Flames coming out of the exhausts...

.... of my gf's Sportster. When she stopped the insides of the exhausts
(both of them) were glowing brightly. She reported a fairly abrupt loss of
power, although she also said the bike had been running roughly the
previous day. It's the carburetted engine, not FI.

What could cause that, then?

 

Paging SirPip, SirPip to the red hot glowing telephone please.

 

Senor Swellero would like a word.

 

If it was the headers that were glowing then I would be thinking air
leak or running way too lean. As it is the silencers that are glowing
inside then I am inclined to think that unburnt fuel is getting into the
silencers and then being burned off within them. At a guess, it is
running on one cylinder. The unburned fuel/air is being pumped into the
exhaust system and being ignited by the hot gasses from the other. It is
affecting both exhausts due to them being linked by a balance pipe.

 

Afterthought - one cylinder may be running intermittently or both may be
misfiring due to an ignition problem, which could also result in unburned
fuel getting into the exhaust system.

 

My old BMW R80 did that just before the exhaust valve 'mushroom' parted
company from the valve stem. Running very, very weak or exhaust valve
clearances too tight?

JB