Motorcycle Repair: Dead cylinder #4 follow-up, blown head gasket, point ignition

Question
Hi Wayne,
I really hope you remember my questions a week, or so, about a cold cylinder on my 1980 GS750. You suggested a number of things(burned valve, blown head gasket, scorched piston, etc.)based upon the things I've tested so I won't go over that again. As I told you, #4 is not working. I tested the compression with a new tester and was very confused by my findings; I expected #4 to be low...it was high. Here are the numbers compared now and 5 years ago.

2002            Now (2007)
#1 was 150      #1 is 150
#2 was 145      #2 is 145
#3 was 150      #3 is 148
#4 was 150      #4 is 170 (This is the cold cylinder!!)

I checked them all twice and the numbers were exactly the same. Wayne, I'm more confused than ever. I expected #4 to be low...but it was high. What could possibly be the problem?? Thank you, so much, for your help!

Brad  

Answer
Brad,

I think at this point you will have to start
checking your carbs and ignition from the
beginning again. It is easy to miss something
unless you double and triple check it.

-See how far the spark will jump,
since your compression is high
you will need a healthy spark on cylinder 4,
probably about a 1/4" at least.
Try pulling the plug cap off and put the
wire directly on the spark plug.

-try spraying fuel in the carb mouth
and see if you notice a change in the
engine rpm.

I know you have tried some of these things,
but try it again when you are ready.

-some early 80's Suzuki's still had points,
this is why I asked about the points.
-------------------------------------
Are you sure this bike doesn't have contact point
ignition? That would definitely be the thing to check
if so.

--------------------------------------------------------
-the high compression number is to be expected as that
cylinder has not been running much and
may have a build up of carbon on the
piston and valves.

-now that you are sure you have good compression,
the next thing is to get some proper fuel
mixture and spark happening.

-have you tried switching cylinder number one and fours
coil wires? See if the problem moves to number 1?

-try replacing number fours coil wire, plug cap
and spark plug. New plugs can be faulty sometimes.

-i forget if you said you have inspected and cleaned
number fours carb thoroughly?

-will number 4 fire if you spray wd40 or carb
cleaner in the carb throat?

-i suspect a problem with the ignition or spark
primarily, with the carb coming in second.

-see if it will respond to a plug wire switch or replacement.
try a timing light on the wire and look for
misfiring.

-hold your hand over the back of the #4 carb and
see if you can feel any suction when it
is running.

-the problem has to be narrowed down to fuel or spark
at this point unless something really weird
is happening like a worn camshaft not opening
the valves. The suction test might help here.