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Old 06-20-2002, 01:03 PM
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Ignition Problems - Cylinders Misfiring

Ignition Problems
Cylinders Misfiring


by David Hansen
If you're experiencing an intermittent missing problem, first check the Coil Pick-up Wire fix in the Repair Tips forum. If that's not the culprit, Paul Streit used the following diagnostic procedure to track down the problem.

Introduction
Paul Streit
This article was written after a long and difficult exercise in tracking down and correcting a problem with no spark in cylinders #1 and #4. The diagnosis process required that I study the XS11 ignition system and create several electrical tests. Other than a few basic tools, all you need is an ohmmeter for these tests.

It began several years ago when the bike would occasionally stop firing in 2 cylinders for a short time. Since it was very brief, there was no way to tell which cylinders had missed or to determine the cause of the problem. As a result, I had no choice but to ride it that way.

Recently, the problem worsened significantly. Within a few days, the it became permanent and the bike could not be ridden. In this state, it was simple to determine that cylinders #1 and #4 were not firing. Shortly after starting it, a quick touch of each header pipe showed that the two outside pipes were cold. (You can also use water spray or window cleaner)

At this time, it would be easier to locate the cause since it had completely failed. Also, it was more urgent now that I had to drive the car to work.

Ignition Overview
There are three major ignition components with wires connecting them:
  • The ignition advance
  • The ignition unit, and
  • The ignition coils.
The pick-up coils, combined with the ignition advance, are under the cover on the left end of the crank. They detect the rotation of the crank using two magnetic pick-up coils (black rectangular things about and inch long).

Out of each coil come two wires. The four wires from the pick-up coils go all the way back to the ignition unit mounted on the rear fender. They come in via a four-wire plug. On my '79F, the mapping is as follows:
Pick-up coilIgnition unit
Upper BlueWhite/Green
Upper RustWhite/Red
Lower BlueYellow/Green
Lower WhiteYellow/Red

The ignition unit does its magic and signals the ignition coils when to fire via three wires in an eight-wire plug. The three wires attach to the ignition coils just behind the steering head under the gas tank.
  • The Red/White wire is common to both coils
  • The Orange wire controls the ignition coil for cylinders #1 and #4
  • The Gray wire controls #2 and #3.
If you take off the gas tank, you can easily get to the connectors for these wires located just above the coils.

Test and Repair Steps
The first thing to do is to check all wires and connectors. Each connector should be disconnected, cleaned by filing lightly or sanding, and lubed with something like WD-40.

A good example of this was a problem I ran into with this job. Instead of cleaning one connector, I simply pushed it together more tightly. Later, the bike had power but would not crank. I eventually tracked it down to the connector I had tightened. It was for the emergency off circuit. Tightening it had broken the connection and the bike acted as if the emergency off switch was on. After cleaning the connector, the bike was fine.

Another thing to check for is cracked spark plug wires, which allow the very high voltage spark current to escape and short to the engine. This problem is typically the worst when the engine is wet.

You can check for this by running the bike in the dark (don't do this in an enclosed area) and throwing water in the wires. Visibly check for sparks anywhere a plug wires nears the engine. Also check for sparks jumping from the plug caps past the plug to the engine.

Try correcting this problem by smearing dielectric grease on the wires, inside the plug caps, around the plug insulator, and anywhere else where shorts occur. Hopefully this solves the problem because new OEM coils and wires were $120 several years ago and are out of production. The spark plug wires are "permanently attached" to the ignition coils. However, the spark plug wires and caps can be replaced by following the instructions in the "New Wires in Old Coils" tech tip in the Repairs forum. Aftermarket brands are another choice, but somewhat pricey.

The next thing to check is the famous broken wire problem in the ignition advance. Since this is such a common problem with the XS11, the odds are pretty good that it may be the source of the problem.

There is a simple way to check the ignition advance wires.
  • Take off the four-wire plug on the ignition unit.
  • Using an ohmmeter, check the resistance between Yellow/Green and Yellow/Red, and between White/Green and White/Red.
  • I measured about 600 ohms. These pairs of wires simply run down to the pick-up coils and loop back to the ignition unit.
  • While the ohmmeter still connected, rotate the ignition advance unit a few times and move the wires around. If you lose continuity at all, or the resistance changes much, there is a problem with the wires and you should repair them.
Since the pick-up coils are nothing more than magnets, there probably isn't much more that can go wrong with them.

If the pick-up coils check out, the next thing to do is check the ignition coils. My Clymer manual says to measure the resistance between the plug wire caps for each coil. It should be 15K ohms +/- 10% at 68F.

The other test is to measure resistance between the colored wires leading to the coils from the ignition unit. At the coils themselves, expect 1.5 ohms +/- 10% at 68F.

In addition to these checks, I suggest that you measure the resistance of the colored wires at the ignition unit.
  • Pull out the eight-wire plug
  • Measure resistance between Orange and Red/White, and between Gray and Red/White.
  • Expect it to be a bit higher (approx 1 ohm higher) than what you got at the coils themselves because there is a resistor in the circuit.
  • If these tests are all OK, then the coils are probably fine.
Advanced Tests
If you've made it this far and have not found the problem yet, chances are pretty good it is the ignition unit. However, it might make sense to be more sure of that before taking the ignition unit apart or replacing it. Just be careful with these steps - if you're not careful, you could break something!
  • First, make sure everything you've unplugged or disconnected is back where it belongs.
  • Next, unscrew one spark plug from a failing cylinder and one from a cylinder that works. Stick them back into the plug caps and lay them so the hex nut part of the plug is touching the engine metal.
  • Crank the engine briefly and confirm that there is no spark on the missing cylinder and there is a spark on the good one.
To completely eliminate the ignition coils as the problem,
  • swap the orange and gray wires leading to the ignition coils.
  • Crank the engine again. If the dead plug now fires and the good one is dead, then you have a bad ignition coil.
  • Otherwise, it is either the ignition unit, the pick-up coils, or the wiring.
  • Put the orange and gray wire back into their proper connector.
The next test is to determine if the problem is in the ignition unit or the pick-up coils. This step is a little risky, so be careful.
  • First, record the wire colors in the four-wire plug on the ignition unit.
  • Now, double-check what you just wrote down.
  • Next, remove each wire connector from the plastic four-wire connector. Under each wire connector, inside the plastic plug, there is a small metal hook. Use a small screwdriver forced in from the back of the connector to unhook the connector and pull it out.
Once all four wires are out of the plug, reconnect them in their original order. Crank the engine and make sure the bad plug is still bad and the good one is still good.

Now, move the upper pair to the bottom and the lower pair to the top. In my case, White/Green and White/Red were switched with Yellow/Green and Yellow/Red. Keep each pair's order the same relative to each other.

Now, crank the bike again. If the bad stays bad and the good plug stays good, then the problem is your ignition unit. If the plug spark changes, then the pick-up coils are the problem. Carefully put the wires back into the plastic plug in their proper place.

In my case, the problem was in the ignition unit. Rather than replace it, I opened it up. The circuitry inside the unit is quite heavy-duty and it looks like it was designed to take a lot of vibration. However, in my case, the problem was obvious. Near the place where the eight-wire connector attaches, one solder connection was cracked and partially gone. A quick check with the ohmmeter confirmed that there was resistance between it and the plug connector. Within a few minutes, I had resoldered the connection. Shortly after that, I had the bike running on all four cylinders again.

It took me almost a week of nights to figure this out. I hope the time invested in recording the details helps all of my fellow XS11 riders out there. Certainly, they've taken the time to help me when I've needed it.
 

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