The short answer
Pull the plug after a good ride and look at the tip. Light tan or milk-chocolate brown is correct. Black and dry means it's running rich or fouling. Black and wet means oil or flooding. White or blistered means dangerously lean, and you should stop and sort it before you melt a piston.
A spark plug is a two dollar part that tells you more about your engine than anything else you can look at without taking it apart.
How to pull the plug
- Engine off and cool. A 2-stroke head gets hot enough to take skin off.
- Pull the rubber boot straight off the plug. Grip the boot, not the wire.
- Brush or blow the dirt out of the recess first. Anything sitting around the plug falls straight into the cylinder the moment you remove it.
- Use the plug socket that came in the tool kit. Turn counterclockwise.
- Look at the electrode end, the part that was inside the engine.
For a meaningful reading, do this right after a proper ride, not after the bike has been idling in the driveway.
What the colours mean
Light tan or light brown
Correct. Fuel mix, oil ratio and jetting are all in the right place. Put it back in.
Black, dry and sooty
Running rich. Usually one of: a dirty air filter choking the intake, the choke left on, too much idling, or a mix with too much oil in it. Clean the filter first, that fixes it most of the time.
Black and wet, smells like fuel
Flooded. Too much choke, or repeated failed start attempts. Clean and dry the plug, then try starting with the throttle held open and the choke off. See the wont start checklist.
Black and wet, smells oily
Too much oil in the mix. Check your ratio. It should be 25 to 1, which is 5.12 oz of oil per gallon. The mixing chart is here. A bike running this rich usually smokes heavily as well.
White, grey, or with a blistered, glazed look
Lean, and this is the one to take seriously. Lean running on a 2-stroke means not enough fuel and therefore not enough oil, which means not enough lubrication, which means a seized piston. Common causes are an air leak at the intake or crank seals, a blocked fuel flow, or too little oil in the mix. Stop riding until you have found it.
Specks of melted metal on the tip, or a broken electrode
Something has come apart. Do not keep running it. That plug has been detonating badly or has physically failed.
Changing the plug
Replace it once a season, or any time it is fouled and the engine runs poorly. They cost a few dollars and it is the cheapest fix on the bike.
- Match the part. Take the old plug to any auto parts store and they will match it, or read the number printed on the porcelain. Most small 2-stroke minis run something in the NGK B-series range.
- Check the gap. Use a feeler gauge or a coin-style gap tool. Typical is around 0.024 to 0.028 in, or roughly 0.6 to 0.7 mm. Adjust by gently bending the outer electrode. Never pry against the centre electrode.
- Thread it in by hand first. All the way down until it stops. If it does not turn freely by hand, back it out and start again. Cross-threading an aluminium cylinder head is an expensive mistake.
- Snug with the wrench. About a quarter turn past hand tight for a plug with a crush washer. Firm, not gorilla tight.
- Push the boot on until it clicks onto the plug terminal.
Testing whether you have spark
If the bike will not start, this takes a minute and tells you which half of the problem to chase.
- Pull the plug but leave it connected to the boot.
- Lay the metal body of the plug firmly against bare metal on the engine. It must have a solid ground.
- Make sure the kill switch is on and the tether is in place.
- Pull the starter and watch the gap. Do this out of direct sunlight, in shade, or you will not see it.
A crisp blue-white snap means spark is fine, so the problem is fuel or compression. Weak orange, or nothing at all, means an ignition problem: kill switch, tether pin, plug, plug cap, coil, or the wiring between them. If the cord will not pull at all while you are testing, that is a different job: how to fix a 50cc pull start.
Keep your hands off the metal part of the plug while you do this. Ignition voltage is high enough to be genuinely unpleasant. And do not do this near where you spilled fuel.
Keep a spare
Buy two plugs, keep one in the tool kit with the plug socket. A fouled plug is the single most common reason a bike quits mid-ride, and swapping it in the field takes three minutes if you have the part. Without it, your afternoon is over.
Questions
How often should I change it?
Once a season for normal use, or whenever it is fouled and cleaning does not restore normal running.
Can I clean a fouled plug instead of replacing it?
Short term, yes. Wire brush the deposits off the electrode, let it dry, re-gap it. It will get you home. It will also foul again sooner than a fresh one, and it does not fix whatever caused the fouling.
My plug is wet every time I try to start it
You are flooding it. Less choke, and once flooded, hold the throttle wide open with the choke off while you pull, so the engine can clear itself.
Does the plug colour change with fuel mix?
Yes, directly. Too much oil reads dark and oily, too little reads pale. It is the fastest feedback you get on whether you are mixing correctly.
Full maintenance walkthroughs at manual.pluggedby50cc.com, or text 623 320 8984.
Plugged by 50CC is an independent retailer. We do not manufacture, assemble, modify or service the bikes we sell. Every bike ships sealed in its factory crate exactly as it reaches us, about 90% assembled from the factory, and final assembly and setup are completed by the buyer. Off-road use only. Riders under 18 should always be supervised by an adult.