Correcting spark plug fouling issues on classic Japanese motorcycles

“The plugs on my bike are sooty and wet – so the carbs are not working properly – right? I better order some carburetor rebuild kits.” 

Our carburetor rebuild kits can smooth out fueling issues and stop your classic Japanese motorcycle spark plug fouling issues. Plugs can foul for many reasons, and an incorrect air-fuel mixture is only one. We’d love to sell you some carb rebuild kits – but only if they solve your problem.

Before you start pulling the carbs apart, understand that black soot on a spark plug does not always mean the fuel mixture is rich. It can also mean the fuel isn’t burning fully, which is a sign of a weak spark rather than too much fuel. Let’s see if we can help determine the root cause of the issue. You will need a test meter — aka multi-meter — to perform these tests.  Having the workshop manual for your make and model handy is a good idea, too.

The first question is, how old are the spark plugs and the plug caps? You should replace these parts periodically to maintain peak performance on any vintage Japanese motorcycle – and now might be a good time to do it. If replacing the spark plugs and caps doesn’t help, you can perform some simple tests to assist in isolating the issue.

Many of the classic Japanese bikes’ spark plug wires are molded into the coils. It common for the plug wires to develop internal breaks. When they do, you really only have one choice: replacing the coil. On some coils, it is easy to check the plug wires, but for some others it is not. Inspect the outside of the wires for cracks in their coatings. If everything appears normal, check the resistance of the coils and wires with your multi-meter.

To check the resistance of the coils and wires:

First, disconnect the negative battery cable, then unplug the wires feeding into the coil(s) from the wiring harness. With the test meter set to ohms, connect the two probes to bare metal on the two coil wires. For bikes with points and condensers, you should measure from 3.5 ohm to 4.5 ohm, plus or minus. Your workshop manual should tell you the range for your model. If you have multiple coils, the make certain you get the same readings on both coils.

Make the next check only on coils with two spark plug wires coming out. Unscrew the plug caps, then insert the probes into the core of the plug wires. You should measure from 6,000 to 13,000 ohms. Wiggle the plug wires around while testing. If the ohm reading is bouncing around as you move the wire, it may be broken inside. Again, compare the reading you get against the workshop manual as the required range changes across brands and models. Before you thread the plug caps back into the plug wires, snip ¼ inch off each plug wire, which allows the plug cap to get better connectivity to the core of the wire. If a coil measures out of the acceptable range, it is best to replace it. Always replace coils as a pair if you have two coils.

Spark plug caps can also go bad. Most factory caps contain a 5000 ohm resistor to reduce radio frequency interference (RFI). When you check the resistance of the caps, you should see around a 5000-ohm reading for each of the caps. If the coils and caps check out okay, check the voltage reaching the coils. If you shortchange the coils on the voltage, they will shortchange you in return with a weak spark. With some electronic ignitions, insufficient voltage can be as bad as too much voltage. It can and will kill the electronic ignition.

With a test meter set on Volts DC, check the battery voltage across the two battery terminals. On a healthy batter, you should measure from 12.5 to 12.75 volts. If your reading registers below this range, either make charge the battery or replace with a new, healthy battery.

To perform the next check, you will need to remove the fuel tank.

Locate the switched power feed to your coils. This wire supplies electricity to each coil and only carries voltage when the ignition and kill switches are set to run. On most multi-coil bikes, you will have a double-female connector with two single-bullet wires, which run to each of the coils.

Insert the positive probe (+) in the switched power feed. Take the negative probe (-) back to the negative (-) terminal on the battery. With the kill switch set to run, turn the ignition on. Don’t leave it on for long or you could burn out a coil. What reading did you get? You need at least 11 volts, and ideally better than 11.5 volts, to get an effective spark. It is not unusual to measure 10 volts or less on vintage motorcycles that have been sitting idle for years.

If you are seeing less than 11 volts, you are losing power in the harness — somewhere in all the connector blocks, bullet connectors, ignition switch or kill switch. This is also a good time to see if there are signs of a previous owner hacking and splicing the wiring harness. Rewire any homespun wiring to eliminate it a possible cause of the fault.

Cleaning all the electrical contacts can help restore voltage. Start by pulling all the connectors apart. Clean them before reconnecting, then test the voltage again. Getting inside the ignition switch and kill switch is often tricky and may be impractical on some designs, but sometimes the act of sweeping the key and kill switch through the different positions can clean the contacts. If you can get the voltage back above 11.5 volts, it will usually provide a much stronger spark.

If you are still stuck under 11 volts, there is a coil relay modification you can do. While we do not recommend this approach, it is sometimes the only cost effective approach.