Changing the spark plugs & brands to avoid
Use only the recommended spark plugs listed here. There have been accounts of plug failure, including detonation, when using Champion or Autolite replacements.
The recommended CR8HSA plugs most likely won't be on the shelf at your local shop. Plan to wait from one to several days for your local parts house to get them for you. Places where ordering has not been a problem include Advance, Napa, Carquest, O'Reilly, and Kragen/Checker/Schuck's.
Why you shouldn't run Champion spark plugs in your bike
Here's a set of Champions taken from a 250. Note the melted electrode on the one on the right. Once the plugs were replaced, everything seemed to return to normal - after a LOT of cranking.
The heat range is not correct on the Champion cross reference chart. Someone at Champion messed up badly when they made the charts and then distributed the list to every parts place in the world.
What happens to the plugs is that as you ride the bike at higher speeds the plugs get damn hot, and the center electrode melts away, since it can't shed off the excessive amount of heat. Eventually there's no electrode, the bike starts to misfire, and you get stuck on the side of the road. If you're lucky.
If you're unlucky, a piece breaks off and destroys your combustion chamber. Traveling 25 miles at highway speed is about how long it takes for Champion spark plugs to die a horrible death.
We don't recommend that anyone use Champion brand spark plugs on their Ninja 250, or any other vehicle, for that matter. If your lawn mower likes them, so be it.
Changing the spark plugs in your Ninja 250
The spark plugs are recessed into the top of the engine. The owner's manual suggests they can be changed without removing the gas tank, but if you've never done this before, it's easier to remove it first.
Now you see the top of the engine. Gently remove the two ignition wires by the boots where they attach to the plugs. Grab the boots, not the wires themselves. Then take your handy spark plug wrench that came with your bike's toolkit and remove the old plugs. The stock plug tool works much better on this bike than a normal plug socket. In fact, there is NO BETTER tool than the stock tool for pulling these plugs, and it is the only way you'll get 'em off without pulling the gas tank, should you wish to try it that way.
If you use another tool, you will find that a) it doesn't bend, making it impossible to get in without removing the tank (or removing the engine) and b) the rubber bit won't grab the plug, so you'll have to fish the plug out with something else (long pliers? parts grabber?) after you've loosened it.
The stock tool doesn't have a socket-drive end. It has a 14mm nut welded to it. You'll need a 14mm socket to turn the tool.
Examine the plugs. See if they are dark, white, bent, oily... plug condition can point to engine problems. See links below.
Set the gap on your new plugs. Don't believe the parts guy when he says they come 'pre-gapped'. Correct gap is .6 - .7 mm. For gapping information, go to Proper Gapping at the NGK Spark Plug 411.
Spark plug gap measuring tools are available at most auto parts stores. The gapping tool (really a feeler gauge) should slide in with little effort and drag slightly when the clearance is correct. Gap your plug so the right gauge slides in and the next size up does not.
Put a light coating of anti-seize on the plug's threads. Try to find the anti-seize that comes in a wax crayon, AGS Thread Magic, rather than the brush-on Permatex variety. It's easy to apply accurately and will aid in keeping the anti-seize off the plug's electrode. Available at fine stores everywhere (see Ace Hardware or Canadian Tire). One member has used it for 7 years without issues. It's a soft wax crayon with nickel in it. You just "color" the threads and screw the plugs in.
One thing to remember when you are installing your plugs is that if you cross-thread the plug into the engine threads you're screwed, so you really need to make sure the plug goes in fairly easily right until the crush washer makes contact. If your plug is not going in easily, turn it back out and start again.
The trick for installing plugs is to rotate them by hand counterclockwise (backwards), and then go forward to feel the threads properly seating. Tighten with the stock tool and your fingers until finger tight, then get out the ratchet and give it 1/2 turn once you can't turn it by hand any more.
Due to the fact that the spark plug threads are recessed so far into the head, you may have trouble getting them started on the threads. Here are some suggestions for starting the plugs:
Torque is finger tight plus 1/2 turn.
Don't use a torque wrench on your plugs. Anti-seize will act as a lubricant and may lead to over-torquing the spark plugs, possibly leading to stripping the threads out of the head. This is why we don't recommend a torque wrench for this procedure. It won't be accurate. So, just use a ratchet and do not turn the plug more than 1/2 turn with your ratchet.
There is always some confusion as to what is meant by “a turn”. One turn = 360 degrees. Therefore, 1/2 turn = 180 degrees, or halfway around.
Used plugs: While it is not recommended, if for some reason you are putting your old plugs back in, the recommendation is to put them in finger tight + just a smidgen with the wrench. The crush washer is already crushed.
For more information about spark plugs, check out: