Topic: Mystery Misfire

[TPIGroove]

Hello all. 1974 CB550, cylinders 2 and 3 have decided to not fire anymore.

The top end of the bike's been rebuilt (new head gasket, rings, tensioner, ect.). Ran fine for over a year so far after that. When I got the bike before that I redid basically everything else electrical as well, new 5ohm coils that have been working fine. I had a condenser issue that caused cylinders 2 and 3 to not fire, the cheapos from 4into1 were the problem. I solved that by putting the original 50 year old ones back on and then getting Hondaman's transitorized ignition module to bypass them. Bike ran fine as of last night. Earlier today I figured I'd clean and tighten up my drive chain, and while I was at it I put the bike into TDC at 1 and re-tightened the cam chain tensioner since it had been a while. These are the ONLY things I have done to the bike since it last ran. For troubleshooting, I pulled the plugs and went ahead and put fresh ones in to eliminate that cause, no fix. Then I swapped the coils, and cylinders 2 and 3 were still cold.

One consistent odd thing is this: I'll run the bike (roughly) for a few moments to test, then shut it off. Next time I would turn the key, flip the switch to power the coils, and I'd get a pop. It was hard to tell, but I think it was igniting the fuel in the chamber and popping it either out the airbox or the exhaust, depending on how loud the pop was.

[scottly]

Have you checked for spark with a spark plug laying against the head for those two cylinders?

[newday777]

Swap the condenser leads to the other cylinders to see if you have another failed condenser.

[TPIGroove]

I figured swapping the coils would have solved the condenser issue. Besides that, doesn't Hondaman's ignition module not require the condensers for actual spark? I swapped the coil wiring, so that should have ruled that out as a potential cause. It's definitely not the plugs, either.

[Bankerdanny]

2-3 is an ignition problem. You need to trace the wiring for that coil and double and triple check all connections are clean and tight. Take a look at the tiny bolt that holds the wires to the points. It passes through 2 piece plastic sleave to keep it from grounding out on the points. I traced an issue like that on my 750 for over a year before I finally figured it out. I finally replaced the insulator with a replacement from another points and plate I happened to have.

[HondaMan]

I might suggest unplugging the 2-3 wires from the Ignition module and plug the wires back to the points directly to see if the 2-3 side will fire. The clue: turning on the key made them fire (that 'pop' you heard) which indicates the Transistor Ignition is working, but it's not 'seeing' the points. This can happen if the points are not fully "cured", especially those made after 2006. If this fixes it, I recommend then unplugging the other side from the box, too, and plug them back together: run the bike about 100 miles with the normal setup to cure the points, then re-plug the box back in.

The points made since 2006 or so are no longer tungsten, but only tungsten-plated copper pads instead. This makes a nice thermocouple, but a poor signal switch for transistors. What needs to happen is: the points must arc for a while to arc-weld the tungsten outer plating into the copper core of the points pad, then it will work normally. I'm seeing and hearing a lot about this since 2 years ago, with riders installing new points (regardless of brand) and then they get intermittent or troubling spark from the box. Once the points have properly arc'd themselves together, it works fine again.

The makers of these points had little choice but to do this because the tungsten market cratered more than 90% when CFL lightbulbs got big, and tungsten mines globally just closed up. Tungsten then became more than 200x as much (2000% price increases) in just 3 years' time, but it is the best metal suitable for arcing applications, like points. So, plating it over copper pads in switches, contactors (i.e. power relays), and points became the way to do it.

[TPIGroove]

Alright, got some more testing today (it's sweltering out there, hard to do anything when you're covered in sweat). Pulled the condensers and put a capacitor tester to them, readings were consistent. Ran the bike with ONLY the transistor ignition module (condensers removed), and the bike did the same thing, cold on cylinders 2 and 3. I unplugged the transistor unit and restored the bike to original wiring with the condensors only. Other than a slightly harder start, same thing. I've already swapped the coils and replaced the plugs, so I'm totally certain it isn't an electrical issue at this point.

