Got hot, won't start..... - Kawasaki KLR 650 Forum
1987 to 2007 Wrenching & Mods For maintaining, repair or modifications of Generation 1 KLR's. 2007 and earlier.

 
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post #1 of 7 Old 06-10-2015, 06:14 PM Thread Starter
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Got hot, won't start.....

Hola!
I was out riding in the back country and stopped for lunch. When I went to start up my bike, no joy. It would crank, and I could smell gas, but I couldn't check for spark as I didn't have a plug wrench. It was hot out, about 85 degrees, and very dry conditions. I was able to coast it to blacktop and tried to bump start it. It coughed, sputtered to life for about 2 seconds, backfired, then died again.
The next morning, when the bike was cool, it started right up with some choke. WTH?!?! By then I had my pickup and loaded the bike up and took it home.
Any ideas what I should be looking at to make sure this doesn't happen again? Thanks!!

Jeff
1993 KLR 650.
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post #2 of 7 Old 06-10-2015, 07:13 PM
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Hot starting problems are often caused by valve clearances being too tight. I don't think that would be the case with your bike though, it's usually a gradual thing. Smarter people than me will be along shortly to offer some guidance .

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post #3 of 7 Old 06-11-2015, 01:55 PM
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Fuel tank vent may be an issue, especially if you have a charcoal canister. If you can duplicate the symptoms, try opening the filler cap.

A spark check is simple enough if you have a spare plug as the wire can be plugged on to the spare to check for spark. Just make sure the plug is against the a good ground as sometimes an extraordinary gap can push voltage high enough to cause damage to plug cap, wire, coil or (rare on KLR) module.

Other likely possibles are vacuum petcock, boiling fuel in the float bowl, on and on. I'd be trying to tie to spark or fuel.
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post #4 of 7 Old 06-11-2015, 05:52 PM
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Quote:
Originally Posted by Normk View Post
. . . sometimes an extraordinary gap can push voltage high enough to cause damage to plug cap, wire, coil or (rare on KLR) module.
New one on me, Normk! How does an excessive spark gap raise voltage, and . . . what is the mechanism of damage?

No argument, just--honestly never heard of this one, and curious about the operational electrical mechanisms involved.
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post #5 of 7 Old 06-11-2015, 06:11 PM
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Let's consider the spark plug gap as most will be familiar with oscilloscope patterns of spark firing. The voltage produced at the spark gap is relative to the resistance of the gap. The higher the resistance, the higher the voltage required to fire the spark/create a high enough voltage to jump the gap. One also notices that the higher the spark voltage required to fire the plug, the shorter will be the spark duration. This is because there is a limited energy stored in the coil's magnetic field (saturated field ignitions) or in the CDI capacitor (CDI ignitions).

Under normal conditions, there is sufficient energy contained in the magnetic field to produce a spark of sufficient duration to provide reliable ignition. (More to this but this simplified example conveys the concepts).

A worn plug requires more voltage to fire so the spark duration will be less. When the spark plug wears sufficiently that the spark required to fire the plug does not allow sufficient remaining energy for a long enough spark duration, misfiring occurs.

One can do the same by taking a new plug and opening the gap to an extraordinary gap and the high firing voltage, reducing duration results in misfire.

That to set the context.

From the example showing that the plug gap both requires increased spark voltage and reduces duration, the following will be seen: of the plug gap is too great for sufficient voltage to develop to begin the spark (ionize the air gap), the spark will not jump despite that the entire magnetic field energy will be involved in driving up voltage.

In an ideal system, there would simply be no spark and the ignition would cycle to the next occurrence. What sometimes happens, especially in an older system, is that the voltage rises high enough to break through the resistance of materials elsewhere in the system. It is not unusual for the spark to jump between adjacent windings of the coil secondary inside the coil which will continue until the arcing burns the wire and then gradually increasing gap will eventually fail to jump. Regardless, if one has pulled the spark plug cap and cranked the engine with "infinite" air gap, the coil can be damaged in this way, eventually to fail later.

It is not unusual for the spark to punch through the insulation on a plug wire or plug cap, to arc down the inside of the plug cap resulting in burning/carbonizing a track which will short the spark.

In CDI systems, the result of leaving a plug cap hanging in the air can be to cause the CDI module to fail.

These are all quite common events which one sees in the profession. That's why one should not crank an engine without either disabling the ignition or providing a ground to the plug lead.

Some will be familiar with the early GM HEI distributors which originally came with black. "blakelite" rotors. If one pulled a plug wire and cranked the engine will the plug lead away from ground, the result was often no spark afterwards. Investigation will show that there is spark from the ignition coil wire and none from the plug wires. This because the voltage developed due to the excessive gap at the disconnected wire, drove the spark voltage high enough to punch straight through the rotor to the distributor shaft.

Last edited by Normk; 06-11-2015 at 06:16 PM.
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post #6 of 7 Old 06-11-2015, 07:56 PM
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Thanks for the explanation, Normk!

Wonder why the engineers/designers don't provide adequate insulation to prevent coil self-destruction. Just wondering how/why a charge would arc over from one winding to a sister winding with no ground path (otherwise provided by a successful spark discharge).

Again, appreciate the explanation; cautions/warnings against ignition system operation with excessive/infinite spark gaps should appear in owner manuals; certainly, in service manuals (maybe they are; just never noticed 'em).
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post #7 of 7 Old 06-11-2015, 08:02 PM
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The insulation issue is a space one due to the need to produce a coil as densely arranged as possible. One can get away with it most of the time with some, all the time with others and never with some types. The issue is why most techs will automatically jumper a spark plug lead to ground when doing compression tests and such unless the coil primary or ignition circuit can be disabled. I was taught that a bit over 55 years ago and one didn't not do what my dad said. Later on I saw the effects of others who didn't know. Try it with an older CDI and one can often watch the spark punch right through the insulation on the plug wire.

Manuals often don't include the very basics because they have to take some base line of assumed technician training. Same is true of fastener knowledge.
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