Motorcycle Repair: single cylinder not firing has fuel, spark, and compression, plug caps, honda 650

Question
Hi Bill,

I have a 1980 CB650 C. Cylinder #4 is not firing at any speed and the exhaust drop remains cold even after a short ride.

I have checked and re-checked the carbs and have verified beyond doubt that fuel is getting into the cylinder. As well, I checked the compression and it is registering at ~120-130 psi. My last thought is that it might be electric. I have swapped coils, cables, boots, and plugs numerous times without change. Additionally, I get a blue spark outside of the cylinder.

Is there anything I'm missing? Do you have any ideas as to what the problem may be?

Thanks for your time,
Peter

Answer
Peter... normal compression for a Honda 650 Four is about 175 psi. You are down 40-50psi, unless you didn't hold the throttle open all the way when making the compression check. Compare all 4 cylinders to verify what is "normal" for your bike. If all have come up at that 120-130 level, I am suspecting valve timing issues if compression is low all the way across, like the cam timing off a tooth. That doesn't explain a dead #4, however.

Adjust the valves first, to see if the compression comes back up. If not, then you could have valve sealing issues that will require removal of the cylinder head to remedy.

Are you counting 1 thru 4 as LEFT to RIGHT sitting on the bike? It looks like the left side #1 carburetor has an air cut valve which could have a bad diaphragm causing an air leak and subsequent misfire. http://www.cmsnl.com/honda-cb650c-80-us_model473/partslist/E++1801.html  #14

As far as electrical testing... check spark plug caps for about 5k ohms. Primary windings of the coils should be around 1 to 1.5 ohms, I believe.

If you have steady spark across both mated spark plugs.. 1-4 and 2-3, then the spark units must be okay, but try swapping them, as well, just as an experiment.
http://www.cmsnl.com/honda-cb650c-80-us_model473/partslist/F++15.html  #2

Bill Silver