Motorcycle Repair: 1974 CB360... one cyl sour, safety caps, point gap

Question
Scenario: 1974 CB360, 13K, last on the road in '95. Clean old bike that basically needed a battery to run.

Work done so far: New battery, flushed gas tank (no rust), changed oil, overall lube and once over.

Problem: At idle, the left cyl doesn't seem to fire. The exhaust is cold on that side. It occasionally emits a muffled backfire, sometimes a loud report. No restrictions in exhaust system. Stumbles on take off, seems to surge at 50 mph, backfires when decelerating. Seems to do all worse once operating temp is reached.

Work done to rectify (unsuccessfully): Added Sea Foam to gas tank, thinking it might be varnish in the jets. Removed black plastic safety caps from idle mix screws and set both to one turn out. Removed CV slide on left carb to make sure the rubber seal was good and the slide was free. Filed, then sanded (2000 grit) the points, reset with a dwell meter while running (30 degrees). Experimented with varying point gap on left cyl and with the timing with no positive results. Removed the points to make sure the leads weren't brushing the base plate, etc. Reinstalled and reset. Dumped a small quantity of sea foam directly into the left exhaust valve's adjustment port thinking it might be a sticky valve. Checked primary resitance of both coils at running temp, got 5.9 ohms on both. Switched leads (L to R) of condensers with no change. Changed spark plugs twice, changed left plug boot, checked all primary leads.

I do have an old set of coils from a CB450 that I can swap out to see if that makes a difference.  

Answer
Hi Scott.

The problem does not seem to be the coils.  It could be a short in the ignition system or varnish in the carbs.

"seafoam is not (contrary to advertising) a repair for 10 years of varnish build-up.  It, like many other fuel system treatments, is for minor (a few weeks worth) varnish build-up, not for months or years worth of it.

 The carbs need to be gone through properly and synchronized. Make sure that the valves are within tolerances as well.

Let me know what you find.
Good luck and ride safe.
FALCON