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  #1  
Old 06-19-2019, 06:21 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, 08:35 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, 11:21 AM
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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, 05:56 AM
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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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Old 08-31-2019, 05:45 PM
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Still (slowly) trying to sort this bike out... haven't touched it in months. I took another look at the valves and I think I found the issue, at least I hope.







Kind of hard to see and photograph, but there's a "T" shaped mark on the flywheel that is supposed to line up with the notch in the ignition cover, and when that is lined up the hash marks on the cam are supposed to be level with the surface of the head. I'm hoping this is the issue... can anyone confirm?
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Old 08-31-2019, 07:15 PM
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Chase:

I've see it before, your cam chain has jumped a tooth or more!

Unless the Bubba you got the bike from did it wrong!

Get the cam aligned with the flywheel & you should be good to go!
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Old 09-01-2019, 12:41 AM
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I agree that it has jumped time. It may be time for a new timing chain and guide. This will not have an effect on the weak spark reported.

Paw Paw
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Old 09-02-2019, 02:21 PM
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Quote:
Originally Posted by PawPaw View Post
I agree that it has jumped time. It may be time for a new timing chain and guide. This will not have an effect on the weak spark reported.

Paw Paw
I'm aware that the weak spark is not related, and through my troubleshooting I don't think it was ever the problem to begin with. Here's to hoping I'm right...

I dug further into the timing issue today and found a couple new concerns...
First being some damage to the cam journals. Nothing I could hang a fingernail in, but still doesn't make me very happy.



Second being the decompressor plunger and pin are missing. I don't think this will affect anything other than making it harder to start without using the manual decompression but I need to research that some more.


But I did adjust the timing and now it looks much better... not going to try and start it until I have time to look into the decompressor pin issue some more.




Someone has definitely been into the top end at some point, what they did is anyone's guess. I noticed today after pulling the header off that the 2 small bolts on the bottom right hand side of the cylinder have been marked and there are definite tool marks on the cylinder from the ratchet. When I compression tested it I got 100psi which is the low end of factory spec so I don't think the top end is new.

Time to go do some more investigating online.
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Old 09-02-2019, 03:16 PM
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100psi is very low. Did you do it with the throttle wide open? You really want 125+.
Cam journals show oil starvation at some point in it's life.
Cam timing sure looks better.

Paw Paw
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Old 09-03-2019, 06:17 PM
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Quote:
Originally Posted by PawPaw View Post
100psi is very low. Did you do it with the throttle wide open? You really want 125+.
Cam journals show oil starvation at some point in it's life.
Cam timing sure looks better.

Paw Paw
Yes I did, but the bike has not run in at least a year so there is little to no oil in the cylinder most likely. The service manual recommends putting a small amount of oil down the spark plug hole to see if compression numbers increase but I wanted to sort out other potential issues first.
The bike almost tried to fire... I was hopeful but it sounds like I will have to replace the decompression spring and plunger in order to get it running.
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