Topic: Some questions about weak spark

[Mattydiah]

I have a 77 CB750K that I'm trying to get running.  I've got the wiring sorted, as far as I can tell.  All electrical functions work as they should, starter turns over, etc.

I am, however having a problem with weak spark.  Seems to be worse on cylinders 1 and 4 than it is on 2 and 3, but all 4 cylinders are pretty weak, spark-wise, no matter how you look at it.

The bike looks to have a stock ignition system, TEC coils, which appear to be OEM Honda, and the stock Reg/Rec.  Points and condensers look almost brand new, but I know that means nothing with regards to how well they actually work.

What is my best plan of attack to isolate the issue, without just throwing money at replacing everything until it works?

[736cc]

Coils should be replaced. Run your bike in garage in the dark and watch the fireworks around the plg caps. Aftermarket coils, wires and caps a big upgrade. Correct ohms I'm not sure; depends on ignition you use.

[TwoTired]

The actual spark voltage is directly related to the voltage provided the coils.  If you are still using a battery charger to power your bike, you had better tell us its charge rating.  The bike loading may be lowering its voltage.  Guaranteed you will get weak spark when the coils are only provided with 9V.

I do not understand the recommendation to replace the coils without proof of actual failure.  I've got over 10 bikes that are still using the stock, original coils.  (Although one set got an insulation crack at the coil lead exit, due to my rough handling of the hardened ignition lead.)  Looking for arcing in a darkened garage is a good technique.

I think your continuity light behavior is as expected.  But, there are different types of test lights.  Is yours powered from the bike or its own internal battery?
In either case, be aware that one point set is always closed.  Therefore a circuit path is provided through the coil to the power lead, where it joins the other coil.  Then current can flow through the other coil, through the closed point set, to engine case.  Since the coils have resistance, the light could be dimmer due to that.  Again, you'll have to better describe the test set up.

Weak spark can be caused by dirty spark plugs, failed plug cap resistors, or crack in the insulation of the ignition wires, as well as low supplied voltage.  Do you know what the voltage is at he B/W leads attached to your coils?

Look for insulation cracks/leakage.
Measure the resistors in your plug caps.
Measure the voltage being delivered to the coils.
Use new spark plugs.

Cheers,

[TwoTired]

I was told all CB750s had 5K ohm plug caps.  All mine do.  Your's are measuring 10K ohm.  This is typically what CB550s use.  But, they have slightly different coils, too.  But, the plug caps look the same externally.

Regardless of cap resistance, 9V is pretty paltry for coil power.  Since battery voltage is usually a little over 12V, and running voltage should be about 13.8V - ish, it is not surprising that your spark is weak.

You still haven't told us your charger's rating.  EG. If it is a 5 Amp charger, and your bike is trying to suck, say, 8 Amps from it, the charger won't be able to keep the voltage at rated output.

What all are you trying to power on your bike?

Cheers,

PS. Maybe it's time to get a battery?

[CB750F2]

Matty, TT has given you good advice. I would just like to add that it is not good practice to use a battery charger connected without a battery installed. Some of the cheaper battery chargers use very little filtering consequently the voltage supply can be very "dirty". The voltage measured by a multimeter will give you the average DC value but in practice there could be quite high peaks of voltage generated by the charger. Having a battery installed and connected will absorb these peaks. Install a good battery and do your testing. Pat

[Bodi]

Using a test light on points:
Disconnect the spark plugs (or the engine may fire and injure you).
With  a 12V test light, connect across the points contacts, ignition ON. Turn the engine over slowly. The lamp should be ON with the points open, OFF with the points closed. Dim with the points closed means the contacts are dirty and a touchup with fine sandpaper or a points file should fix them.
Light connected between battery + and the moving contact: light should be ON with points closed and OFF with points open. On with points open suggests a bad condensor (disconnect it and repeat the test) or a bad points plate assembly.
Only 9V will give a pretty weak spark at best. Get the battery voltage up to float (14V or so) and measure the voltage at the coil's black power wire with one or both points sets closed (to draw coil power). It should be within 2V of the battery voltage - ie for battery at 14V, coil reading should be above 12V. More than 2V of lost voltage and I would clean/tighten all the harness bullet connectors and spray some contact cleaner into the ignition and kill switch contacts.