Topic: "Soft backfiring"

[tonian]

I finally got it to idle! Onto the next problem...

 I have a '78 550k. Cylinder 1 is "soft backfiring" as someone's thread defined it. I've never heard the term but there's a lot I don't know. Basically, it sounds like a hissing pop. Similar to releasing a quick release nozzle at the end of an air compressor's hose.
Examining the plug revealed it to be very black. Too rich?
 I adjusted the carb's air mixture screw in, so that it is less than 1 turn from seating. Same results only this time there was a traditional BANG! of a backfire as well. I wanna believe it's still the carb and not valves, but what other adjustments would you all recommend?

[mattcb350f]

A soft hiss out the exhaust on one or more cylinders, usually only noticable at idle is a leaking exhaust valve. However, one or more of the valves could also be adjusted too tight and this would cause the same thing.

Have you performed a proper valve adjustment?

 Matt.

[tonian]

nope. I'll look into that. I just didn't want to overlook a carb problem.

[mystic_1]

I've heard that noise before.  Sorta of a short, quick FFFP!  I've always equated that with what I've heard called "spitting back through the carbs".  Not sure precisely what causes it but I'd start with valve clearances and timing followed by a full carb sync.


mystic_1

[TwoTired]

The carbs are the last thing to tune in the tune up sequence, unless you like doing work twice. 

I don't remember your bike's history, but if a float valve is leaking and and making the bowls too full, it can make those sounds and black plugs, too.

Hope you didn't use 22MM for a float height on the 78's PD carbs.

Cheers,

[tonian]

nope. I adjusted them at 12.5mm but I had to tweak some of them a bit because the bowls seemed to overflow anyway. When I bought the bike, it had been stored in a barn since 1989 so I started with a carb rebuild. A leaky float valve is gotta be the problem. I have a hunch about those suckers anyway. thanks everyone.