[Honda BF75] Ignition question: Spark plug cap resistance ?

I have some trouble with heavy carbon deposit on the spark plugs causing the engine to misfire. Ignition is main suspect as compression is good.

I measured resistance of ignition system components, but one thing that is not mentioned in the service manual is the resistance of the spark plug caps.

I'm measuring 9kOhm and 11.5kOhm for the two caps. Is this a normal value? Also are Honda BF75 spark plug caps supposed to have a resistor since the spark plugs them selfs are already of the resistor type? (DR5HS or DR4HS) I thought it was either resistor in spark plug or cap, but not both? It is a breaker-point ignition.

Any help would be greatly appreciated, thanks!

Secondary ignition resistance is measured with one probe in one spark plug wire end and the other probe in the end of the other plug wire end. Total resistance should be 31.5k ohms for CDI or breaker point ignition.
The measurement is taken this way because the coil fires both plugs simultaneously.

Primary winding resistance is as follows:
Breaker point coil= 1.5 ohms
CDI coil.............= 0.8 ohms

I hope this is what you were looking for. If not, ask away.
You might want to take a look at the capacitor if the coil checks out. It is .24mfd

Cheers,
jimmyd

Hi Jimmyd,

thanks for you feedback
I have measured the resistance of all components of the ignition system. I found different values for the two spark plug caps. I was wondering what the standard value of spark plug caps resistance would be, because i can't find a number for these anywhere.

BTW, the Honda service manual I have supplies other numbers for the ignition system resistances than you mentioned in your post. Do you know where your numbers come from?

I have measured:

ignition coil (between wire ends, excluding spark plug caps): 14.6 kOhm (given: 8kOhm +-20% according to manual)

from ground to terminal located on ignition coil: 1.4 Ohm (given: 0.56 +-10% Ohm)

primary coil (the coil under flywheel): 2.0 Ohm (given: 2.0 Ohm)

spark plug caps 9 and 11.5 kOhm (given:??)

It looks like the values for the ignition coil are way of compared to the numbers in my service manual. Could this be a bad coil?
I also suspect the spark plug caps, but i'm not sure if i have to replace them with caps with or without resistors (and what resistance)?

Cheers

It's a 7.5HP from 1978 btw with breaker point ignition.

Once again, with feeling. Two "high tension" spark plug wires exit the body of the ignition coil. One of your test probes connects to the end of one of these wires and the other test probe to the end of the other wire. The value should be 31.5k ohms. This will depend on ambient temperature.

This is an "empirical" value that we have kicked around here on this forum before. The 8k ohms value in the service manual appears to be incorrect as all of the "known good" coils I've tested have not been 8k ohms but closer to the 31.5k ohm value, +/- 5%. Doesn't seem to matter if it's breaker or CDI ignition. This measures the resistance of both spark plug wires and the secondary windings of the coil simultaneously. I know of no individual test for each of the spark plug wires or "caps" independently.

Also, I don't know what test you are referring to when you say; "from ground to terminal on ignition coil: 1.4 Ohm (given 0.56 +- 10% Ohm)"

If you obtained either of these values, you have a grounded coil. Any reading from ground to any coil terminal should be AT LEAST in megaohms.

I hope this makes sense to you.