All I did beforehand to the engine was redo the cam chain tensioner. Is it possible to have it overtightened and cause this, since it's in the middle near those two cylinders? I can't fathom why it might, although even though I did a top end rebuild the tensioner spring has always been a bit rough, so typically I'll undo the nut and jiggle the set screw so it can try to find its tension, and then I'll tighten the nut while holding it with a screwdriver, but that may have overdone it a little.

[HondaMan]

Alright, got some more testing today (it's sweltering out there, hard to do anything when you're covered in sweat). Pulled the condensers and put a capacitor tester to them, readings were consistent. Ran the bike with ONLY the transistor ignition module (condensers removed), and the bike did the same thing, cold on cylinders 2 and 3. I unplugged the transistor unit and restored the bike to original wiring with the condensors only. Other than a slightly harder start, same thing. I've already swapped the coils and replaced the plugs, so I'm totally certain it isn't an electrical issue at this point.

All I did beforehand to the engine was redo the cam chain tensioner. Is it possible to have it overtightened and cause this, since it's in the middle near those two cylinders? I can't fathom why it might, although even though I did a top end rebuild the tensioner spring has always been a bit rough, so typically I'll undo the nut and jiggle the set screw so it can try to find its tension, and then I'll tighten the nut while holding it with a screwdriver, but that may have overdone it a little.

No, the tensioner won't cause things like this to happen.
What can make two cylinders feel 'weak' or run 'cold' are:
1. Bad ignition components (it seems you've already nailed this one down, though).
2. Bad sparkplug cap(s). If one of the two on a given coil is more than 2x the resistance of the other, both will misfire with a weak spark. In the dawn of these bikes the caps were 7.5k ohms (of it it is a Canadian bike, 10k ohms) and they need to be within 800 ohms of each other (at 7500 ohm level, aka 10% max difference) or the spark can be weak on both plugs of a coil.

What kind of sparkplugs are in use?

One other thing comes to mind, given the 'quality' of some modern points (notably Daiichi), is: check the timing using a strobe light to see if the 2-3 coil is firing, and if the timing might be 'late'. This will make the cylinders cooler for those 2. I have seen (on Japanese-made Daiichi and FEW from Italy, both) when a chunk of the rubbing foot on the points chipped off all of a sudden. This led (on one bike) to no spark on one coil, and the other bike would fire once in a while on that coil because the gap had closed to about 0.004", not nearly enough. Above 2500 RPM it would stop firing, but it would fire at idle.

[scottly]

Everything but the points themselves have been changed. Is there 12v on the wire from the coil when the points are open, and does the voltage drop to zero when the points are closed? Are the points set to the proper gap?

[TPIGroove]

Before I went crazy and dug into the carbs, I went and checked the points. I had to rotate the nut to accurately set the cam chain, but the 2-3 point seems to have been rubbed by my socket and thrown just barely out of alignment. I initially noticed the point plate wasn't sitting flush with the timing plate, it was bent outwards slightly at the post. I went ahead and disassembled it, straightened it out, and reassembled it with the correct gap, and the bike was hot on all four cylinders (seems to be running a bit smoother now, too). The top of the post sticks out slightly past the nut, so if you've got a wrench or socket on it it's liable to smack the point, very annoying.

I guess while I'm here, my plug boots are actually 5ohm boots, with 5ohm coils too. The plugs are D7EA. Is this an acceptable combo, or should I get a higher resistance boot?

[HondaMan]

That's a good plug/coil/cap combo for in-town riding. If you decide to hit the interstate for longer rides, try a little cooler plug like the ND X24ES-U (or if you can find them, the D8ES-L or DR8ES-L). The D8ES-L was the original plug, but our modern gasolines burn much cooler and those plugs tend to foul in city riding. The D7 (or X22ES-U) are a bit hotter and will help them stay cleaner longer.

[PeWe]

I could run HM ignition module without condensers, only a few misfirings at higher rpm.

Very important that the green wire from HM module is connected to the point plate via one of the condenser screws. Misfires if not and maybe bonus firings as well.

Is the point plate of same brand as the points?

TEC 2:3 point has a guide pin that TEC plate has, the cheap Daiichi plate not.