting.I feel at this point it is likely the thing is running too rich and collecting unburned gas and oil in the muffler.It will collect a certain amount until it starts spilling out again. My question is this: Can anyone tell me the factory settings for all three adjustments so the thing runs normal again. I do not live above sea level and no it does not have a catalytic converter.

My bad, I was thinking float type.
Anywho, g-mann I tried sending you a PM but the postmaster keeps sending them back. I have the info from my manual on carb settings. I'll try later.

Neal

Are there any "tricks" to removing and reinstalling the muffler? Or is it just unscrewing the two bolts that hold it in place? I notice that there appears to be some kind of flexible rubber gasket material under the heads of the bolts.

Similar issue ...I have a bg55 with oily residue clogging the spark arrestor...after cleaning ... it cloggs after 30 min of use at max rpm....i us 95 octane gas... standsrd certified 2 cyc oil ...2.6 oz per gal...unit 4 yrs old... use..1-2hrs per week...have another same model 2yrd old with no issue...it seems like its not burning the fuel / oil mix completely..anybody got any ideas

721 , The most important issue is to ensure you have proper air / fuel ratio . This will ensure a optimium fuel burn . Your high and low jets should be set between 1-1/4 to 1-1/2 out from seated . Also your air filter must be clean to ensure a non rich fuel air mixture . Secondarily fuel filter and exhaust screen must be inspected and cleaned at least seasonally . Last but not least a Premium Synthetic oil will provide maximum engine protection with the least oily film and carbon accumulation . Your current ratio of 2.6 ounces of oil to a gallon of fuel ( 50:1 ) is the best ratio for today's new 2 cycle engines . I recommend Amsoil Sabre however Ultra or Opti- 2 are other fine oils . A new spark plug annually will provide a colour ( medium brown to tan ) indication of proper engine protection . Any darker and either to much oil or fuel !