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Old 06-19-2019, 05:21 PM
cowboyona426's Avatar
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Default 2001 XR400 - weak spark

     

Still working through diagnosing a no start condition on the bike I recently bought. According to what I've found in the service manual I have, which I believe is genuine Honda, almost everything checks out electrically. Exciter coil voltage is 100v minimum, I have around 150v. Ignition coil voltage is also 100v minimum, again I'm seeing around 150v. Pulse coil voltage is 0.7v minimum, I have around 1.4v. I was told on the very informative platform of Facebook (insert sarcasm) that the exciter coil should have around 90 ohms resistance, I measured 94.

Now, in talking with other XR4 owners I should be able to remove the plug boot from the plug and let it dangle near the engine, and when kicking the bike over there should be a rather loud POP and it should be able to arc about 1/4" of spark... I have nowhere near that much spark. I can see a spark when kicking the bike over with a plug in the boot and grounded, but it's inconsistent and not very big.

I have read elsewhere that the resistor in the plug cap could be the problem. What I haven't been able to find is resistance specs for the primary and secondary sides of the coil. Hoping PawPaw will chime in as he's the resident expert on electrical stuff Anyone else have suggestions or pointers?
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Old 06-19-2019, 07:35 PM
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I do not have the exact OHM readings for the coil, but in general here are some numbers that should tell you if it is usable or not.
On the primary side... You want a reading of 15 or better. ( if lower the coil spark energy will break down under load.)
The secondary side... You want a number below 10 to get a good spark at the plug.
Have you tried removing plug cap and cutting the plug wire off about 1/4" and then putting the plug cap back on?
Have you checked the coils mounts for rust or dirt? The mounts are the ground and need to be clean. The same goes for the engine frame mounts.

Paw Paw
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Old 06-20-2019, 10:21 AM
cowboyona426's Avatar
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I did trim the coil wire, and I did remove the coil and cleaned the mounts. I did not go so far as to remove the paint beneath where the coil mounts, the bolts being threaded into the frame should be sufficient to get a good ground I would hope. I have not removed or checked any engine mounts other than the head stay.
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Old 06-21-2019, 04:56 AM
Big Maico 393's Avatar
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Cowboy,

Due as PawPaw said & clean the paint both the mounting tab on the frame & the coil.

Add some dielectric grease between the mounts, to keep it from rusting.

You could also run a ground wire from the front engine bolt to the motor. Check with a continuity tester.

Retest your spark, if not better it’s not a grounding problem.
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