Tuesday, May 26, 2015

Honda CX500 - "fantastinen" Finnbox FBG47 CDI ignition

Problem Statement:

Honda doesn't produce the CDI ignition module for the early CX500 motorcycles.  The capacitors and semiconductors are now over 30 years old and probably not performing as they did when they were new.

These are the symptoms my CX500 was beginning to experience:
  • Hard Starting
  • Misfire while on choke
  • "Flat" spots in the rev range
  • Hesitation from idle
  • Misfire while accelerating hard
  • Total loss of power in the rain
As a benchmark, the bike was getting around 48mpg with backroad and commuting to work type of driving.

Naturally the carburetors were eliminated as a culprit.  They have been cleaned, rebuilt, balanced, and tweaked repeatedly.  The spark coils were replaced with new modern ones from an ATV.  The spark plugs were changed to a resistor type plug and the resistors were eliminated from the caps.  These upgrades only provided marginal improvement.  The bike no longer ran on one cylinder at start, but still had all the other problems.

I also checked the stator and source coil winding to make sure they were healthy.  
I had the whole engine out over the winter, so checking the resistance readings was easy.  The insulation was good, so I had no doubts that the G47 stator was in good health.

I started reading about the ignition module and found this post on the CX/GL500 forum.  This seemed like the appropriate solution.

(There was also a Ignitech option, which was computer programmable, and cost several hundred dollars.  This wasn't something that I was interested in.)

The Solution:

I contacted "Andy62" from the forum and he replied swiftly confirming that he could happily make a Finnbox FBG47 for me.  Like any good engineer he had decided to make what he could not purchase.  The result is a re-engineered version of the CDI ignition module similar to the original, but unique with modern components.  A few short weeks later it showed up in the mail from Finland.  The exchange of emails between Aki and myself was pleasant and I'm very grateful for his dedication and stewardship to support the Honda CX500.

... and was promptly installed
It is a well engineered product, for something that wasn't supposed to be a commercial venture.  As enthusiasts we are indebted to people like this that use their talents to support old equipment.  The board is professionally created, and the components are thoughtfully arranged and packaged.  The whole module is enclosed in a tough water-tight enclosure, with the leads coming out sealed nicely.

Hook up was easy, and he even supplied a set of mating barrel connectors for the flying leads.
From top to bottom:
White = Right Coil
Yellow = Left Coil
Blue = Kill Switch

The source wire pairs were White and White / Blue.
If you put a thin screwdriver down alongside the original twin connector you can push the original spades out and re-use the housing with the new connectors.

The ground was obvious, and went under the original hold-down screw on the left side of the bike. 
(The mounting screw on the right side of the bike was eliminated so that the new Finnbox would sit properly.)

The stator connector was also obvious, and fit directly into the original place, and secured with hook-loop "Velcro" to the inner fender.

The whole Finnbox enclosure was secured to the bike frame with "Velcro" and also zip-tied securely in place.  (Belt and Suspenders) Wires were just the right length to be routed to the original locations, and tied securely in place.

(Ignore that little cube relay, that's for my tail-lamp flasher module.)


The bike fired up easily after the final termination.  I couldn't wait to test it so I went out right away.

The initial results were very noticeable.  The bike ran evenly on both cylinders while warming up.  There was no misfire present at all.  The requirement for choke was perhaps even shortened as I rode off up the road.  The acceleration at around 4500 rpm went straight through past 5000 rpm with no flat spot or hesitation.  These are all very good results and brought a wide grin to my face.
  • Improved - Less misfires when cold
  • Solved - No flat spot on acceleration from idle
  • Solved - No flat spot or misfire at mid-range engine speed.
  • Solved - Pulls like a mule when accelerating hard.... (now I notice the clutch slips a little)
  • I haven't been in the rain yet, but I'm fairly sure that was due to weak spark, and should be fine now.
The 300 mile trip I took over this past weekend yielded a respectable 60 miles to the gallon with mostly highway riding.  I don't think I ever could do better than 55 mpg before, and I was keeping up with traffic and the rest of the motorcycle group who all had >900cc bikes.

Long term results of fuel economy are improved, but negligible.  The around-town fuel economy remains about the same in the mid 50's mpg.

Overall a successful and simple upgrade for the average rider, with all the really hard work done by friend and benefactor in Finland Mr. Aki "Andy62" Lötjönen.


  1. Hi how do I get a finnbox47 for my cx500?

  2. Read carefully and contact "Andy62"