Motorcycle Repair: 1980 CB750F supersport, honda cb750f, plug caps

Question
My 1980 CB750F supersport is not firing on cylinders 2&3, i cleaned the carbs, they were quite dirty total blockage. Now #2 is firing every so often but not as well as 1&4 and #3 is still not firing at all. fresh gas, clean carbs, all four wires have spark. I just can't think of what is wrong can you help me?
       Thanks.

Answer
Blaine.  see below for illustrations of the parts on your bike.
http://www.cmsnl.com/honda-cb750f-750-super-sport-80-us_model7241/partslist/

Go to the basics, first....  Do all cylinders have good compression? Do all cylinders have clean carbs/jets and working float systems? Are there air leaks at the intake manifolds?

From there you need to check all four spark plug caps for similar resistance readings on an ohm meter. Should be about 5k ohms, generally.

The coils are paired 1-4 and 2-3, of course, so there seems to be a common problem on the middle two cylinders. Assuming that the compression is okay, and the spark plugs and caps are good, then you may have to consider the spark units, located back by the battery. Try swapping them, and see if the problem moves to the other coil set. If so, then the failing spark unit need to be replaced. You have to watch the spark outputs across the plugs to see if the sparks are all consistent on all cylinders. Sometimes, you can check for spark and see it intermittently and assume that just because you see spark that it is coming in constantly and at the right time.

Honda used the spark units on most all late 650s, DOHC 750s, 900, 1000 and 1100s, plus the CBX, so most wrecking yards should have good used ones available.

There is a vacuum nipple on the #2 carbs on the bikes with the automatic fuel valve setup. Supposedly yours doesn't have that, according to the microfiche, but they did use that feature on other 1980 machines, so check your carbs to be sure. If the fitting is left open to atmosphere, the you will have a big air leak on #2 cylinder.

As always, on vintage Hondas, check all wiring connections in the harness, have a fully charged battery in the system and be sure that the charging system is functioning properly. The rotors on these engine series fail quite a lot, then you get a low voltage condition that interferes with the spark units and ignition system output.

Bill Silver