Spark Plug

Spark Plug

If the owner's manual specifies a spark plug, I have not found mention of it. (They do, however, specify the gap should be 0.7mm and say to keep a spare with you). I am sure my bike came with an NGK “5” heat range plug as other 2T trials bikes do.

Although the spark plug cap has an internal resistance, I think it is a good idea to use a resistor spark plug as well. Resistance there lengthens the rise-time of the spark event which helps minimize radiated electromagnetic interference that can upset the workings of the ECU.

My favorite 2T spark plug cap is the NGK TB05EMA. It is a nice waterproof cap with a 5K-ohm resistance.

I like spark plugs with a fine-wire center electrode as a matter of principle. Electrons prefer to jump off a sharp point rather than a blunt edge because a sharp point increases the concentration of the electric field.

I have been mostly using NGK's BCPR5EGP. It is a fine-wire gold-palladium resistor plug with a 5/8" hex and a solid top terminal (screw-on terminals can become loose over time). You can see that I modified the ground strap of the plug on the right. I removed some metal to decrease its thermal mass and possibly keep it hotter. Does it help? I do not know.

I almost never replace a spark plug, but I do burn off the carbon with a propane torch and grit-blast the insulator at least once a season.

Below you can see two spark plugs. The one on the left is an NGK BRK4EIX with one event of run-time. The spark plug on the right is a BCPR5EGP with one season of run-time. There are no high-speed transfers at any of my riding spots.

Trials spark plugs always seem to look wet. I have learned that carburated trials bikes seem to run their best (i.e., produce the best low-speed torque) if the mixture is quite rich. If you lean the mixture off enough for a clean plug, the engines do not perform to my liking. I do not want my trials bike to run like an MX bike.

Left NGK BRK4EIX (one event) Right NGK BCPR5EGP (one season)