'82 XJ750J Possible Cam Chain (and Owner) Issues

The mixture screws were uncapped when I got the bike and the mechanic mentioned adjusting them. The rack wasn't split as far as I know so no new shaft seals. I have checked and rechecked for vacuum leaks on this bike many times and the only one I found on a manifold I sealed with silicone. I know, not the best fix, but it works. I can check for vacuum leaks once I have it running again.

I just drained a good amount of gas out of carbs 3 and 4, the only two with functioning drain screws. Both flowed continuously with the petcock set to prime and the gas that came out was perfectly clean. No rust, dirt, water, or other contaminants visible.

I just removed the plugs and I'm about to do a spark test on all of them. The only thing of note here is that number 4, the dead cylinder, was practically dripping wet. It looks like it doesn't have spark so it never lit any of its gas. I'll know more once I check for spark.

 

Make sure to only check one plug for spark at a time, or you could kill the TCI.
If #4 does not have spark, suspects are: the plug, the resistor in the plug boot, the plug wire.
Swap plugs and see if the problem follows the plug, then swap #1 and #4 plug wires and see if the problem follows the wire.

 

Hah. Unfortunately I just finished testing for spark, on all four cylinders at once. I won't do that again.

The result was perfect bright blue/white spark on all four cylinders.

 

Swap the #4 plug around to #1. Just because it sparks in open air dosen't mean that it will under compression.

 

OK time to add another problem to the list.

I swapped plugs 1 and 4. I also unscrewed the plug end on 4 to check for corrosion or other connection issues. It looked perfectly fine.

Next I checked compression on 4. I was able to quickly get up to 140 psi before the starter motor stopped engaging. It would turn the engine over for a moment then I could hear it freewheeling. Not the standard clattering sound of the clutch disengaging, but a whirring sound of the motor.

Then, when I tried hitting the starter button it engaged, turned the engine a bit, and just stopped. I hit the button a few times quickly but it just clicked (new solenoid btw).

Battery is about 6 months old and has a charge of 12.68 volts, so I don't think it's a voltage issue.

I put the bike into 5th gear and turned the rear wheel to rotate the engine. It turned over just fine with obvious resistance on the compression strokes.

My only guess is that either the motor momentarily locked up (unlikely) or the starter has failed. My only gripes with this explanation is: I've had the starter apart already a I know it's clean and greased inside and the commutator and brushes are clean and well within spec. Also, the bike had a similar issue turning over yesterday although I failed to mention it because it started eventually, but just a few days ago it was spinning over strong with the starter.

How could tightening the cam chain have messed up this engine/starter so badly? It's not like I tore the carbs apart or rebuilt the electrical system or anything that could really mess things up.

 

Have the starter brushes been replaced?

The engine turns with the rear wheel; the chain IS NOT the culprit.

Here's what's going on. You have a relatively neglected 30+ year old motorcycle, and there is maintainance that must be caught up on. You are reaping the rewards of every previous owner who just gassed it up and road it.

 

Starter brushes are pretty long. I think they're only about 1/3 into their wear range. I might try jumping the bike with our minivan (engine off of course) just to rule the battery out.

 

the solenoid clicking really means nothing, the click is the contacts moving into position to make contact but if they will, that's another story.
if it clicks again and nothing happens try crossing the big terminals with a screwdriver.

 

Bad news. Starter motor is completely dead. Even hitting it with 12v from a battery charger (now removed from bike) just sparks a little. I'm going to open it and check the windings.

 

After some intense cleaning, the commutator bars on the starter motor are all shorted together. Looks like one of the seals failed and oil got in. The oil then gummed up the commutator with graphite paste and burned up the motor. What's odd is it looks perfect. No burned resin on the windings or anything like that.

I guess it's of to eBay for a new starter!

 

Would the starter motor from an 83 XJ900 Seca or an 82 XJ650 Turbo work in an 82 XJ750J?

Scratch that. http://xjbikes.com/forums/threads/xj750-starter-motor.43727/ This thread says the 900 will work.

Just ordered a starter for $26 shipped. Hopefully it works out as that price is perfect. I'm assuming it would be a good idea to order a starter rebuild kit and just rebuild this unit straight away.

 

If the windings are good you can jut throw a rebuild kit into it. Less than $30. Any used starter will need a eebuild kit anyway, so if that dosen't work you won't be out any money. Just swap the kit over to the replaement starter (if you end up needing one).

 

What I meant is every bar on the commutator is shorted to every other bar. I checked this with the rotor outside of the motor using a continuity tester. With the motor assembled, checking from the positive post to the casing shows 0ohms. I don't think a rebuilt kit is going to fix a fused rotor, but I'll order one for the new starter.

 

OK, that'll kill the starter. It can be repaired, but not worth it since you found another already. It is worth keeping around in case you need it though. re-cutting a commutator is fiddly, but not difficult.

 

I may try to do that some time. It would be nice to have some spare parts lying around.

Do you have any good sources for the process of fixing a commutator?

 

That's basically what I've already done but with 500 grit. I was going to move to a higher grit after fixing the short. The only difference is I also scraped a bunch of graphite gunk out from between the bars. Even then the commutator is still completely shorted out.

Beyond resurfacing the copper, is there anything I can do to possibly isolate the windings again?

 

don't you find it odd that all the coils got shorted all at the same time? i think when you're measuring them, maybe you see .01 ohms and call it 0. not all meters can even measure .01 ohms.
scrape the carbon from between the copper pads a bit and try it

 

As far as I know, the individual bars of a commutator should only be connected to certain other bars on the same commutator. What I know from tests is that I have 0 ohms (or rather 0.3 ohms: the resistance produced by touching my meter's leads together) between any given bar and every other bar. I also know that the motor has failed to perform its standard function and will not turn over whether powered by the motorcycle's battery or a battery charger outputting ~6 amps max at 12 volts, plenty to spin the motor outside of the engine.

However, I do find it very strange that the entire commutator would short out all at once, but I have scraped the material out from between the bars and sanded the commutator. The motor still does not turn, even if rotated to different positions.

I don't want to give up on this motor, but I think it's reasonable to say that something dramatic happened which damaged at least the commutator.

If you have any specific testing procedures I would be more than happy to try them. Saving the motor would be a great help to this project. I'm going to try cleaning the commutator again, being as thorough as I can, just to rule that out.

 

I'd like to apologise for being presumptuous in my last post. I took the motor apart again and thoroughly sanded the commutator down to perfect copper. This time however, I also sanded off the faces of the brushes.

After reassembling I jammed the entire motor against the terminals of my old battery and, after throwing some nice sparks, the motor jumped! I held it more firmly against the battery and I got it to spin continuously! Clearly it needs a rebuild and I'm not going to cancel that new motor order, instead I'll likely rebuild the new motor and use it and keep the old one for parts.

For anyone who sees this in the future, large starter motors have essentially zero resistance on their windings and every commutator bar is actually connected. Only on small DC motors will there be a noticable increase in resistance across the different bars of the commutator.

Although I will be replacing this motor, at least now I can continue diagnosing the engine until next weekend when the motor should arrive. I'm pretty busy during the week, so expect my next update on Tuesday.