78 GS550 air leak maybe?

[Guest]

I have a 78 GS550. Over the winter I did a dyna electronic ignition.. And shimmed the valves. New plugs etc.. Has been running pretty nicely all spring. About a week ago I notice a couple of random backfires every now and again. Usually off the line. Just when I hit the throttle.. Pulled the plugs today.. Every plug looks a bit black and and souty.. Except for cylinder 4. Which had a white electrode and ceramic. So air leak on 4?? Where would be the most likely place for an air leak on a individual cylinder?? Carb boot?

[tom203]

I don't like souty plugs.. What you got for airfilter box,stock setup? Is filter element clean?

[Jmcb]

Yes.. Stock air box and filter. The filter was new last summer. Maybe pull it and give it a clean. But perplexed about the 1 lean looking one? What could cause 1 cylinder to run lean? Assuming all 3 carbs are set up the same?

[Big T]

Never assume anything on a 40 year old bike
Pull the carbs and check everything

[Steve]

One prime suspect would be the o-ring where the intake boot meets the cylinder head.

Another prime suspect would be a broken tip on a pilot fuel screw, but that would be rather noticeable, as it would appear to idle on just three cylinders, then the fourth one would kick in about 1/4 throttle.

.

[tom203]

I'd drop #4 bowl and hope that something might have got sucked up into pilot jet area and can be extracted - you might get lucky!