Firing on one cyl. weak spark

Could be #1 spark plug, or ignition wire to # 1 plug. Can also be lack of proper compression in #1 cylinder, caused by bad or worn piston rings, or valves not sealing properly. When you check spark to #1 it should be a nice blue color not yellow, yellow is a weak spark and doesn't fire properly under compression

What make, model, year is the bike??? The coil likely has no gap to set. Do you have a points type ignition system?? The spark plug and any other gap(s) will always be critical so do not rely on a bit of business card or other thing unless you know it is of the correct or close to the correct thickness to achieve the proper gap. With points if the gap is to large the dwell will not be sufficient to allow an adequate voltage rise and if the spark plug gap is too small it will allow a weak spark to jump too soon and if it is too large the spark may never reach a high enough voltage to bridge the gap. It could be so weak, so low voltage that it will not bridge a spark plug gap or you will not see it or otherwise evidence it because it is so weak. If there is spark but weak and the bike will not fire this is exacerbated with increasing compression pressure as it will be harder for the spark plug to fire/spark as required with increased pressure. With some bikes if you have both of two spark plugs disconnected ie wires not grounded at the plug end there will be no spark produced. That is another reason why bike information is needed to properly address your question. Also one would like to know how you are testing for spark? You also should provide some history of the problem and why you have done whatever you have done so far?



John

Error Code: P0303
Description:
Cylinder 3 - Misfire Detected
Possible Cause:
Wiring harness, igniter and fuel injector connectors.
Ignition system. (Spark plugs gaps)
Fuel injector No. 3.
Fuel pressure.
Running out of fuel.
EVAP canister purge valve.
Evaporative emission system.
Low compression, cylinder No.3. (check valve clearance and throttle plate for excessive wear)
Engine control module (ECM).

Something else wrong with that cylinder. Check compression and swap injectors, etc.

getting gas may be the fuel pump it weak and you are not getting the gas you need to the motor

The body of the spark plug has to be "grounded" to the metal on the engine when you remove it and test it. If you did it that way, OK. If there is no spark then (and it should be a nice blue color, not orange or yellow), the the "air-gap" between the ignition coil and the flywheel needs to be checked. Some disassembly is required to get to it as these parts are under the front engine cover. The air gap should be around .010" (or about the thickness of a standard business card). If the gap is too wide, the spark will be weak. If the gap is too close, the legs of the ignition coil may slightly rub on the flywheel. If you indeed have no spark, after properly testing, then check the air-gap.
If this is a fuel related issue, post back and we will address that as well.

Test the plug either by using a spark tester tool or by having the plug removed from the cylinder, then plug it into the coil wire, then place it somewhere against the metal on the engine, It has to be metal to ground the plug to complete the circuit. Dim the light and pull the cord! You should see a nice little blue spark...yellow or orange is weak and the coil may have issues or the air gap needs to be adjusted. Don't let the plug just hang in mid-air while connected to the coil and pull the cord. That may damage the coil.

There are several possibilities.
Based on the year (2002) and low miles (19,000) it probably has the original Plugs, Wires and Coil. Rubber insulation on Wires does dry rot over a period of time. Spark Plugs should be changed about every 3 to 4 years regardless of miles. Wires should be replaced every other Spark Plug change. Translated you should be on you 3rd set of Spark Plugs and 2nd set of wires.
When working in the engine compartment and with the electrical system remember: ther is 40,000 to 75,000 volts present. Bad Wires could mean an electrical shock!
You can cheaply purchase Ignition (Spark Plug/Wire/Coil) Test Kits at most auto parts stores or even at Harbor Freight.
1) Remove the #2 Wire connection to the #2 Coil position.
2) Insert the tester into the #2 Coil position. Clipping the other end onto the engine block or onto a good ground.
3) Start the Engine.
4) You will be looking for a good strong (blue) and rhythmic spark. If it is not strong (yellow or orange) nor rhythmic, the Coil is malfunctioning and should be replaced.
5) If it is a good rhythmic spark, Turn OFF the Engine.
6) Disconnect the Tester and reconnect the #2 Wire to the #2 Coil position.
7) Disconnect the #2 Wire from the #2 Spark Plug.
8) Connect the Tester into the end of the #2 Wire. Clipping the other end to the engine block or a good ground.
9) Start the Engine.
10) Again you are looking for a strong (blue) and rhythmic spark. If is not strong (yellow or orage) and rhythmic, the Wire is bad and should be replaced. Wires should always be changed as a Set is the standard rule.
11) If it is a good rhythmic spark, Turn OFF the Engine.
12) Remove the #2 Spark Plug out of the engine.
13) Check the Igntion end of the Plug for carbon, dirt, or being fouled. Check the Gap.
14) Insert the #2 Spark Plug back into the #2 Wire.
15) Using the Spark Plug Tester. Connect the Tester to the #2 and clip the other end to the engine block or a good ground.
16) Start the Engine.
17) Once again you should be checking for a strong (blue) and rhythmic spark. If it is not strong (yellow or orange) nor rhythmic, the Spark Plug could be malfunctioning.
18) Clean the #2 Spark Plug.
19) Regap the #2 Spark Plug.
20) Retest the Spark Plug.
21) If there is now a strong (blue) and rhythmic spark, then the #2 Spark Plug is now good.
22) If there is not a strong (yellow or orange) nor rhythmic spark the #2 Spark Plug is malfunctioning and needs to be replaced. Again the standard rule is to replace the Spark Plugs as a set.

The several other possibilities relate to sensors. However without additional error codes or the check engine light on, we will assume that the problem can be solved with the solution above.

Let me know if this helped, or if you have additional information or questions. Feel free to contact me at FixYa.com!

Yes. I would start with the plug wire. If your gap is correct and you know that the coil is good. Then it should be the wire.

From karjunkie:

If the plugs get wet, then you have a spark problem somewhere in the ignition system. Pull a spark plug and hold it against a ground while you turn the engine over. You need to see a fat blue spark. If not, check the distributor, spark plug wires and coils. An easy way to test for spark is with an inexpensive spark tester. This is a device that is readily available at most auto supply stores. If the plugs do not fire properly, check the spark plug wires for fraying or cracking. Next, you need to check the coil(s). If firing voltage is low (weak spark), measure the coil primary and secondary resistance with an ohmmeter. If it is out of specifications, replace the coil. Also, make sure the coil is receiving normal voltage from the PCM or ignition module. If it is low, check the charging system for anything that may be reducing alternator output (battery and alternator cable connections, ground connections and voltage output).