How-To: Read Spark Plugs
Reading spark plugs is an easy way to determine how an engine is running. Now, I’m going to cover the 3 most common conditions, lean, normal, and rich. Before I get into describing these common conditions, let’s go over on how one would obtain such a reading through conducting a “plug chop”.
What is a plug chop?
A plug chop is referred to taking a motorcycle out on the road at certain speed and RPM for a few minutes, without changing the throttle position. Once the few minutes are up, the rider quickly pulls in the clutch, shuts the engine off, and coasts to a stop. Once pulled over, all plugs are removed and the spark plug color is inspected.
What should I do before a plug chop?
Simply put, the motorcycle should be in great running condition.
- The electrical system should be putting out over 13V to the battery.
- Valve lash must be within specification for even compression.
- Proper point gap and ignition timing if applicable.
- No vacuum leaks.
- Carburetors must be synchronized.
- Install brand new spark plugs.
What color am I looking for on the insulator tip?
A light tan color is perfect. Note that sometimes, depending on the fuel and additives, the color may be grey instead. If it is, light grey is acceptable. No color, or white, indicates a lean condition, or lack of fuel. Dark brown or black color indicates a rich condition, or excess fuel.
Normal spark plug color
Normal spark plug color should show a light tan on the insulator tip. You can see the light brown color begin to appear on the left side of the insulator in the image below.
Lean spark plug color
A lean spark plug will have little to no color on the insulator tip. Symptoms of running lean are overheating, bogging or loss of power, and delay of returning to idle from higher RPM’s.
Rich spark plug color
The image below shows a plug off a motorcycle that was running pig rich. This plug eventually fouled and the bike would not start. A symptom of running rich is black smoke exiting from the exhaust.
1983 xj750 Yamaha,wont start?
Pulled one plug out.
Black color ,around tip,but plug is in good shape.
turning over ,
battery & starter good shape,
but no spark,
Rod,
Maybe you can provide some more information. Has this bike been sitting a while? Have you checked the other 3 plugs for spark? Have you tried new plugs?
1993 Honda Shadow 600vlx
V-twin….4 spark plug,front chamber 2spark plug (rich spark plug colour) but other chamber two spark plug seems normal…bike hestiate/jerking while accelarating but good while idle ):
seldom use it and due to that i always close the carb petcock (suggest by mechanic).
Brad, you need to take a closer look at the carb feeding the front cylinder. You can adjust the fuel screw and see if things improve, however you may have to take carb apart. Maybe it is a sticky slide or float that is causing the rich condition.
Hi there matthew,i did do the carb maybe 4 times already,but the problem still…until i met a friend the other day with same bike as mine and having the same problem…it’s due to bad ICM,he fix it & the bike run like normal,BTW thanks for the response mate.
Brad, glad you figured it out!
Spark plugs are turning white after 50k dealer says it needs carb kit. Does that sound right.
I am going to need more info than that.
Hi, Just found your blog and videos because I’m having a carb problem? I have a 1985 Kawasaki ZL900 Eliminator. Love this bike had it for about 4/5 yrs now. Bought it from a used bike dealer in Denver (high altitude) and shipped it down to Dallas. Ran great until last year. Started to emit black smoke, harder and harder to start and will only start and run on full choke, I might add that a connecting exhaust gasket underneath the bike is leaking exhaust about the same time it started blowing out black smoke. Also the bike came with pods instead of the air box. It won’t idle but revs strong and will then die and hard to start back up. I know this is long but I’m trying to include everything I can. Got any ideas?
If it will run on choke only, your pilot jets are clogged. It’s blowing black smoke since the choke is on.
First, check for vacuum leaks: https://howtomotorcyclerepair.com/how-to-diagnose-motorcycle-vaccum-leaks/
Then pull the carbs and clean them, this is your main problem.
hi Admin? am from Kenya, my motorcycle 125cc won’t start easily, it is kick start only and not electric start. so sometimes would kick like hundred times but won’t start.
so yesterday l decided to check the spark plug.. the plug had turned black, so l scratched it with a sand paper for that black colour to vanish.
then l returned it back to where it was, tight. To my surprise, just 1 kick and the motorcycle started..
please note that the spark plug is barely 1 month old.
my motorcycle would start easily today, tomorrow won’t start easily..
please help… thanks. Blessings.
Sounds like it is running too rich. What year make model?
Thanks for the reply, whats the meaning of TOO RICH? the bike is 2014 model,, tvs star , 125cc
It means that there is too much fuel. Send me a pic of the plug after some ride time.