Motorcycle Repair: 1980 GS750 with a dead cylinder, head gasket problem, puff of smoke

Question
QUESTION: Hi Wayne,
I am so glad to be able to follow up with you about my dead #4 cylinder. I appreciated your advice and am going to get a better compression tester followed by a leak down test if necessary. Here's a little more information and a couple of questions.
The carbs are like new, inside and out and the problem existed exactly the same prior to rebuilding them a couple of weeks ago.
If I pull the plug wire from #4 while it is running (whether cold or not), it makes zero difference; it is dead.
You mentioned the head gasket; I do recall once instance when I was revving it up (slightly) in the summer and I heard a loud noise at the engine site and saw a puff of smoke on the right side. It didn't seem to affect performance (that I noticed) and I haven't thought about it since. Was that my head gasket and is it possible that it WASN'T blown...or is that almost a certainty based on the noise and the smoke?
One more question, please: If there was a leak between the header and the cylinder, would that affect compression? Would it keep the cylinder from firing? Also, if the cylinder isn't firing, why does the plug look so good? (It seems it would be oily and black).
Wayne, thanks so much for your patient advice!

Brad

ANSWER: Brad,

I suspect your engine may have a burnt valve or
piston possibly. It may have burnt an
exhaust valve if the header was loose or
leaking.

-The loose header can lean out
the fuel mixture by sucking cool
air in and possibly warping or
burning the exhaust valve slightly.

-the plug may be clean because the
the cylinder may still be running
at higher speeds as a leaking valve will
more affect the idle.

-a good compression test or leak down
can confirm if this is true.
It can also tell if it might be a
head gasket problem.

-have you checked if the valve clearance
is okay on that cylinder?

---------- FOLLOW-UP ----------

QUESTION: Thanks, again, Wayne! I'm only guessing about the header leaking; upon closer inspection, I don't think that's the case. I rode it today and it runs REALLY good...but it pops on deceleration and, as near as I can tell, #4 never fires, even when I rev it up and pull the plug wire. I'm going to re-check the compression with a new gauge and let you know what I find. It may take a few days so I hope you remember my situation.
You didn't comment on me hearing a very loud "bang" and seeing smoke puff out on the right side of the engine. I'm curious to know if that would indicate a blown head gasket or could that happen without causing damage?

Thanks for all your help, Wayne. I'm confident you can help me figure this out.

Brad  

Answer
Brad,

-it is possible the head gasket blew,
depends where the smoke came from
exactly. If it came from the exhaust
header it could be a piston, valve
etc. If it came directly out the side
of the engine it probably was the
gasket.

-compression or leak down test will
tell you if it is engine damage or not.
Then you can decide your next step.

-your welcome for the help.